Heat Index Calculator

NOAA Rothfusz formula — calculate feels-like temperatures using humidity or dew point inputs, with OSHA safety guides.

Environmental Inputs

Air Temperature

Unit

Humidity Source

Relative Humidity (%)

Heat Index Reference Grid (°F)

Air Temp	40% RH	50% RH	60% RH	70% RH	80% RH	90% RH
80°F	81°	83°	85°	87°	90°	94°
85°F	86°	89°	92°	97°	103°	109°
90°F	92°	97°	103°	110°	119°	129°
95°F	100°	107°	116°	127°	140°	154°
100°F	111°	121°	133°	147°	164°	183°
105°F	123°	136°	152°	171°	192°	216°
110°F	137°	154°	174°	198°	225°	254°

Feels-Like Temperature

116.3°F

46.9°C

Calculated RH:60.0%

Risk Level:Danger

OSHA / NWS Protective Guide

Heat cramps or exhaustion likely; heat stroke possible. Reduce heavy physical activity, monitor workers, stay in air conditioning.

Dew Point Conversion: The calculator applies the Magnus-Tetens formula to determine relative humidity from dew point and air temperature variables.

NOAA Rothfusz Heat Index Methodology

The heat index is derived using a multi-parameter regression formula developed by the U.S. National Weather Service. It measures how the human body experiences temperature in high-humidity conditions, where sweat evaporation is slowed.

At temperatures below 80°F (27°C), a simplified linear index is applied. OSHA safety guidelines recommend scheduling hydrated breaks in shaded shelters when index limits trigger Caution thresholds.

Built and maintained by Meet Shah · Last updated 1 August 2026

What this tool is used for

Working out how hot it will feel given the humidity.
Checking whether conditions are into a warning band.
Comparing two days at the same temperature but different humidity.
Deciding whether outdoor activity is advisable.
Checking a forecast's feels-like figure against the inputs.

Frequently Asked Questions

What formula does this use?: The Rothfusz regression the US National Weather Service publishes, with both of its official adjustments — a subtraction for very dry air below 13% humidity and an addition for very humid air above 85%, each within their stated temperature bands.
Why is there a different formula below 80°F?: Because the regression is only fitted above that point and misbehaves below it. Under 80°F the simple average form is used instead, which is exactly what the NWS specifies — a single formula across the whole range would be wrong at the cool end.
Why does humidity make heat dangerous?: Because sweating cools you by evaporation, and humid air slows evaporation down. At high humidity your body's main cooling mechanism stops working, which is why 95°F at 70% humidity feels far worse than 95°F at 20%.
Can I enter dew point instead of humidity?: Yes — relative humidity is derived from it using the Magnus formula. Dew point is often the more useful reading because, unlike relative humidity, it does not change as the temperature does.
Does the heat index assume shade?: Yes, shade and a light wind. In full sun the effective value can be roughly 15°F higher, which is why an index in the Caution band can still be dangerous for anyone working outdoors.
Do the risk categories apply to everyone?: No. They are general guidance for a healthy adult; older people, young children, anyone on medication affecting thermoregulation, and anyone doing heavy work are at risk at lower values. Treat the bands as a floor, not a threshold.

Common errors and gotchas

Using the formula below its valid range, where it is not defined.
Confusing heat index with wind chill, which apply at opposite ends of the scale.
Assuming it accounts for sun exposure, which it does not — it assumes shade.
Applying it to a dry climate, where the index barely rises and the risk is dehydration instead.
Reading it as an air temperature rather than an equivalent sensation.