Heat Index Calculator

What the heat index actually measures

The heat index — the "feels like" number on your weather app — is an estimate of how hot the air feels to a human body once relative humidity is taken into account. It comes from Robert Steadman's 1979 work on thermal comfort, which modelled a walking adult in shade and asked what dry-bulb temperature would produce the same physiological strain. In 1990 Lans Rothfusz fitted a polynomial regression to Steadman's table so a forecaster could compute it in one line. That regression is what the National Weather Service still publishes, and it is what this calculator runs.

The physics behind it is evaporative cooling. You shed heat mainly by sweating; sweat only removes heat when it evaporates, and evaporation slows as the surrounding air approaches saturation. At 30% humidity a US summer afternoon is survivable work weather. At 70% the same thermometer reading pushes your core temperature up because the sweat has nowhere to go.

Worked example: 92°F and 60% humidity

Take a typical July afternoon in Washington DC or the US Midwest: 92°F air temperature, 60% relative humidity. Feeding T = 92 and R = 60 into the Rothfusz regression gives a heat index of about 105°F. The humidity has added roughly 13°F to what your body experiences. Neither the low-humidity adjustment (which applies below 13% RH) nor the high-humidity adjustment (above 85% RH between 80 and 87°F) kicks in here, so the base polynomial stands.

At 105°F the NWS category is Danger. Heat cramps and heat exhaustion are likely for anyone exerting themselves, and heat stroke becomes a real possibility with prolonged activity. If that same reading were taken in full sun rather than shade, the effective load could reach 120°F — squarely in the Extreme Danger band.

How much difference does humidity make?

Read that middle row across. Five degrees on the thermometer is nothing; forty points of humidity turns a pleasant 91°F into a life-threatening 113°F. That is the entire argument for checking a heat index rather than a temperature before a run, a match or a shift on a roof.

The four NWS bands and what they mean for your day

80 to 90°F — Caution

Fatigue is possible with prolonged exposure or physical activity. Most healthy adults are fine; schedule water breaks and stop treating hydration as optional.

90 to 103°F — Extreme Caution

Heat cramps and heat exhaustion become likely with sustained exertion. This is the band where high school athletic associations start mandating breaks and shortened practices, and where outdoor crews should be on a work-rest cycle.

103 to 124°F — Danger

Heat exhaustion is likely and heat stroke is possible if activity continues. Strenuous work should move to dawn or dusk. In the UK this band is rare but no longer unknown; in the US Southeast and Gulf Coast it is a normal July afternoon.

125°F and above — Extreme Danger

Heat stroke is highly likely. There is no safe way to exert yourself outdoors. Air conditioning, or at minimum a cooling centre, is the only reliable answer.

Acclimatization: the same number, different bodies

People adapt to heat over about 10 to 14 days of repeated exposure. An acclimatized body starts sweating sooner, sweats more, and loses less salt in the process — measurably lowering strain at the same heat index. This is why the first heat wave of the season kills more people than a hotter one in August, and why OSHA guidance flags new and returning workers specifically: roughly three quarters of serious heat illness on job sites happens in the first week.

The corollary is that the heat index is a population average, not a personal verdict. Adults over 65, infants, pregnant women, people on diuretics, beta blockers or antipsychotics, and anyone with heart or kidney disease all cross into danger earlier than the chart implies. Alcohol and a poor night's sleep move the line too.

Limitations worth knowing

The regression assumes shade, a light 6 mph breeze, a 5'7" 147 lb adult walking at about 3 mph, and clothing typical of hot weather. Full sun adds up to 15°F. Still air makes it worse than the model says; strong wind above roughly 100°F makes it worse again, because moving air hotter than skin delivers heat rather than removing it. Rothfusz's fit also has an error band of about ±1.3°F against Steadman's original table, and it degrades outside the 80 to 112°F range — which is why readings below 80°F here fall back to the simpler Steadman average and are labelled as such.

For work planning, the more rigorous instrument is wet bulb globe temperature, which measures sun, wind and radiant heat directly. Its numbers are not comparable to heat index numbers and should never be swapped for them. Use the heat index for what it is good at: a fast, honest read on whether today's humidity has turned an ordinary hot day into one that deserves a plan.