Wind Chill Calculator

How to use this tool

Type the air temperature and choose °C or °F, then the wind speed in km/h, mph, m/s or knots. The result updates as you type: the hero line gives the wind chill in your unit with the other unit beside it, and the cards under it show the Canadian frostbite risk level and how much colder than the thermometer it feels. Further down are the arithmetic, a chart against wind speed at your temperature and a table shaded by risk level.

The input people most often get wrong is the wind. The index wants the sustained wind at the standard 10 m (33 ft) height in open ground, which is what a forecast or an airport observation gives you, not the gust and not a hand held reading at head height, which is already sheltered and slower. Enter the forecast wind and the result will match what Environment and Climate Change Canada or the US National Weather Service publishes for the same conditions.

What the calculation is doing

The calculator uses the JAG/TI wind chill index that Canada and the United States adopted on 1 November 2001. In metric form, wind chill = 13.12 + 0.6215 T − 11.37 V^0.16 + 0.3965 T V^0.16, with T the air temperature in °C and V the wind in km/h at 10 m. In imperial form the same index is 35.74 + 0.6215 T − 35.75 V^0.16 + 0.4275 T V^0.16 with T in °F and V in mph. The two forms agree within 0.06 of a degree over the chart domain; the calculator uses the one that matches your units and converts the answer for the other.

The index is defined only for air at or below 10 °C (50 °F) and winds above 4.8 km/h (3 mph). Above 10 °C the tool says so and gives no number. Below 5 km/h the formula misbehaves: at −20 °C with a 1 km/h wind it returns −18.6, warmer than the air. For light winds the calculator therefore uses the straight line that Environment and Climate Change Canada applies to its own hourly records, wind chill = T + ((−1.59 + 0.1345 T) ÷ 5) V, which equals the air temperature in calm air and meets the main formula at 5 km/h. Both agencies round to a whole number; the calculator does too and shows the unrounded value.

Why the curve flattens, and what the index really measures

The 2001 index is a heat transfer model of a bare human face, not a survey of how cold people say they feel. Randall Osczevski and Maurice Bluestein modelled a face with a fixed skin tissue resistance losing heat to moving air, then calibrated it in 2001 against twelve volunteers who walked at 4.8 km/h (3 mph) on a treadmill in a chilled wind tunnel with heat flow sensors on their faces. The output is the temperature of calm air that would remove heat at the same rate. Walking into still air already puts a 4.8 km/h breeze on your face, which is why the index equals the air temperature at that speed.

Convective heat loss grows far more slowly than the air speed, and once the face is modelled with its own tissue resistance the effect on the equivalent temperature comes out close to V^0.16. That exponent is the whole story of the diagram: going from 5 to 20 km/h quadruples the wind and raises V^0.16 by 25%, and going from 20 to 80 km/h quadruples it again for another 25%. Each doubling buys the same small step, so the curve is steep in the first 20 km/h and nearly flat past 40. Air temperature enters in a straight line instead: every 5 °C of air is worth about 6 points of wind chill at 20 km/h.

The model also fixes things you cannot change here: a clear night sky, so bright sun can make a day feel several degrees warmer than the index, and skin resistance from the more susceptible end of the trial group, so the freezing times are for people who frostbite easily. If you run rather than walk, the wind on your face is higher than the index assumes.

The air temperature at which each risk level starts

This table turns the index round: for a given wind, it gives the air temperature at which the wind chill reaches each Canadian risk threshold. At 20 km/h the high risk level starts once the air drops to −18.0 °C; at 40 km/h it starts at −15.4 °C. Read it as "how cold does the forecast have to be before I need to cover up for this wind".

A worked example you can check on paper

The calculator opens at −10 °C with a 20 km/h wind. Start with the wind term: 20^0.16 = 1.615. Then wind chill = 13.12 + 0.6215 × (−10) − 11.37 × 1.615 + 0.3965 × (−10) × 1.615 = 13.12 − 6.215 − 18.362 − 6.403 = −17.86, which rounds to −18. In Fahrenheit, −17.86 × 9 ÷ 5 + 32 = −0.15 °F, so the NWS chart would show 0 °F. The wind makes −10 °C feel 7.9 °C colder, and −18 is moderate risk: frostbite only over long periods outdoors without protection.

The imperial form is a check on the same conditions. −10 °C is 14 °F and 20 km/h is 12.43 mph, so V^0.16 = 1.4966, and wind chill = 35.74 + 0.6215 × 14 − 35.75 × 1.4966 + 0.4275 × 14 × 1.4966 = −0.10 °F. The metric route gave −0.15 °F; the few hundredths between them come from the rounding of the published constants, and both round to 0 °F, or −18 on the Canadian scale.

Mistakes that catch people out

Using the gust. The index is built on the sustained wind at 10 m, not the gust a forecast quotes beside it. A 60 km/h gust at −20 °C gives −36 instead of −30 for the 20 km/h sustained wind, a risk level higher than the weather justifies.

Mixing units. Typing 15 in the wind box while the unit reads km/h, when the forecast said 15 mph, understates the wind by a third. The same slip with °F and °C is worse: 0 °F is −18 °C. Check both unit selectors before you read the answer.

Reading wind chill as an air temperature. A wind chill of −48 at −30 °C with a 40 km/h wind does not mean the air is −48 °C. Antifreeze, batteries, fuel and pipes respond to the −30 °C; the index is about your skin.

Expecting a number above 10 °C. The formula gives 1 at 5 °C with a 20 km/h wind and the calculator shows it, but it stops at 10 °C, where the agencies stop. On a cool windy day the difference from the air temperature is the useful figure; there is no frostbite risk to rate.

Trusting an old chart. Charts printed before November 2001 use the 1945 Siple and Passel index, based on how fast water cooled in a container hung from a pole in Antarctica. Water freezes faster than flesh, so the old index overstated the effect of the wind and reads much colder in a gale. A poster showing −50 for a wind this calculator rates in the thirties is the old index.

Popular calculations

Each address opens the calculator with those figures filled in. The Celsius list reproduces cells of the Environment and Climate Change Canada chart and the Fahrenheit list cells of the NWS chart.

Wind chill in Celsius and km/h

Wind chill in Fahrenheit and mph

Wind in m/s, knots or mixed units

Light winds below 5 km/h

How this was checked

The engine was run against all 66 cells of the chart on the Environment and Climate Change Canada wind chill page, 0 to −50 °C and 10 to 60 km/h, and matched 65 after rounding. The exception is −30 °C at 30 km/h, printed as −45 on canada.ca, where the formula gives −45.62 and rounds to −46; the neighbouring cells at 20 and 40 km/h (−43 and −48) match the formula, so the chart cell looks like a slip and the calculator follows the formula. All 216 cells of the NWS chart, 40 to −45 °F and 5 to 60 mph, matched, including nine within 0.02 of a rounding boundary.

As an independent route, the imperial form was fed converted inputs at 20,000 points from −60 to 10 °C and 5 to 160 km/h and compared with the metric form: the largest gap was 0.07 °C, and 0.06 °C on the chart domain. The named agency examples were reproduced: −20 °C and 30 km/h gives −33, 0 °F and 15 mph gives −19 °F in the 30 minute frostbite band, and 5 °F and 30 mph gives −19 °F. The low wind line meets the formula at 5 km/h within 0.003 and equals the air temperature in calm air, and every risk level and frostbite band boundary was tested at its edges. The script is verify.js in the build files.

Common questions

What is wind chill?

Wind chill is the temperature of still air that would take heat from a bare face at the same rate as the actual air and wind together. It measures how fast you lose heat, expressed as a temperature, not a temperature the air reaches. At −20 °C with a 30 km/h wind the index is −33: your skin cools as it would in calm air at −33 °C, but the thermometer still reads −20 °C.

How is wind chill calculated?

Canada and the United States both use the 2001 JAG/TI index. In metric units, wind chill = 13.12 + 0.6215 T − 11.37 V^0.16 + 0.3965 T V^0.16, with T in °C and V the wind in km/h at 10 m. In °F and mph the constants are 35.74, 0.6215, 35.75 and 0.4275. The result is rounded to a whole number and applies at 10 °C (50 °F) or below with winds above 4.8 km/h (3 mph).

At what wind chill does frostbite happen?

Environment and Climate Change Canada rates −28 to −39 as high risk, where exposed skin can freeze in 10 to 30 minutes; −40 to −47 very high, 5 to 10 minutes; −48 to −54 severe, 2 to 5 minutes; and −55 or colder extreme, under 2 minutes. On the NWS chart the 30 minute band starts at −18 °F, the 10 minute band at −32 °F and the 5 minute band at −48 °F, for bare skin on the most susceptible people.

Why does the Canadian wind chill have no degree sign?

Because it is an index, not a temperature. Environment and Climate Change Canada reports wind chill as a plain number on the Celsius scale, "wind chill −33", to stress that the air is not that cold. The US National Weather Service reports the same index as a temperature in °F. This calculator shows both.

Can wind chill freeze pipes or a car radiator?

No. Wind chill describes heat loss from warm skin. An object with no heat source of its own cools towards the air temperature and stops there, whatever the wind. Wind makes a pipe in −5 °C air reach −5 °C sooner, never colder, and if the air is above freezing nothing freezes however low the wind chill reads.

Why does the calculator give no answer above 10 °C?

The index was fitted to wind tunnel trials at cold temperatures and both agencies define it only at or below 10 °C (50 °F). Above that the formula still produces numbers, but nobody tested them against people, so the calculator says no wind chill is defined rather than print a figure neither agency would quote. Between 0 and 10 °C it gives a value; Canada only puts wind chill in forecasts at 0 °C or below.

Related Calculators and Reference

• Humidex Calculator: the Canadian summer counterpart, how hot humid air feels.
• Heat Index Calculator: the NWS apparent temperature for hot weather.
• Speed Converter: km/h, mph, knots and m/s for a wind reported in another unit.
• Pace Calculator: the index assumes a 4.8 km/h walk; work out the pace of your winter run.
• Dew Point Calculator: the moisture figure behind humidex, and where frost forms.
• Snowboard Size Calculator: size your board before a cold day on the mountain.
• Ski Size Calculator: size your skis before a cold day on the mountain.
• Environment and Climate Change Canada: Wind chill and the Wind Chill Index: the official chart, risk levels and freezing times used here.