Heat Index Calculator — Feels-Like Temperature & Work Safety

Q: What does 90°F at 70% humidity feel like?

About 105°F — squarely in the danger band. At that level, heat cramps and exhaustion are likely with prolonged activity, and heat stroke becomes possible.

Two days can read 90°F on the thermometer and feel nothing alike. The variable hiding in plain sight is humidity.

What the heat index is really measuring

Your body's main cooling system is sweat. As it evaporates off your skin, it carries heat away with it. That trick works brilliantly in dry air — and barely at all in humid air, because the surrounding moisture leaves little room for more water vapour. So on a muggy day the sweat just sits there, your body keeps its heat, and the temperature your nerves report is far higher than what the thermometer claims.

The heat index — also called the apparent or "feels-like" temperature — folds those two numbers, air temperature and relative humidity, into a single figure that tracks how hot the air feels to a person standing in the shade with a light breeze. It's the same model the U.S. National Weather Service uses to decide when to issue heat advisories.

A quick gut-check: at 90°F with 70% humidity, it feels like about 105°F. That 15-degree jump is entirely down to moisture in the air — the thermometer never moved.

The formula doing the work

There's no tidy physics equation for "feels like." Instead, the index comes from a regression that Lans Rothfusz of the NWS fitted to Robert Steadman's 1979 human-comfort research. It takes temperature in Fahrenheit and humidity as a percentage and returns the apparent temperature:

// full Rothfusz regression (T in °F, R = % humidity)
HI = -42.379 + 2.04901523·T + 10.14333127·R
- 0.22475541·T·R - 0.00683783·T² - 0.05481717·R²
+ 0.00122874·T²·R + 0.00085282·T·R² - 0.00000199·T²·R²

The calculator above runs the real thing, including the two corrections the NWS applies at the edges — a small subtraction in very dry air, and a small addition in very humid air — plus the simpler equation used below about 80°F. It carries a stated accuracy of roughly ±1.3°F, and isn't meant for conditions far outside the range Steadman studied.

Why the "in full sun" switch matters

Here's the catch most people miss: the heat index is a shade number. It assumes you're out of direct sunlight, with a gentle 5 mph wind. Step into full sun and the felt temperature can climb by as much as 15°F beyond what the formula reports. That's why the toggle exists — flip it on and the reading shifts to reflect open, exposed conditions, which is closer to reality for anyone working a roof, a field, or a parking lot at midday.

How to use this calculator

Four quick steps to go from a thermometer reading to a feels-like number you can act on.

Enter the air temperature

The plain number your thermometer or weather app shows. Type it in, or drag the slider. Not sure if it's °F or °C? Use the toggle on the right to switch.

Add the humidity

Enter the relative humidity percentage (your weather app has it). Don't know it but know the dew point? Tap "From dew point" and enter that instead — same result.

Flip the sun switch if needed

Standing in direct sunlight? Turn on "In full sun" and the number climbs to match — the basic reading assumes you're in the shade.

Read the result

The big number is the feels-like temperature. The colour and label tell you the danger level. Working outside? Scroll to the work/rest planner.

Heat index, humidex, wet-bulb — which is which?

"Feels-like" isn't one universal number; different countries and uses reach for different yardsticks. They mostly agree on the idea and differ in the details.

Metric

What it is & where it's used

Heat index

The U.S. NWS standard. Temperature + humidity, in the shade. What this page calculates.

Humidex

Canada's version. Same goal, but built from the dew point rather than relative humidity, so values differ slightly.

WBGT

Wet-bulb globe temperature. Adds sun, wind and radiant heat — measured in the sun, not the shade. The gold standard for athletics and the military.

Apparent temp

Australia's measure, and the umbrella term for any felt-temperature figure, sometimes folding in wind too.

The short version: the heat index is the everyday, widely-understood one for hot weather. If you're managing genuine heat-stress risk for people exerting themselves outdoors in the sun, WBGT paints a fuller picture — ideally from an on-site meter.

Heat index reference table

A quick lookup if you'd rather scan than type. Find your air temperature down the left, your humidity across the top, and read the feels-like temperature where they meet. Colours mark the danger band — hover any cell to zoom in.

Feels-like temperature (°F), shade conditions

Calculated with the same NWS formula as the tool above. Values are rounded; the calculator handles any exact combination, plus the full-sun adjustment.

Temp ↓ Humidity →	40%	50%	60%	70%	80%	90%	100%
80°F	80	81	82	83	84	86	89
84°F	83	85	88	90	94	98	104
88°F	88	91	95	100	106	113	121
92°F	94	99	105	112	121	131	143
96°F	101	108	116	126	138	152	168
100°F	109	118	129	143	158	176	195
104°F	119	131	145	161	181	202	226
108°F	130	144	162	182	205	231	260

Comfortable Caution (80–90°) Extreme caution (91–103°) Danger (103–124°) Extreme danger (125°+)

Common questions

What does 90°F at 70% humidity feel like?

About 105°F — squarely in the "danger" band. At that level, heat cramps and exhaustion are likely with prolonged activity, and heat stroke becomes possible. Try those exact numbers in the calculator above to see it land on the arc.

Can the heat index be lower than the actual temperature?

Yes. When humidity is low (below about 40%) and temperatures are moderate, dry air helps sweat evaporate efficiently, so it can feel slightly cooler than the thermometer reads. The formula reflects this by returning a value at or below the air temperature.

Why does my weather app show a different "feels like"?

Many apps blend the heat index with wind chill or use their own proprietary formulas that factor in sun and wind. Some, like "RealFeel," are entirely separate models. This page sticks to the official NWS heat index, which is shade-based and uses only temperature and humidity.

Does the heat index account for being in the sun?

Not by default — the standard figure assumes shade. Full sun can add up to 15°F to how it feels. Use the "in full sun" toggle to apply that adjustment for open, exposed conditions.

What heat index is dangerous?

Caution starts at 80°F, but the index becomes genuinely hazardous from about 103°F (the "danger" band), where heat stroke is possible. Above 125°F it's "extreme danger" and strenuous activity should be avoided altogether.

Is heat index the same as wet-bulb temperature?

No. The heat index estimates how hot it feels in the shade from temperature and humidity. Wet-bulb globe temperature (WBGT) also accounts for direct sun, wind and radiant heat, making it more accurate for outdoor exertion — but it needs more inputs, ideally an on-site meter.

What the heat actually feels like