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Heat Index Calculator

See how hot it really feels from the air temperature and humidity, using the US National Weather Service heat index method.

Level: Grades 9–10

Printable worksheet

Heat Index Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Units

Result

Heat index (feels like)

99.7 °F

At 90 °F and 60% relative humidity, it feels like 99.7 °F (Extreme caution).

In Celsius
37.6 °C
Difference from air temperature
9.7 °FPositive means it feels hotter than the thermometer reads.
NWS heat index category
Extreme cautionThe weather service's general classification, for shade and light wind. Informational only.
Formula used
Rothfusz regression

See the step-by-step solution ↓

Step-by-step solution

  1. First, make a quick estimate from the air temperature T and the humidity RH. It tells us if the full formula is needed

    0.5×(90+61+1.2×(90−68)+0.094×60)=91.52 °F

  2. Average it with the air reading. That's 80 °F or more, so we need the full Rothfusz formula

    91.52+902=90.76 °F= ≥ 80

  3. Put the air temperature (in °F) and the humidity (in %) into the Rothfusz formula. Then add up its nine parts

    −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282T·RH² − 0.00000199T²RH²=−42.379+2.04901523×90+10.14333127×60+−0.22475541×90×60+−0.00683783×8,100+−0.05481717×3,600+0.00122874×8,100×60+0.00085282×90×3,600+−0.00000199×8,100×3,600=99.6777179 °F

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how hot it actually feels outside once humidity is counted. On a humid day the air feels hotter than the thermometer says. That's because sweat dries more slowly, so your body has a harder time losing heat. The heat index turns that into one number. It's the temperature that would feel just as hot if the humidity were at a moderate, standard level. It's the 'feels like' figure weather services quote in summer.

This calculator follows the steps published by the US National Weather Service. It first makes a simple estimate. If that points to a warm, humid day, it switches to a more detailed formula, the Rothfusz regression. Then it applies the small corrections the weather service uses for very dry or very humid air. Enter the temperature in °F or °C. The answer is general information about the weather. It's not a safety guarantee or medical advice.

Where it's used

Why this math matters outside the classroom:

  • Outdoor work. Employers check the heat index to schedule rest breaks and water for crews working outside in summer.
  • Sports practice. Coaches move practice earlier in the day when the afternoon heat index forecast runs high.
  • Humid days. At 32 °C (90 °F) and 70% relative humidity, the heat index is about 41 °C (105 °F).
  • Outdoor events. Festival organizers use heat index forecasts to plan shade, water stations and first-aid tents.
  • Heat advisories. Forecasters issue heat advisories when the heat index is expected to pass local thresholds.

How to use it

  1. Choose Fahrenheit or Celsius.
  2. Enter the air temperature, measured in the shade, and the relative humidity.
  3. Read the heat index, how much hotter than the air it feels, and the weather service's general category.
  4. Open the working to see which formula was used and every part of the calculation.

What the inputs mean

Air temperature
Air temperature in the shade, from 40 °F to 150 °F. Below about 80 °F, the heat index stays close to the air temperature.
Relative humidity
Relative humidity in percent, from 0 to 100. It's how much moisture the air holds compared with the most it could hold. If you only know the dew point, find the relative humidity first with the dew point calculator.

Formula and method

Simple: HI = 0.5 × (T + 61 + 1.2(T − 68) + 0.094 RH)   (use it if (HI + T) ÷ 2 < 80 °F)
Rothfusz: HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282T·RH² − 0.00000199T²RH²
If RH < 13% and 80 ≤ T ≤ 112 °F: HI − ((13 − RH) ÷ 4) × √((17 − |T − 95|) ÷ 17)
If RH > 85% and 80 ≤ T ≤ 87 °F: HI + ((RH − 85) ÷ 10) × ((87 − T) ÷ 5)

HI is the heat index. T is the air temperature in degrees Fahrenheit, and RH is the relative humidity in percent. |T − 95| means the gap between T and 95, ignoring any minus sign.

The heat index comes from R. G. Steadman's 1979 model. It describes how a person in light clothing, walking in the shade, trades heat with the air. Steadman's results were tables. In 1990, L. P. Rothfusz fitted a formula to them, the multiple regression above. The US National Weather Service adopted it.

That formula was only fitted to hot weather, so the weather service first works out the simple estimate. If the average of that estimate and the air temperature is below 80 °F, the simple value is used. Otherwise the full formula is applied. It gets a downward correction for very dry heat. It gets an upward one for humid air in the low to mid 80s °F, where the plain formula is least accurate.

Worked example

The air is 90 °F with 60% relative humidity.

  1. Simple estimate: 0.5 × (90 + 61 + 1.2 × 22 + 0.094 × 60) = 91.52 °F. Its average with 90 °F is 90.76 °F, so the full regression is used.
  2. The nine regression terms are −42.379, 184.4114, 608.5999, −1,213.6792, −55.3864, −197.3418, 597.1676, 276.3137 and −58.0284.
  3. Their sum is 99.6777 °F. Humidity is between 13% and 85%, so no adjustment applies.
  4. In Celsius: (99.6777 − 32) × 5/9 = 37.6 °C.

Result: It feels like about 99.7 °F (37.6 °C), roughly 9.7 °F hotter than the air. That's in the weather service's 'extreme caution' band.

Understanding the result

The National Weather Service groups heat index values into four bands: caution (80–90 °F), extreme caution (90–103 °F), danger (103–124 °F) and extreme danger (125 °F and above). They describe the general level of heat stress for people outdoors. They are not a measure of any one person's condition.

The heat index is worked out for shade and a light breeze. Full sunshine can make it feel a lot hotter than the number shown. So treat it as a low estimate when you're in direct sun.

Dry heat can give a heat index a little below the air temperature, because sweat dries quickly. That's expected, not an error.

Assumptions and limitations

  • A person of average build, lightly clothed, walking in the shade with a light breeze.
  • Temperature and humidity are measured together, in the shade, at about head height.
  • The NWS formula numbers and cutoffs are used exactly as published, in °F. Celsius values are converted in and out.
  • The formula is a curve fitted to tables. It's most accurate from roughly 80–110 °F with moderate humidity, and less certain at extremes.
  • It ignores sunshine, wind, clothing, activity and personal factors, and all of these change how heat affects people.
  • This is an informational weather estimate. It is not a forecast, a guarantee of safety or medical advice. Follow official heat warnings where you live.

Frequently asked questions

Why does the calculator sometimes use a simpler formula?

The Rothfusz formula was fitted only to hot, humid conditions, and it can give odd values in mild weather. So the weather service's own steps use a simpler estimate first. It keeps that estimate when the result is below about 80 °F.

Can the heat index be lower than the air temperature?

Yes. In very dry air, sweat dries quickly and cools the skin well. So the heat index can come out a degree or two below the thermometer reading.

Is the heat index the same as the 'feels like' temperature in weather apps?

In hot weather, most apps show the heat index or something close to it. Some use other measures that also include wind and sunshine, so values may differ by a few degrees.

How does the heat index differ from wet-bulb globe temperature?

Wet-bulb globe temperature is measured with instruments that also respond to sun and wind. It's often used for work and sports guidance. The heat index uses only air temperature and humidity, and it assumes shade.

How this calculator is checked

Method: US National Weather Service heat index procedure: simple Steadman-based estimate, Rothfusz (1990) multiple regression, and the NWS low-humidity and high-humidity adjustments; classification bands as published by the NWS. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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