Heat Index Calculator
Find the feels like temperature from air temperature and relative humidity or dew point, with the NWS risk level, chart and every step of the formula.
Heat stroke, heat cramps or heat exhaustion are possible with prolonged exposure or physical activity.
Scale from 70 to 140 °F. Marks show where Caution, Extreme Caution, Danger and Extreme Danger begin.
Extreme Caution: 90 to 102 °F
What to do at this level, from NWS, CDC and OSHA guidance.
- Move hard work or exercise to the cooler early morning or evening.
- Rest in shade or air conditioning and drink water often.
- New or returning outdoor workers need time to acclimatize (OSHA).
Danger starts at 103 °F, 3 °F above this value.
Heat stroke signs: confusion, slurred speech, loss of consciousness, seizures or very high body temperature. Call 911 and cool the person right away.
Your calculation
The NWS Weather Prediction Center algorithm, run with your numbers.
- Simple formula: 0.5 × (90 + 61 + (90 − 68) × 1.2 + 60 × 0.094) = 91.52 °F.
- 91.52 is 80 or more, so apply the Rothfusz regression: −42.379 + 2.04901523 × 90 + 10.14333127 × 60 − 0.22475541 × 90 × 60 − 0.00683783 × 90² − 0.05481717 × 60² + 0.00122874 × 90² × 60 + 0.00085282 × 90 × 60² − 0.00000199 × 90² × 60² = 99.68 °F.
- No adjustment: the low humidity rule needs RH below 13% with T from 80 to 112 °F, and the high humidity rule needs RH above 85% with T from 80 to 87 °F.
- Rounded to the nearest degree: 100 °F, in the Extreme Caution band (90 to 102 °F).
Heat index chart
Heat index in °F by air temperature (columns) and relative humidity (rows), from the same algorithm.
Outlined: the cell nearest your inputs, 90 °F and 60% humidity.
The heat index assumes shade and light wind. It is an estimate for planning, not a diagnosis: follow local NWS alerts and get emergency care for heat stroke symptoms.
How the Heat Index Calculator Works
This heat index calculator finds the feels like temperature from the air temperature and the relative humidity, or from the air temperature and the dew point. It uses the algorithm published by the National Weather Service (NWS) Weather Prediction Center, accepts Fahrenheit or Celsius, and shows the result, the NWS risk band, the branch of the formula that produced it and each step of the arithmetic.
The heat index, also called the apparent temperature, describes how hot the air feels when humidity is combined with temperature. Sweat cools the body only when it evaporates, and humid air slows evaporation. At the same thermometer reading a humid afternoon therefore feels hotter than a dry one, and very dry air can feel slightly cooler than the air temperature.
The values trace back to a 1979 model by Robert G. Steadman. According to the NWS office in Wichita, it assumes an adult of 147.7 lb and 5 ft 7 in, walking at 3.1 mph in long pants and a short sleeved shirt, in the shade. Because of the shade assumption, NWS notes that exposure to full sunshine can increase heat index values by up to 15 °F.
Typical uses include planning outdoor work, sports practice, runs or yard work, reading a heat forecast, choosing when to hold an outdoor event, and checking a homework answer against the NWS chart.
Heat Index Formula: The NWS Algorithm Step by Step
The NWS computes the heat index in up to three stages. T is the air temperature in °F and RH is the relative humidity in percent. The simple formula is computed first; it already averages the air temperature with Steadman's estimate, because 0.5 × (T + X) is the mean of T and X.
Worked example: 90 °F and 60 percent humidity
Rothfusz regression explained
Lans P. Rothfusz fitted the regression in a 1990 NWS Technical Attachment (SR 90‑23) so that one equation reproduces Steadman's tables. It is a second order polynomial in temperature and humidity with cross terms. The NWS Amarillo office gives its error as plus or minus 1.3 °F, and the WPC states that it is not valid for extreme temperature and humidity beyond the range of Steadman's data.
Adjustment and low value examples
When the simple result is under 80 °F, the heat index stays close to the air temperature and no NWS risk band applies. The NWS online calculator returns the air temperature itself at or below 40 °F, so this calculator asks for an air temperature above 40 °F. For cold, windy weather use the Wind Chill Calculator.
Heat Index Chart
Find the column for the air temperature and the row for the relative humidity. Values are in °F, rounded to the nearest degree, from the same algorithm the calculator runs. The results panel shows the full NWS style grid from 80 to 110 °F in 2 degree steps, with the cell nearest your inputs outlined.
| RH | 80 °F | 85 °F | 90 °F | 95 °F | 100 °F | 105 °F | 110 °F |
|---|---|---|---|---|---|---|---|
| 40% | 80 | 84 | 91 | 99 | 109 | 121 | 136 |
| 50% | 81 | 86 | 95 | 105 | 118 | 134 | — |
| 60% | 82 | 89 | 100 | 113 | 129 | — | — |
| 70% | 83 | 93 | 106 | 123 | — | — | — |
| 80% | 84 | 97 | 113 | 134 | — | — | — |
| 90% | 86 | 102 | 122 | — | — | — | — |
| 100% | 89 | 108 | 132 | — | — | — | — |
A dash marks a heat index above 137 °F. The official NWS chart leaves those cells blank and labels the area extreme danger, so this page does not print extrapolated numbers there.
Of the 135 numbered cells on the 2024 NWS heat index chart, 125 match this calculator exactly. Eight of the other ten sit in the humid corner from 80 to 86 °F at 90 to 100 percent humidity, where this calculator reads 1 to 2 °F higher. Those eight match once the WPC high humidity adjustment is left out, so the printed chart appears to show the regression without it. The last two, 90 °F at 45 and 70 percent, differ by 1 °F, inside the stated error of the regression.
Heat Index Risk Levels: What Is a Dangerous Heat Index?
NWS groups heat index values into four classifications based on the likely effect on the body with prolonged exposure or physical activity.
| Classification | Heat index | Effect on the body (NWS) |
|---|---|---|
| Caution | 80 to 89 °F (26.7 to 31.7 °C) | Fatigue possible |
| Extreme Caution | 90 to 102 °F (32.2 to 38.9 °C) | Heat stroke, heat cramps or heat exhaustion possible |
| Danger | 103 to 124 °F (39.4 to 51.1 °C) | Heat cramps or heat exhaustion likely; heat stroke possible |
| Extreme Danger | 125 °F or higher (51.7 °C or higher) | Heat stroke highly likely |
NWS lists the bands as 80 to 90, 90 to 103, 103 to 124 and 125 °F or higher, so the published edges overlap. This calculator rounds the heat index to a whole degree and places 90 in Extreme Caution and 103 in Danger, which matches the shading on the NWS chart. A heat index of 103 °F or more is the point NWS describes as leading to dangerous heat disorders.
Heat advisory and extreme heat warning criteria
There is no single national heat index trigger for alerts. Each NWS forecast office sets criteria for its own climate. The Caribou, Maine office, for example, starts alert procedures when the heat index is expected to exceed 105 to 110 °F, depending on local climate, for at least 2 consecutive days. Check the criteria and current alerts from the office that serves your area.
On March 4, 2025, NWS renamed the Excessive Heat Watch and Excessive Heat Warning to the Extreme Heat Watch and Extreme Heat Warning as part of its Hazard Simplification Project. Older articles that mention an excessive heat warning refer to the same product.
Heat Index in Celsius
The NWS equations were fitted in Fahrenheit. To get the heat index in Celsius, convert the temperature, run the formula, then convert the answer back.
| RH | 27 °C | 31 °C | 35 °C | 39 °C |
|---|---|---|---|---|
| 40% | 27 | 31 | 37 | 46 |
| 50% | 27 | 33 | 41 | 52 |
| 60% | 28 | 35 | 45 | — |
| 70% | 29 | 38 | 50 | — |
| 80% | 30 | 41 | 57 | — |
| 90% | 31 | 45 | — | — |
| 100% | 33 | 49 | — | — |
Values are heat index in °C; a dash is above 137 °F (58.3 °C). Select °C above to see the full Celsius grid. For other unit changes, use the Conversion Calculator.
Heat Index with Dew Point
Dew point measures the moisture in the air directly. To use it, convert the dew point and air temperature to relative humidity, then run the normal formula. This calculator uses the vapor pressure formula from the NWS online heat index calculator, with temperatures in °C.
The dew point can never be higher than the air temperature; when the two are equal, relative humidity is 100 percent. The NWS online calculator rounds the converted humidity to a whole percent before computing the heat index. This calculator keeps the decimals, which can shift the result by a few tenths of a degree. In RH mode the dew point appears as a stat tile; for dew point on its own, use the Dew Point Calculator.
Heat Index vs Temperature vs Wet Bulb Globe Temperature
The air temperature is what a shaded thermometer reads. The heat index adds humidity, so the gap between the two grows quickly in moist air. At 90 °F the heat index is 91 °F at 40 percent humidity, 100 °F at 60 percent, 113 °F at 80 percent and 132 °F at 100 percent. In dry air it can fall below the air temperature: at 100 °F and 15 percent humidity the heat index is 96 °F.
Wet bulb globe temperature (WBGT) is a different measure of heat stress. It adds sun angle, cloud cover and wind to temperature and humidity, and is measured in direct sunlight:
The NWS Wichita office lists an example at 90 °F with a 65 °F dew point and 5 percent cloud cover: the heat index is 92 °F, while WBGT is 89 °F in a 3 mph wind and 83 °F in a 13 mph wind. The heat index ignores wind and sun, so it stays the same. WBGT values are usually lower than heat index values, so a WBGT of 89 does not mean less risk than a heat index of 92.
For daily activities, NWS Wichita says the heat index serves well; for outdoor work or vigorous activity in full sun, it recommends WBGT. OSHA uses WBGT to determine whether a workplace heat hazard was present and calls an on site WBGT meter the most accurate way to measure environmental heat.
Heat Exhaustion vs Heat Stroke
Most heat index warnings aim to prevent these two illnesses. The CDC National Institute for Occupational Safety and Health (NIOSH) lists the following signs and first aid.
| Heat exhaustion | Heat stroke | |
|---|---|---|
| Signs | Headache, nausea, dizziness, weakness, irritability, thirst, heavy sweating, elevated body temperature, less urine | Confusion, slurred speech, loss of consciousness, hot dry skin or profuse sweating, seizures, very high body temperature |
| First aid | Move out of the heat, remove extra clothing, cool the head, face and neck with cold water, give frequent sips of cool water, get a medical evaluation | Call 911, move to a cool shaded area, remove outer clothing, cool fast with a cold water or ice bath or cold wet cloths, circulate air, stay until help arrives |
Heat stroke is the most serious form. NIOSH notes that body temperature can rise to 106 °F or higher within 10 to 15 minutes, and that heat stroke can cause permanent disability or death without emergency treatment.
Working in the heat
OSHA promotes water, rest and shade as both prevention and treatment. Its guidance includes letting new and returning workers acclimatize, providing shade, scheduling work earlier or later in the day, using work and rest schedules, limiting strenuous tasks and using relief workers. Federal OSHA relies on the General Duty Clause, and Washington, Minnesota, California, Oregon and Colorado have their own occupational heat rules. OSHA issued directive CPL 03‑00‑024, a National Emphasis Program on outdoor and indoor heat hazards, on April 10, 2026. The CDC adds that people 65 or older, infants and young children, pregnant women, people with heart disease or asthma, and anyone working or exercising outside face higher risk. To estimate fluid needs, see the Water Intake Calculator.
How to Use the Calculator
- Unit. Pick °F or °C. Switching converts the values you entered.
- Air temperature. Use a shaded reading from a forecast or weather station.
- Humidity. Enter relative humidity in percent, or switch to dew point.
- Read the result. The hero shows the heat index and band; the cards below give actions, the steps and the chart.
- Examples. The chips load typical cases, including both humidity adjustments.
Common Mistakes
- Measuring in the sun. The formula expects shade air temperature.
- Mixing units. 32 °C entered as °F is below the 40 °F limit.
- Adding 15 °F every time. Sunshine can add up to 15 °F; it is a ceiling, not a fixed correction.
- Comparing with WBGT. The scales differ and WBGT reads lower.
- Treating under 80 °F as safe. Hard work in the sun can still cause heat illness.
Frequently Asked Questions
How do you calculate the heat index?
The National Weather Service first applies a simple formula: HI = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094), with T in °F and RH in percent. If that result is 80 °F or higher, it switches to the Rothfusz regression, a nine term equation in T and RH, then applies a small adjustment when humidity is below 13 percent or above 85 percent. At 90 °F and 60 percent humidity the simple formula gives 91.52, so the regression applies and returns 99.68, which rounds to a heat index of 100 °F.
What is a dangerous heat index?
The NWS classifies a heat index of 103 °F or higher as Danger: heat cramps or heat exhaustion are likely, and heat stroke is possible with prolonged exposure or physical activity. At 125 °F and above the category is Extreme Danger, where heat stroke is highly likely. Values from 90 to 102 °F are Extreme Caution and 80 to 89 °F are Caution. These values assume shade and light wind, and NWS notes that full sunshine can raise the heat index by up to 15 °F, so time in direct sun moves you toward a higher band.
How do you calculate heat index in Celsius?
The heat index equations were fitted in Fahrenheit, so convert first, calculate, then convert back. Multiply the Celsius temperature by 1.8 and add 32, run the NWS formula with relative humidity in percent, then subtract 32 from the result and divide by 1.8. For 32 °C and 70 percent humidity, 32 × 1.8 + 32 = 89.6 °F. The simple formula gives 91.55, so the Rothfusz regression applies and returns 104.74 °F. That converts to (104.74 − 32) ÷ 1.8 = 40.4 °C, which falls in the Danger band.
What is the difference between heat index and temperature?
Air temperature is what a thermometer in the shade reads. The heat index, also called the apparent or feels like temperature, adds the effect of humidity: moist air slows the evaporation of sweat, so the body sheds heat less effectively and the air feels hotter. At 90 °F the heat index is about 91 °F at 40 percent humidity but 132 °F at 100 percent. In very dry air it can fall below the air temperature, for example 96 °F at 100 °F and 15 percent humidity.
Can you calculate heat index from dew point?
Yes. Convert the dew point and air temperature to relative humidity first, then use the normal heat index formula. The NWS calculator computes vapor pressure as e = 6.11 × 10^(7.5 × Td ÷ (237.7 + Td)) with temperatures in °C, repeats it with the air temperature to get the saturation vapor pressure, and divides the two. For 90 °F air with a 75 °F dew point, relative humidity is 61.6 percent and the heat index is 100.6 °F, which rounds to 101 °F.
What is the difference between heat index and wet bulb globe temperature?
The heat index uses only air temperature and humidity and assumes shade and light wind. Wet bulb globe temperature (WBGT) also accounts for sun angle, cloud cover and wind, and is computed as 0.7 × natural wet bulb + 0.2 × black globe + 0.1 × dry bulb temperature. OSHA uses WBGT to judge workplace heat hazards, and the NWS Wichita office recommends it for outdoor work or vigorous activity in full sun. WBGT values are usually lower than heat index values, so the two scales should not be compared number for number.
Sources & References
- NOAA National Weather Service, Weather Prediction Center. The Heat Index Equation (simple formula, Rothfusz regression, adjustments, validity). wpc.ncep.noaa.gov
- NOAA National Weather Service, Weather Prediction Center. Meteorological Conversions and Calculations: Heat Index Calculator (dew point to relative humidity method, 40 °F floor). wpc.ncep.noaa.gov
- National Weather Service. Heat Forecast Tools (heat index chart, up to 15 °F in full sunshine). weather.gov
- National Weather Service Amarillo. What is the heat index? (classification table, regression error of 1.3 °F). weather.gov
- National Weather Service Wichita. Wet Bulb Globe Temperature vs Heat Index (Steadman assumptions, WBGT formula and examples). weather.gov
- National Weather Service Caribou. Excessive Heat Information (local alert criteria). weather.gov
- National Weather Service. National Weather Service revamps Heat Watch and Heat Warning products, March 2025. weather.gov
- CDC, National Institute for Occupational Safety and Health. Heat Related Illnesses, updated March 3, 2026. cdc.gov
- CDC. About Heat and Your Health, updated July 20, 2026 (fans above 90 °F, groups at higher risk). cdc.gov
- U.S. Occupational Safety and Health Administration. Heat: Working in Outdoor and Indoor Heat Environments. osha.gov
Results are for informational purposes only and are not a diagnosis or medical advice. Call 911 or local emergency services for signs of heat stroke, and follow alerts from your local National Weather Service office. Every number on this page was recomputed with the same code the calculator uses. Methodology reviewed: 2026.