How to use it

I would start with one of the four scenarios above the form, then change one thing at a time. Clothing and activity move the answer far more than most people expect, and the chart makes that plain. The bar on the left is the same hour with no cold stress at all, so the difference between the first two bars is what the cold is adding. The bar on the right is the coldest you could go in the same clothes before shivering starts. That is the bar I would aim for.

If you are using this for the house, set the time to the hours you are home and awake, and the usual temperature to where your thermostat sits now. The plan then gives you the extra calories per day of the change, and what that equals in brisk walking.

What the terms mean

Basal metabolic rate (BMR)
What your body burns lying still and warm, from the Mifflin-St Jeor equation.1 Divided by 24, it is your hourly base, and everything else on the chart is measured against it. The BMR calculator explains it in full.
MET
A multiple of your resting rate. Sitting quietly is about 1.3, brisk walking 4.8, running at 6 mph about 10, from the 2024 Compendium of Physical Activities.2 Activity calories here are BMR per hour × (MET − 1) × hours. The minus one keeps the resting part from being counted twice: it is already in the base.
Clo
The unit of clothing insulation. One clo was defined in 1941 as the insulation of a typical business suit, enough to keep a seated person comfortable at about 21 °C (70 °F).3 Shorts and a T-shirt are about 0.3, trousers and a long-sleeved shirt 0.6 to 0.7, add a sweater and you are near 1.0, a heavy winter coat with hat and gloves about 2.4,5
Effective temperature (Te)
The temperature your body is facing once your clothes and your own heat are accounted for. Here it is the room or air temperature, plus 6 °C for every clo you wear, plus 4 °C for every MET above rest, plus a little for body fat, less a correction for wind outdoors. The 6 °C per clo is the standard comfort-engineering rule of thumb; the 4 °C per MET is coarser, and I would not lean on it above a jog.
Thermoneutral zone and lower critical temperature
The band of temperatures in which you hold your core temperature with blood flow alone, at no extra calorie cost. Its bottom edge is the lower critical temperature. In lean young adults wearing light clothing it was measured at about 22 to 23 °C (72 to 73 °F); slightly lower in women and in people with more body fat, who are better insulated.6,7 The model puts it at an effective temperature of 27 °C (81 °F).
Comfort line and shiver line
The two numbers the plan gives you. Above your comfort line, the cold costs nothing. Below your shiver line, brown fat has run out of room and your muscles start to shiver. Between the two is the zone this calculator is built to find.
Brown fat and non-shivering thermogenesis
Brown adipose tissue burns fat and glucose to make heat rather than to store energy. In adults it sits mainly above the collarbones and along the spine.12 It switches on in mild cold, without any shivering, and that is non-shivering thermogenesis. At the coldest lean men could tolerate without shivering it added 17% to their resting rate on average, about 300 calories a day; in men with obesity, 6%.6
Shivering thermogenesis
Muscles contracting against each other, not to move you but to make heat. It can drive metabolism to about five times resting for short periods in cold water,17 and it is the main source of heat in serious cold.16 It burns muscle glycogen quickly, it is tiring, and nobody keeps it up.
Acclimation
How your body has adapted to cold you have already had. There are three patterns: more heat production (the one you want), tighter blood vessels to hold heat in, and habituation, a blunted response all round.18 On the calculator, high acclimation means more brown fat and a later shiver; low means less of both. Most adults who live indoors are low or medium.
Uncompensable cold
Cold beyond what even maximal shivering can make up for. Core temperature then falls. The calculator flags it in red; it is hypothermia, not a weight-loss setting.

How the calculator works

I built the model from the logic of the physiology studies, in five steps. Every number it uses is on this page, so you can check any result by hand, and I would rather you did than took it on trust.

  1. Base. BMR per hour × hours.
  2. Activity. BMR per hour × (MET − 1) × hours.
  3. Cold stress. ΔT = 27 − Te, and never below zero.
  4. Brown fat. Rises in a straight line with ΔT until shivering starts, where it reaches its ceiling: 18% of BMR at medium acclimation, half that at low, one and a half times at high. Above a BMI of 25 the ceiling is scaled down, to 40% of itself at a BMI of 32, because that is roughly what was measured in men with obesity.6
  5. Shivering. Starts when ΔT passes 6, 8 or 10 °C for low, medium and high acclimation, then climbs by 15% of BMR for every further degree, to a ceiling of three times BMR. Beyond that, the cold is uncompensable.

Both cold responses take time to build, so they are averaged over the exposure with a 15-minute time constant. An hour counts for about three quarters of a full hour's effect; ten minutes for very little.

I checked the model against the two room-calorimeter studies that best fit everyday life. Dropping from 24 to 19 °C (75 to 66 °F) raised 24-hour energy expenditure by 6% at the NIH, and by 5% in a second group there;8,9 the model gives about 7.5% for the same conditions. A German study found 22 °C cost about 73 calories a day more than 28 °C;10 the model gives about 70. Close enough to trust the direction and the size, not the last ten calories.

A worked example

A 45-year-old woman, 5 ft 8 in and 175 lb, has a BMR of 1,487 calories a day, or 62 an hour. She sits at home for 4 hours in trousers and a T-shirt (0.5 clo, 1.3 MET) with the thermostat at 60 °F (16 °C).

  • Effective temperature: 16 + 3 + 1.2 + 0.7 for body fat = 20.9 °C. ΔT = 27 − 20.9 = 6.1.
  • Brown fat ceiling, medium acclimation, BMI 26.6: 18% × 0.87 = 15.6%. She is 6.1 of the 8 degrees to her shiver line, so 76% of that: 11.9% of BMR.
  • Over 4 hours, allowing for the build-up: 62 × 4 × 0.119 × 0.94 = about 27 extra calories.

At 72 °F she would have burned none of those. Twenty-seven calories is about seven minutes of brisk walking. Done every day for a month it is 800 calories.

Indoors: mild and sustained

Shivering burns calories fast, and in my view it is the wrong target. It drains muscle glycogen, it is exhausting, and it makes people miserable enough to quit. What you want is the band just above your shiver line, where brown fat is working hard and your muscles are not, held for hours rather than minutes. I would rather see someone hold 64 °F all afternoon than sit at 55 °F for twenty minutes and give up.

  • 60 to 64 °F (16 to 18 °C), for part of the day. The studies that switched brown fat on used 16 to 19 °C (60 to 66 °F), from two hours a day up to six.11,12,14 The measured gain was about 5 to 6% of resting metabolism at 19 °C, and up to 17% in lean men at the coldest they could stand without shivering. In men with obesity it was nearer 6%.6,8 Figures of 10 to 20% are the top of what lean young men achieved, not what most of us should expect.
  • Dress one layer lighter than is comfortable. The cold has to reach your skin. A sweater is worth about 2 °C (4 °F), so at 60 °F a heavy sweater puts you most of the way back to a warm room. The calculator's plan tells you the lightest clothing that still keeps you above your shiver line.
  • Where brown fat sits. Above the collarbones, around the neck and along the upper spine.12 Some people put a cool pack or a cooling vest there. That is plausible, since it is the skin over the tissue, but it has not been tested as a weight-loss method; the studies cooled the whole body. If you try it, wrap the pack and take it off after 20 minutes.

If you are over 65, have heart disease or have small children in the house, the WHO's floor of 18 °C (64 °F) for a home in the cold season is the one to keep.22 Below that, use the outdoors, for a limited time.

Outdoors: dress for the effort, not the doorstep

  • Start cold, finish warm. You should feel distinctly chilly for the first ten minutes of a walk or a run. If you are comfortable the moment you step out, you are over-dressed. When you pick an activity of 3 METs or more, the plan gives you a clothing level for the effort.
  • The harder you work, the less the cold adds. At a jog your own heat moves your comfort line down by about 24 °C, so in most winter weather the cold share of the burn falls to near zero. That is not a reason to slow down. The total burn is still higher at the faster pace; it is only the cold's share that disappears. The cold matters most on easy days and when you stand still.
  • Wind adds cold. A breeze strips the layer of warm air off your skin and out of your clothing. Above freezing, on a walk you can cut short, that helps. Below freezing it is the main frostbite risk, and wind on sweat-damp clothing is how a walk becomes hypothermia. The American College of Sports Medicine advises particular care below a wind chill of −27 °C (−18 °F).19 The calculator's wind setting is a rough correction, not a wind-chill model.

The three ways your body takes it back

The body does not like losing heat, and it has ways of getting even. Some of what you will read about them online is stronger than the evidence, and I have tried to keep to what the studies show.

1. Eating it back

In one trial, men cycled for 45 minutes in water at 20 °C (68 °F) or at 33 °C (91 °F). They burned the same, about 505 calories, either way. After the cold water they ate 877 calories, 44% more than after the warm.20 So cold water did not add to the burn, and it did add to the plate. Mild cold air seems different: two and a half hours at 18 °C (64 °F) raised energy use without changing hunger or how much people ate afterwards.21

My practical answer is the same either way. Warm up with a hot drink that has no calories, black coffee, plain tea or broth, and do not reward yourself with a 300-calorie mocha. One of those cancels more than a week of a cooler thermostat.

2. Moving less

It makes sense that a cold person curls up and sits still, and that the drop in everyday movement could erase the gain. In the one calorimeter study that measured it, though, people at 19 °C did not move any less than at 24 °C.8 Even so, I think moving is the better trade. Standing instead of sitting burns more than the cold does, even though it warms you and shrinks the cold's share. A standing desk in a cool room, or ten squats on the hour, beats a blanket.

3. Adapting the wrong way

Repeated cold changes the body in one of three ways: more heat production, tighter skin blood vessels to hold heat in, or a blunted response all round.18 Only the first helps you. Which one you get depends on how cold your skin and core get and for how long. The trials that built brown fat used mild cold, daily, for 10 days to 6 weeks, and one group concluded that a variable indoor climate with frequent cool spells might be the practical way to do it.13,14,15 So pulse it: a few cool hours most days, varied, then warm.

Contrast routines, a sauna then a cold plunge, are popular for the same purpose. I know of no trial that tested them for weight, and sudden cold-water immersion is the riskiest form of cold for anyone with heart disease. If you do it, do it with someone else there.

What to expect on the scale

Not much, on its own. In a Japanese study, two hours a day at 17 °C (63 °F) for six weeks increased brown fat activity and reduced body fat mass in young men who started with little brown fat.14 That is the encouraging end. Ten days of mild cold in men with obesity increased brown fat activity but was not designed to change weight.15 I would treat a cooler house as one more lever alongside eating less and walking more, not as a strategy in itself. Fifty to one hundred calories a day matters over a winter, and I would not talk anyone out of it. It does not survive a single extra snack a day.

If you want to see what a steady small deficit does over months, and why it slows, the weight loss timeline shows it. For the walking side of the bargain, the calories burned by activity calculator covers more than 1,500 activities.

Who this is wrong for

I left these out of the model on purpose, because I could not find numbers good enough to put in it.

  • Anyone in cold water. Water draws heat about 25 times faster than air. This model is for air, and it badly underestimates cold plunges and open-water swims.
  • People on beta-blockers. Brown fat is driven by adrenaline-type signals, which beta-blockers dampen. I would expect less brown fat effect than shown. Do not stop the medicine to get it.
  • Untreated thyroid disease. An underactive thyroid makes you colder and burns less; an overactive one the reverse. Neither is captured.
  • Very lean or very large bodies. The fat-insulation and brown fat corrections are averages from a few dozen people in two studies.
  • Children. Different BMR equations, different body proportions, and a much lower threshold for harm.
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Questions people ask about this

Can I lose weight by being cold?

A little, if you stay cool for hours and do not eat it back. In a room calorimeter, 19 °C (66 °F) instead of 24 °C (75 °F) raised energy use by about 6%. For most adults that is 50 to 100 calories a day if the house stays cool from morning to night, roughly the same as a 15 to 25 minute walk. It helps. It will not replace eating less or walking more.

What temperature activates brown fat?

In the studies, 16 to 19 °C (60 to 66 °F) in light clothing, held for one to six hours. Brown fat switches on below your comfort line and works hardest just above the point where you would start to shiver. Wearing a sweater moves both lines down by about 2 °C (4 °F).

How many calories does shivering burn?

A lot, briefly. Peak shivering in cold water reaches about five times resting metabolism. Mild shivering in a cool room might add 50 to 100% of your resting rate. Nobody can keep it up, it drains muscle glycogen, and it is followed by hunger, so it is a poor weight-loss tool.

Does exercising in the cold burn more calories?

Barely. Exercise makes so much heat that the cold adds very little on top once you are warmed up. The calories come from the exercise. The cold matters most when you are sitting still.

Do cold showers burn fat?

Not in any amount you would notice on a scale. A cold shower lasts minutes, and both brown fat and shivering take several minutes to get going. If cold showers help with weight, it is by building tolerance for longer, milder exposures, and that has not been tested.

Is cold exposure safe?

Mild cold is safe for most healthy adults. If you have heart disease, high blood pressure that is not controlled, Raynaud's, or you are over 65, pregnant or have small children in the house, keep your home at 18 °C (64 °F) or warmer and ask your own doctor before using cold deliberately. Never plunge into cold water alone.

References

The equations, the clothing values, the calorimeter studies the model is checked against, the brown fat and shivering studies, and the safety guidance.

  1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247. doi:10.1093/ajcn/51.2.241 The BMR equation the base and every percentage are built on.
  2. Herrmann SD, Willis EA, Ainsworth BE, et al. 2024 Adult Compendium of Physical Activities: a third update of the energy costs of human activities. J Sport Health Sci. 2024;13(1):6–12. doi:10.1016/j.jshs.2023.10.010 The MET values on the activity slider, including 4.8 for brisk walking.
  3. Gagge AP, Burton AC, Bazett HC. A practical system of units for the description of the heat exchange of man with his environment. Science. 1941;94(2445):428–430. doi:10.1126/science.94.2445.428 Where the clo comes from.
  4. ANSI/ASHRAE Standard 55-2020. Thermal Environmental Conditions for Human Occupancy. Atlanta: ASHRAE; 2020. Clothing insulation of typical ensembles, and the rule of thumb that each 0.1 clo offsets about 0.6 °C.
  5. ISO 9920:2007. Ergonomics of the thermal environment — Estimation of thermal insulation and water vapour resistance of a clothing ensemble. Geneva: International Organization for Standardization; 2007. Clothing values, and the principle that wind and movement reduce them, behind the rough wind setting.
  6. Brychta RJ, Huang S, Wang J, et al. Quantification of the capacity for cold-induced thermogenesis in young men with and without obesity. J Clin Endocrinol Metab. 2019;104(10):4865–4878. doi:10.1210/jc.2019-00728 The brown fat ceiling and its BMI scaling: 300 kcal/day (17%) in lean men and 125 kcal/day (6%) in men with obesity at the coldest tolerable temperature without shivering. Lower critical temperature 22.9 °C lean, 21.1 °C with obesity.
  7. Brychta RJ, McGehee S, Huang S, et al. The thermoneutral zone in women takes an “arctic” shift compared to men. Proc Natl Acad Sci U S A. 2024;121(19):e2311116121. doi:10.1073/pnas.2311116121 Lower critical temperature 21.9 °C in women and 22.9 °C in men, driven mainly by body fat insulation: the reason the model adds a little for body fat.
  8. Celi FS, Brychta RJ, Linderman JD, et al. Minimal changes in environmental temperature result in a significant increase in energy expenditure and changes in the hormonal homeostasis in healthy adults. Eur J Endocrinol. 2010;163(6):863–872. doi:10.1530/EJE-10-0627 19 °C against 24 °C for 12 hours in a room calorimeter: energy expenditure up 5.96%, with no change in spontaneous movement. The model's main calibration point.
  9. Chen KY, Brychta RJ, Linderman JD, et al. Brown fat activation mediates cold-induced thermogenesis in adult humans in response to a mild decrease in ambient temperature. J Clin Endocrinol Metab. 2013;98(7):E1218–E1223. doi:10.1210/jc.2012-4213 The same comparison overnight: up 5.3%, tracking brown fat activity on PET.
  10. Henkel S, Frings-Meuthen P, Diekmann C, et al. Influence of ambient temperature on resting energy expenditure in metabolically healthy males and females. J Nutr. 2025;155:862–870. doi:10.1016/j.tjnut.2025.01.013 Half an hour at 22 °C cost 73 kcal/day more than 28 °C, and 18 °C cost 96 more: the second calibration point.
  11. van Marken Lichtenbelt WD, Vanhommerig JW, Smulders NM, et al. Cold-activated brown adipose tissue in healthy men. N Engl J Med. 2009;360(15):1500–1508. doi:10.1056/NEJMoa0808718 Brown fat active at 16 °C in 23 of 24 men, and less active in those who were overweight.
  12. Saito M, Okamatsu-Ogura Y, Matsushita M, et al. High incidence of metabolically active brown adipose tissue in healthy adult humans: effects of cold exposure and adiposity. Diabetes. 2009;58(7):1526–1531. doi:10.2337/db09-0530 Brown fat above the collarbones and along the spine, switched on by 2 hours at 19 °C, far more often in people under 35, and more active in winter.
  13. van der Lans AA, Hoeks J, Brans B, et al. Cold acclimation recruits human brown fat and increases nonshivering thermogenesis. J Clin Invest. 2013;123(8):3395–3403. doi:10.1172/JCI68993 Ten days of cold acclimation raised brown fat and non-shivering thermogenesis, with less shivering, and the suggestion of a variable indoor climate.
  14. Yoneshiro T, Aita S, Matsushita M, et al. Recruited brown adipose tissue as an antiobesity agent in humans. J Clin Invest. 2013;123(8):3404–3408. doi:10.1172/JCI67803 Two hours a day at 17 °C for six weeks raised brown fat and cold-induced thermogenesis, and lowered body fat, in men who started with little brown fat.
  15. Hanssen MJ, van der Lans AA, Brans B, et al. Short-term cold acclimation recruits brown adipose tissue in obese humans. Diabetes. 2016;65(5):1179–1189. doi:10.2337/db15-1372 The same in ten men with obesity, over ten days.
  16. Haman F, Blondin DP. Shivering thermogenesis in humans: origin, contribution and metabolic requirement. Temperature (Austin). 2017;4(3):217–226. doi:10.1080/23328940.2017.1328999 Shivering as the main source of heat in serious cold, and how brown fat changes how much of it is needed.
  17. Eyolfson DA, Tikuisis P, Xu X, Weseen G, Giesbrecht GG. Measurement and prediction of peak shivering intensity in humans. Eur J Appl Physiol. 2001;84(1–2):100–106. doi:10.1007/s004210000329 Peak shivering at 4.9 times resting metabolism, in cold water. The model's ceiling is lower, at four times, because sustained shivering in air does not reach the peak.
  18. Castellani JW, Young AJ. Human physiological responses to cold exposure: acute responses and acclimatization to prolonged exposure. Auton Neurosci. 2016;196:63–74. doi:10.1016/j.autneu.2016.02.009 The three patterns of cold acclimatization: habituation, metabolic and insulative.
  19. Castellani JW, Young AJ, Ducharme MB, et al. American College of Sports Medicine position stand: prevention of cold injuries during exercise. Med Sci Sports Exerc. 2006;38(11):2012–2029. doi:10.1249/01.mss.0000241641.75101.64 Safe exercise in the cold, and the −27 °C wind-chill threshold for frostbite.
  20. White LJ, Dressendorfer RH, Holland E, McCoy SC, Ferguson MA. Increased caloric intake soon after exercise in cold water. Int J Sport Nutr Exerc Metab. 2005;15(1):38–47. doi:10.1123/ijsnem.15.1.38 The same 505 calories burned in cold and neutral water, then 44% more eaten after the cold.
  21. Langeveld M, Tan CY, Soeters MR, et al. Mild cold effects on hunger, food intake, satiety and skin temperature in humans. Endocr Connect. 2016;5(2):65–73. doi:10.1530/EC-16-0004 Two and a half hours at 18 °C raised energy use without changing hunger or intake.
  22. World Health Organization. WHO Housing and Health Guidelines. Geneva: WHO; 2018. who.int 18 °C as the minimum indoor temperature for a home in the cold season, higher for vulnerable groups.