Heat Index Calculator

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard temperature conversion formulas

What this calculator does

This heat index calculator estimates how hot the air feels to people when warm air and high relative humidity act together. It is useful for interpreting summer conditions that feel much more stressful than the dry-bulb temperature alone suggests.

The tool is designed for quick weather interpretation, communication, and planning. It helps explain why a day at 90 F can feel manageable in dry air but much more dangerous when moisture prevents sweat from evaporating efficiently. Heat index is a comfort and risk indicator, not a replacement for measured air temperature.

Inputs explained

  • Air temperature: Enter the ambient dry-bulb air temperature in either F or C.
  • Relative humidity: Enter the percentage of moisture in the air relative to saturation at that temperature.
  • Unit selector: Choose the temperature unit that matches the value you entered so the calculator can convert internally without changing the displayed result.

How it works / method

The calculator follows the same logic used in common NOAA and National Weather Service guidance. It starts with a simpler warm-weather estimate and then applies the Rothfusz regression for hotter and more humid conditions. The result is mapped to a practical risk band so the number is easier to interpret during planning, reporting, or safety conversations.

Formula used

HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541TRH - 0.00683783T^2 - 0.05481717RH^2 + 0.00122874T^2RH + 0.00085282TRH^2 - 0.00000199T^2RH^2

In this page's engine, T is handled in F for the main regression and RH is relative humidity in percent. The calculator also applies NOAA-style low-humidity and very-high-humidity adjustments in the standard warm-weather range.

Practical note: Heat index assumes shaded, light-wind conditions. Direct sun, heavy clothing, poor acclimatization, work rate, and individual health can make real heat stress worse than the number shown here.

Estimates human-perceived heat based on NOAA NWS standards.

Safe
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Risk Level: --

Step-by-step example

Suppose the air temperature is 95 F and the relative humidity is 55 percent. The result should be interpreted as a practical warning signal rather than a literal thermometer reading.

  1. Enter 95 for air temperature and choose F.
  2. Enter 55 for relative humidity.
  3. The calculator estimates a heat index a little above 107 F.
  4. That falls into a dangerous range where heat illness becomes more likely during outdoor activity.
  5. If those same conditions occur in direct sunshine, the real outdoor burden on the body can be even higher.

Use cases

  • Checking whether planned outdoor work or sports sessions should be shortened, rescheduled, or supplied with extra cooling breaks.
  • Explaining why a modest rise in humidity can materially change summer discomfort even when the thermometer barely changes.
  • Comparing forecast days when the air temperature is similar but the moisture burden is very different.
  • Adding context to local weather reports, preparedness pages, and plain-language safety messaging.

Assumptions and limitations

  • Heat index is built for warm weather and human heat stress, not for cold weather, engineering heat transfer, or indoor HVAC sizing.
  • The regression is most relevant in the normal warm-humid operating range used by NOAA guidance and is not a universal law of human physiology.
  • The value is less informative when unusual clothing, radiant heat, intense physical work, or limited airflow dominate the actual exposure.
  • A comfort index can support decisions, but site safety policies may require WBGT or direct field measurements instead.

If the question is about direct sun, gear, or work-rest scheduling, compare this page with the WBGT calculator before making a risk decision.

NOAA Heat Index Calculator

Use noaa heat index calculator or heat index calculator noaa when you want the apparent temperature from air temperature and relative humidity. The National Weather Service describes heat index as how hot it feels when humidity is combined with air temperature, generally for shady, light-wind conditions.

National Weather Service. Heat Index: How To Compute And Use It[R]. Noaa, 2020. style citations usually point to the same caution: full sun can make conditions feel hotter than the shaded heat-index value.

Frequently Asked Questions

Use the Rothfusz regression — the same one the US National Weather Service uses. The full polynomial takes T in °F and RH as a percent and returns the apparent temperature. For T below 80°F or RH below 40%, simpler formulas apply. Inputs are just two: dry-bulb air temperature and relative humidity. The model assumes shade, light wind, and a 1.7 m tall, 67 kg adult. Direct sun can add another 8°C to perceived heat, so do not treat the calculator as a sunlit prediction.
The Rothfusz equation is HI = c1 + c2T + c3R + c4TR + c5T2 + c6R2 + c7T2R + c8TR2 + c9T2R2, with T in °F, R in %RH, and nine fitted coefficients. It is messy by design — it is a regression fit to Steadman's full physiological model. Most calculators use it because typing nine constants once is easier than coding the underlying biophysics. Valid range is roughly T ≥ 80°F and RH ≥ 40%; below that, return T itself.
NWS bands give the working scale: 27–32°C (80–90°F) caution, 32–41°C (90–105°F) extreme caution, 41–54°C (105–130°F) danger, and above 54°C (130°F) extreme danger. In the danger zone, heat-stroke becomes likely with prolonged exposure. Cancel outdoor sport and physical labour, hydrate aggressively, and watch for confusion and dry skin in colleagues — those are heat-stroke red flags. For official advisories, lean on your country's meteorological service rather than just the calculator number.
At 90°F and 70% RH, the Rothfusz formula returns about 105°F (40.6°C), which sits at the boundary of NWS's "danger" band. That means heat-stroke is plausible with sustained exposure or strenuous activity. In real terms — cancelled afternoon construction shifts, water-break protocols every 15 minutes, watch for nausea, dizziness, no sweating. A 90°F day in dry Phoenix at 15% RH would feel like 86°F. Same air temperature, dramatically different physiological load. That is why humidity matters.
The effect grows sharply with temperature. At 27°C, going from 30% RH to 90% RH adds maybe 2°C to perceived heat. At 35°C, the same humidity jump adds nearly 12°C — your apparent temperature goes from 35 to 47. The reason is evaporative cooling. Your body sheds about 80% of its excess heat through sweat evaporation. When the air is already saturated, sweat sits on your skin and the cooling fails. So the muggier it gets, the more dangerous a hot day becomes.
Above roughly 40% RH at warm air temperatures (over 27°C), humidity starts noticeably raising the perceived heat. Below 40% RH the heat index basically equals the air temperature. The slope steepens as both temperature and RH climb — at 35°C, every 10% RH adds about 1.5–2°C. This is why the Sahara at 45°C and 10% RH is uncomfortable but survivable, while Karachi at 40°C and 70% RH becomes life-threatening fast. Humidity, not just temperature, is the killer in tropical heatwaves.
A heat index chart is a two-axis lookup table. Temperature runs along one side (usually 80–110°F), relative humidity along the other (40–100%). The cell where they intersect gives you the apparent temperature, often colour-coded into NWS bands — yellow caution, orange extreme caution, red danger, purple extreme danger. To read it, find your air temperature row, slide across to your RH column, read the value. It is the same calculation as the Rothfusz formula, just pre-computed and printed.
Practical guidance from NWS and OSHA: above 40°C (105°F) heat index, scale back vigorous outdoor work and increase rest cycles. Above 46°C, stop strenuous activity entirely except essential operations with cooling and medical standby. For school sport, many associations cancel above 32°C heat index (90°F). These are floors, not ceilings — acclimatisation, hydration, age, and clothing all shift personal limits. Always defer to local advisories from your meteorological service for the legal threshold in your jurisdiction.

Sources & references

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