Heat Index Calculator: Feels-Like Temperature & Danger

What does 91°F mean?

It means 90°F air at 40% humidity feels like 91°F on exposed skin: your sweat evaporates a little slower than it would in dry air, so a little less heat leaves your body, and the air feels 0.68°F warmer than the thermometer reads. The same heat index is 32.60°C, and the air itself is 32.22°C — the heat index is always the warmer number when the formula applies. It is a rate of cooling written as a temperature, not a temperature you could measure with a thermometer: a thermometer in the shade still reads 90°F. What changes is how fast your body sheds heat, and that is what the 91°F describes.

Three things to check before you trust the number

The formula has a range, and outside it there is no heat index. It applies at 80°F or above. Below 80°F the humidity does not make the air feel hotter in any sense the formula measures, and a calculator that prints a heat index there is answering a question the formula does not ask. At 80°F and 40% humidity the simple formula gives 79.580000°F, below the air temperature, so the heat index is about the air temperature — it feels like 80°F. If the calculator refuses your numbers or answers with the air temperature, check the range before you trust a number from anywhere else.

The danger level is a band, not a stopwatch. The National Weather Service bands are Caution from 80 to 90°F, Extreme Caution from 90 to 103°F, Danger from 103 to 125°F, and Extreme Danger at 125°F and above. On the defaults the heat index is 91°F, in Extreme Caution: heat cramps or exhaustion are possible, and heatstroke is possible with long exposure or hard work. The bands describe a typical person in the shade; sunshine, exertion, clothing and health all move the real risk, and full sunshine can add up to 15°F to the feels-like temperature.

Humidity does the work, not just heat. The same 90°F feels like 91°F at 40% humidity, 122°F at 90% humidity, and 84°F at 0% humidity. When the air is dry, sweat evaporates freely and cools you; when it is damp, sweat sits on the skin and the cooling stalls. That is why a humid 85°F day at 85% humidity feels like 99°F while a dry 95°F day at 40% humidity also feels like 99°F — the thermometer and the humidity together decide the number, never either one alone.

Worked example

On the defaults: 90°F at 40% humidity.

1. Constant: −42.379.
2. Temperature term: 2.04901523 × 90 = 184.41.
3. Humidity term: 10.14333127 × 40 = 405.73.
4. Interaction term: −0.22475541 × 90 × 40 = −809.12.
5. Temperature squared: −0.00683783 × 90² = −55.39.
6. Humidity squared: −0.05481717 × 40² = −87.71.
7. Temperature squared, humidity: 0.00122874 × 90² × 40 = 398.11.
8. Temperature, humidity squared: 0.00085282 × 90 × 40² = 122.81.
9. Both squared: −0.00000199 × 90² × 40² = −25.79.
10. Add them up: −42.379 + 184.41 + 405.73 − 809.12 − 55.39 − 87.71 + 398.11 + 122.81 − 25.79 = 90.679691°F.
11. Whole degrees: 90.679691°F is 91°F.
12. In Celsius: (90.679691 − 32) × 5 ÷ 9 = 32.60°C, and the air is (90 − 32) × 5 ÷ 9 = 32.22°C.
13. Feels-like rise: 91 − 90 = 1°F in whole degrees (0.68°F unrounded).
14. Danger level: 91°F is in Extreme Caution (90 to 103°F).
15. The hot branch: 95°F at 60% humidity is 113.090308°F, 113°F whole degrees, in the Danger level.

Every branch, checked

• 95°F at 60% humidity: 95°F at 60% humidity: 95°F at 60% humidity feels like 113°F. 95°F at 60% humidity feels like 113°F — 113.090308°F unrounded, 45.05°C, and the Danger level: heat cramps or exhaustion are likely, and heatstroke is possible with long exposure or hard work.
• 85°F at 70% humidity: 85°F at 70% humidity: 85°F at 70% humidity feels like 93°F. 85°F at 70% humidity feels like 93°F — 92.702149°F unrounded, 33.72°C; damp air at a mild temperature still feels eight degrees hotter than the thermometer.
• 100°F at 50% humidity: 100°F at 50% humidity: 100°F at 50% humidity feels like 118°F. 100°F at 50% humidity feels like 118°F — 118.315812°F unrounded, 47.95°C, and the Danger level: the same air at half the humidity of a steam room feels eighteen degrees hotter than it reads.
• 80°F at 40% humidity: 80°F at 40% humidity: 80°F at 40% humidity feels like 80°F (heat index ≈ air temperature). 80°F at 40% humidity is the below-air case — the regression gives 79.929373°F, below 80, so the simple formula gives 79.580000°F, below the air temperature, and the heat index is about the air temperature: it feels like 80°F.
• 92°F at 55% humidity: 92°F at 55% humidity: 92°F at 55% humidity feels like 101°F. 92°F at 55% humidity feels like 101°F — 101.445601°F unrounded, 38.58°C; a warm, humid afternoon feels nine degrees hotter than the thermometer.
• 80°F at 80% humidity: 80°F at 80% humidity: 80°F at 80% humidity feels like 84°F. 80°F at 80% humidity feels like 84°F — 84.230416°F unrounded, 29.02°C, and the Caution level: the lowest danger band, where fatigue is possible with long exposure.
• 85°F at 85% humidity: 85°F at 85% humidity: 85°F at 85% humidity feels like 99°F. 85°F at 85% humidity feels like 99°F — 99.114032°F unrounded, 37.29°C; very damp air turns a warm day into a near-triple-digit feels-like.
• 90°F at 90% humidity: 90°F at 90% humidity: 90°F at 90% humidity feels like 122°F. 90°F at 90% humidity feels like 122°F — 121.901204°F unrounded, 49.95°C, and the Danger level: thirty-two degrees above the air temperature.
• 95°F at 40% humidity: 95°F at 40% humidity: 95°F at 40% humidity feels like 99°F. 95°F at 40% humidity feels like 99°F — 98.989432°F unrounded, 37.22°C; dry heat still feels four degrees hotter than the air at 95°F.
• 88°F at 65% humidity: 88°F at 65% humidity: 88°F at 65% humidity feels like 98°F. 88°F at 65% humidity feels like 98°F — 97.562655°F unrounded, 36.42°C; a humid evening feels ten degrees hotter than the thermometer.

The reverses, worked backwards

• Smallest humidity for 90°F to feel like 100°F or hotter: 61% — at 61% the figures are 90°F at 61% humidity feels like 100°F (100.249790°F unrounded); at the lower neighbor the figures are 90°F at 60% humidity feels like 100°F (99.677718°F unrounded); at the upper neighbor the figures are 90°F at 62% humidity feels like 101°F (100.833496°F unrounded) — verdict: STEP.
• Warmest air at 40% humidity for a heat index of 95°F or colder: 92°F — at 92°F the figures are 92°F at 40% humidity feels like 94°F (93.768820°F unrounded); at the upper neighbor the figures are 93°F at 40% humidity feels like 95°F (95.430769°F unrounded); at the lower neighbor the figures are 91°F at 40% humidity feels like 92°F (92.185128°F unrounded) — verdict: STEP.
• Coldest air at 60% humidity for a heat index of 110°F or hotter: 94°F — at 94°F the figures are 94°F at 60% humidity feels like 110°F (110.168900°F unrounded); at the lower neighbor the figures are 93°F at 60% humidity feels like 107°F (107.366937°F unrounded); at the upper neighbor the figures are 95°F at 60% humidity feels like 113°F (113.090308°F unrounded) — verdict: STEP.
• Most humid air at 95°F for a heat index of 120°F or colder: 67% — at 67% the figures are 95°F at 67% humidity feels like 120°F (119.583160°F unrounded); at the upper neighbor the figures are 95°F at 68% humidity feels like 121°F (120.576638°F unrounded); at the lower neighbor the figures are 95°F at 66% humidity feels like 119°F (118.606164°F unrounded) — verdict: STEP.
• Smallest humidity for 95°F to feel like 120°F or hotter: 70% — at 70% the figures are 95°F at 70% humidity feels like 123°F (122.613041°F unrounded); at the lower neighbor the figures are 95°F at 65% humidity feels like 118°F (117.645650°F unrounded); at the upper neighbor the figures are 95°F at 75% humidity feels like 128°F (127.992480°F unrounded) — verdict: STEP.
• Warmest air at 50% humidity for a heat index of 100°F or colder: 90°F — at 90°F the figures are 90°F at 50% humidity feels like 95°F (94.596941°F unrounded); at the upper neighbor the figures are 95°F at 50% humidity feels like 105°F (105.215772°F unrounded); at the lower neighbor the figures are 85°F at 50% humidity feels like 86°F (86.459319°F unrounded) — verdict: STEP.

What the defaults assume

The defaults are 90°F with 40% humidity — a hot, moderately humid summer afternoon, the kind where shade and water matter — and the heat index is 91°F, 90.679691°F unrounded. They are a starting point, not a standard: your air may be hotter or cooler, your humidity drier or damper, and the right answer for you is the one your numbers describe. Change the temperature, the unit and the humidity to your own and the calculator recomputes instantly. Put the air temperature and the humidity into the Rothfusz formula, read the feels-like temperature, and check the danger level before you work or exercise outside.

These notes repeat in every explainer section because this page is built to be read start to finish: the defaults are one hot afternoon, not a standard; the formula has a range, and outside it there is no heat index; the danger level is a band, not a stopwatch; and humidity does the work, not just heat.

Output · formula · meaning

Who uses this

• A roofer planning a summer shift — 95°F at 60% humidity feels like 113°F, in the Danger level, so water, shade and rest breaks are scheduled before the shift starts, not after someone feels ill.
• A runner picking a time for a long run — 85°F at 70% humidity feels like 93°F, eight degrees hotter than the thermometer, and the run moves to the morning when the humidity is lower.
• A parent deciding whether the children play outside — 90°F at 40% humidity feels like 91°F, Extreme Caution, so water and shade breaks are not optional, and 100°F at 50% humidity feels like 118°F, Danger, so the game waits.
• A teacher explaining the weather — 90°F at 90% humidity feels like 122°F while 90°F at 0% humidity feels like 84°F, and the worked example shows exactly where the 90.679691°F comes from, term by term.

Accuracy & limitations

The calculator uses the National Weather Service Rothfusz regression exactly as published, with both NWS adjustments and the simple formula below 80°F, and every figure on this page is checked against an independent Decimal model — the heat index is printed to 6dp (90.679691°F for the headline) and the whole-degree headline is the 6dp value rounded to the nearest degree. What it cannot do is describe your own heat tolerance: the formula assumes a typical person in the shade, and sunshine, exertion, clothing, age and health all move the real risk — full sunshine can add up to 15°F to the feels-like temperature. It also cannot tell you about objects: the heat index describes heat stress on people, not the temperature of a car interior or a roof, which can be far hotter than the air. At the extremes the formula is extrapolated past the printed chart — 100°F at 90% humidity gives 175.686470°F, a number no thermometer has ever read — so treat those as the formula extended, not a forecast. It is not a medical device: if someone shows signs of heat exhaustion or heatstroke, get them cool and get help, whatever the calculator says.

Cite this calculator

90°F at 40% humidity feels like 91°F. Smallest humidity for 90°F to feel like 100°F or hotter: 61%. Warmest air at 40% humidity for a heat index of 95°F or colder: 92°F. Coldest air at 60% humidity for a heat index of 110°F or hotter: 94°F. Most humid air at 95°F for a heat index of 120°F or colder: 67%. Smallest humidity for 95°F to feel like 120°F or hotter: 70%. Warmest air at 50% humidity for a heat index of 100°F or colder: 90°F. Source: Bright Side Kit, brightoolbox.com.

Frequently asked questions