Wind Chill Calculator
How fast the cold takes heat off exposed skin, by the 2001 index the National Weather Service and Environment Canada both use — with the published frostbite exposure times, and a refusal to answer outside the range the formula was fitted for.
Temperature and wind
The air temperature a thermometer reads, in the shade. The index stops at 50 °F.
The sustained wind, as a forecast reports it — not the gusts.
In this wind, bare skin loses heat as fast as it would in still air at 6.2 °F — 13.8 °F below what the thermometer says.
- Wind chill
- 6.2 °F
- Air temperature
- 20 °F
- Colder by
- 13.8 °F
- Frostbite risk
- No published exposure time
| Wind (mph) | 40 °F | 30 °F | 20 °F | 10 °F | 0 °F | -10 °F | -20 °F | -30 °F |
|---|---|---|---|---|---|---|---|---|
| 5 | 37 | 25 | 13 | 1 | -11 | -22 | -34 | -46 |
| 10 | 34 | 21 | 9 | -4 | -16 | -28 | -41 | -53 |
| 15 | 32 | 19 | 6 | -7 | -19 | -32 | -45 | -58 |
| 20 | 31 | 17 | 4 | -9 | -22 | -35 | -48 | -61 |
| 25 | 29 | 16 | 3 | -11 | -24 | -38 | -51 | -64 |
| 30 | 29 | 15 | 1 | -12 | -26 | -39 | -53 | -67 |
| 35 | 28 | 14 | 0 | -14 | -27 | -41 | -55 | -69 |
| 40 | 27 | 13 | -1 | -15 | -29 | -43 | -57 | -71 |
- Wind chill describes how fast bare skin loses heat, not how cold things get. Your car cools to the air temperature and stops there however hard the wind blows — the wind only gets it there sooner.
- The index assumes bare skin, no sun and dry conditions. Direct sunshine can feel like 10 to 18 °F of relief; wet skin or wet clothing wipes out far more than that.
The 2001 wind chill index, in both unit systems
One fit, published in two forms. Temperature enters twice — once on its own and once multiplied by the wind term — which is why wind bites harder the colder it already is.
°F: Tᵣ = 35.74 + 0.6215T − 35.75V⁰·¹⁶ + 0.4275T·V⁰·¹⁶- °C form
- Tᵣ = 13.12 + 0.6215T − 11.37V⁰·¹⁶ + 0.3965T·V⁰·¹⁶
- T
- Air temperature — °F in the first form, °C in the second
- V
- Wind speed — mph in the first form, km/h in the second
This tool runs whichever form matches the units you picked rather than converting your input and running one of them. Both sets of constants are published rounded, so the two disagree by up to about 0.05 °C across the range — small, but it is the difference between agreeing with your own weather service and not.
The limits are part of the formula
The index is defined only at or below 50 °F (10 °C) and only above 3 mph (4.8 km/h). Below the wind threshold the expression turns into roughly 1.049T − 0.01, which is warmer than the air it started with: a wind chill that says a still, cold room feels milder than it is. That is not a small error at the edges, it is a sign change, and it is why this calculator stops rather than printing a number.
20 °F in a 15 mph wind, term by term
First the wind term, which everything else leans on: 15⁰·¹⁶ = 1.5423. Then the four pieces of the formula:
- 35.74 — the constant, which is what keeps a 50 °F day mapping close to 50
- 0.6215 × 20 = +12.43, the temperature acting on its own
- −35.75 × 1.5423 = −55.14, the wind stripping heat away
- 0.4275 × 20 × 1.5423 = +13.19, the cross term — warmer air gives the wind less to take
Adding them: 35.74 + 12.43 − 55.14 + 13.19 = 6.2 °F. So bare skin loses heat as fast as it would in still air at 6 °F — 13.8 °F below the thermometer, and the published chart prints exactly 6 in this cell.
Drop the temperature to 0 °F at the same 15 mph and the answer is −19.4 °F, a 19.4 °F penalty rather than a 13.8 °F one. That widening gap is the cross term at work, and it is the reason wind matters far more on a bitter day than on a merely cold one.
From water freezing in Antarctica to a model of a face
The first wind chill index was fieldwork. In 1940 and 1941, on the United States Antarctic Service Expedition, Paul Siple and Charles Passel hung plastic cylinders of water outside and timed how long they took to freeze at assorted temperatures and wind speeds. The index they published in 1945 measured something genuine — a rate of heat loss, in kilocalories per square metre per hour — and it was later recast as an “equivalent temperature” that stuck in the public mind for half a century.
The trouble is that a plastic cylinder full of water is not a face. It has different geometry, different thermal properties, no blood supply and no skin, and the wind speeds were measured well above head height. The resulting numbers were dramatic and not especially useful: on the old index, a bitter but survivable winter day could read −50.
In November 2001 the US and Canadian weather services replaced it together. The new index computes the wind at about five feet — the height of an adult face rather than the ten metres an anemometer stands at — applies modern heat transfer theory to a model of the human face, adopts a standard for skin tissue resistance, assumes a clear night sky so no sunshine flatters the result, and lowers the calm-wind threshold to 3 mph. The volunteer trials behind it had people walking at 3 mph in a chilled wind tunnel with sensors on their faces. The numbers it produces are milder than the old ones and describe the actual risk far better.
What the exposure times mean
The National Weather Service shades its wind chill chart into three bands, and this calculator reports which one you are in: frostbite possible within 30 minutes at a wind chill of −18 °F, within 10 minutes at −32 °F, and within 5 minutes at −48 °F.
Read them as thresholds for bare skin on a healthy adult in dry conditions, not as a timer. Wet skin, gusts above the sustained wind, alcohol, dehydration, fatigue, age and any condition that restricts circulation all shorten the time, and the face, ears, fingers and toes go first because they are furthest from a warm blood supply. Nothing here speaks to hypothermia, which is a separate and slower danger and can develop over hours at temperatures well above the frostbite bands — including above freezing, if you are wet.
This page is general information about a published weather index, not medical advice. If skin has gone white, waxy or numb, treat it as an emergency.
What this calculator assumes
- Bare skin, on the face, about five feet above the ground. Not your hands in gloves and not the back of your neck under a scarf.
- A clear night sky and no sunshine. Direct sun can feel worth 10 to 18 °F that the index does not credit you with.
- Dry conditions. Wet skin and wet clothing lose heat far faster than the index allows for, because water conducts heat roughly twenty-five times better than still air.
- Sustained wind, as a forecast reports it, not gusts. A gust to 40 mph in a 15 mph wind is briefly a much colder number.
- The formula is applied only inside its published domain — at or below 50 °F (10 °C), and above 3 mph (4.8 km/h). Outside it the tool refuses rather than extrapolating.
- Wind chill is a rate of heat loss from skin, not a temperature anything in the world reaches. Objects cool to the air temperature and no further.
- Results are shown to a tenth of a degree. Weather services publish whole degrees, so a forecast may differ from this page by half a degree purely in rounding.
Wind chill FAQ
Does wind chill make my car, my pipes or my plants colder than the air?
No, and this is the most common misreading of the number. Wind chill is a rate, not a temperature the world reaches. It describes how quickly bare skin loses heat, expressed as the still-air temperature that would strip heat away just as fast. An object with no internal heat source — a car engine, a water pipe, a plant, a parked bicycle — cools until it matches the air temperature and then stops. Wind gets it there sooner; it cannot take it below the air temperature. At 20 °F with a 15 mph wind, the wind chill reads 6 °F and your engine block still bottoms out at 20 °F.
Why does the calculator refuse to answer on a mild day?
Because the index is not defined there. The 2001 formula is a regression fitted for temperatures at or below 50 °F (10 °C) and wind speeds above 3 mph (4.8 km/h), and outside that range it does not merely lose accuracy, it inverts. Feed it a 1 mph wind and the arithmetic returns a wind chill warmer than the air, which is nonsense. Plenty of calculators will print that number anyway. This one stops and says why.
How long before frostbite sets in?
The National Weather Service publishes three exposure bands for bare skin on a healthy adult: frostbite is possible within 30 minutes at a wind chill of −18 °F, within 10 minutes at −32 °F, and within 5 minutes at −48 °F. Treat those as warnings rather than countdowns. Wet skin, wind gusts, alcohol, poor circulation, fatigue and age all shorten the time, sometimes drastically, and the bands say nothing about hypothermia — which develops over hours at temperatures far above the frostbite range.
Where did the wind chill formula come from?
The original came out of Antarctica. In 1940 and 1941 Paul Siple and Charles Passel measured how long it took water in plastic cylinders to freeze at various temperatures and wind speeds, and published a wind chill index in 1945. It was a real measurement of a real thing — but a plastic cylinder of water is not a face, and the resulting equivalent temperatures were notoriously alarming. The current index replaced it in November 2001 after a joint US and Canadian working group ran volunteer trials in a chilled wind tunnel and rebuilt the number from heat transfer theory instead.
Why does the 2001 index give warmer numbers than the old one?
Because it models something different and models it better. It computes the wind at face height — about five feet — rather than at the ten metres where an anemometer sits, which alone knocks a large slice off the effective wind speed. It uses a human face rather than a cylinder of water, incorporates a standard for skin tissue resistance, assumes a clear night sky with no solar warming, and lowers the calm-wind threshold to 3 mph. The net effect is that a day the old index called −40 the new one usually calls something in the −20s.
Does wind chill account for sunshine, humidity or what I am wearing?
None of the three. The index assumes bare skin, dry conditions and a clear night sky. Direct sunshine can feel worth 10 to 18 °F of relief; wet skin or damp clothing wipes out far more than that, because water conducts heat away roughly twenty-five times faster than still air does. Clothing is deliberately outside the model — it is the variable you control, and the whole point of the number is to tell you how hard your clothing has to work.
Sources and review notes
- NWS, “Wind Chill Chart” — the formula, the five-foot face model, and the statement that the index is defined only at or below 50 °F and above 3 mph
- NWS wind chill chart (PDF) — the shaded 30, 10 and 5 minute frostbite bands reproduced by this calculator
- NWS cold weather safety — hypothermia and frostbite guidance beyond the index itself
The formula has not changed since the joint US and Canadian revision took effect in November 2001, and the constants above are the published ones. What is worth rechecking is the safety guidance rather than the arithmetic: exposure advice is revised far more often than indices are.