Heat Index Calculator
Compute the 'feels-like' temperature when high air temperature is combined with humidity, using the National Weather Service Rothfusz regression. Use it to assess heat stress, plan outdoor activity, and trigger safety precautions in hot weather.
Last updated: May 2026
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About this calculator
Heat index (HI) — also called apparent temperature — quantifies how hot the air feels to a human body when humidity is included alongside air temperature. The body cools primarily by evaporating sweat; high humidity slows evaporation, so the same air temperature feels hotter. The formula used here is the Rothfusz regression adopted by the US National Weather Service: HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH², where T is air temperature in °F and RH is relative humidity in percent (entered as a number between 0 and 100). The result is heat index in °F. The model is valid for T ≥ 80 °F and RH ≥ 40%; below those thresholds the NWS uses a simpler fallback (HI ≈ T) and the regression should not be applied. Edge cases: the regression slightly underestimates extreme cases (T > 110 °F and RH > 85%) and is corrected by NWS adjustments. It does not account for direct sun exposure, which can add 10–15 °F to the perceived temperature; the wet bulb globe temperature (WBGT) is the better metric for sun-exposed athletic and occupational settings. It also does not account for wind, which can reduce perceived heat by aiding sweat evaporation. The Rothfusz model assumes a standardized adult subject in light clothing; children, elderly, and people on certain medications experience heat stress at lower thresholds. NWS warning categories: 80–90 °F caution, 91–103 °F extreme caution, 103–124 °F danger, 125+ °F extreme danger.
How to use
Example 1 — humid summer day. T = 90 °F, RH = 70%. Substitute into the Rothfusz formula: −42.379 + 2.04901523 × 90 + 10.14333127 × 70 − 0.22475541 × 90 × 70 − 0.00683783 × 8,100 − 0.05481717 × 4,900 + 0.00122874 × 8,100 × 70 + 0.00085282 × 90 × 4,900 − 0.00000199 × 8,100 × 4,900. Term by term: −42.379 + 184.411 + 710.033 − 1,415.959 − 55.387 − 268.604 + 696.696 + 376.094 − 78.991 = 105.9 °F. NWS classifies this as "danger" — heat exhaustion likely with prolonged exposure. Verify: an alternative simplified estimate (Steadman 1979 lookup) gives ~106 °F at 90/70 — within 0.1 °F. Example 2 — borderline applicability. T = 82 °F, RH = 55%. Substitute: −42.379 + 2.04901523 × 82 + 10.14333127 × 55 − 0.22475541 × 82 × 55 − 0.00683783 × 6,724 − 0.05481717 × 3,025 + 0.00122874 × 6,724 × 55 + 0.00085282 × 82 × 3,025 − 0.00000199 × 6,724 × 3,025. Computing: −42.379 + 168.02 + 557.88 − 1,013.65 − 45.98 − 165.82 + 454.45 + 211.55 − 40.49 ≈ 83.6 °F. The "feels like" is only 1–2 °F above air temperature here because both inputs are at the lower edge of validity. Verify by extreme: at the NWS validity floor (80 °F, 40%), the model returns ~80 °F — consistent with the boundary condition that low humidity in mild heat barely changes perception.
Frequently asked questions
What is the difference between heat index and Wet Bulb Globe Temperature (WBGT)?
Heat index is calculated from air temperature and relative humidity only, assuming a person in shade with normal indoor or shaded outdoor exposure. WBGT additionally incorporates wind speed, sun angle, and radiant heat from surroundings, making it more accurate for outdoor athletic and occupational settings. WBGT is the metric used by the military, OSHA recommendations, and sports governing bodies for activity restrictions because direct sun exposure can raise the effective heat stress by 10–15 °F beyond what heat index reports. For backyard, indoor, or shaded settings, heat index is sufficient; for outdoor athletes, construction workers, or anyone in direct sun, prefer WBGT or add a 10–15 °F mental adjustment to the heat index. NOAA and the National Weather Service publish WBGT forecasts at weather.gov/wbgt.
Why is the heat index formula only valid above 80 °F and 40% humidity?
The Rothfusz regression was fit to physiological data above these thresholds because that is the range where humidity meaningfully impairs the body's evaporative cooling and the heat-index concept becomes useful. Below 80 °F, the body easily compensates for humidity changes and the apparent temperature is approximately equal to air temperature. Below 40% relative humidity, evaporation is fast enough that even hot air does not feel proportionally hotter; the formula can return values lower than air temperature in dry-air conditions, which is mathematically possible but not how the NWS reports heat index in practice. The NWS uses a simpler model for the borderline cases and only switches to Rothfusz above both thresholds. Reporting heat index for cool or dry conditions is misleading and not standard practice.
How quickly does heat-related illness develop at different heat index levels?
Risk and timing depend on heat index, exertion level, hydration, and individual susceptibility. At 80–90 °F (caution), heat exhaustion is possible after several hours of strenuous outdoor work or exercise. At 91–103 °F (extreme caution), heat cramps and heat exhaustion can develop within 1–2 hours of prolonged exposure. At 104–124 °F (danger), heat exhaustion is likely and heat stroke is possible within 30–60 minutes; outdoor workers should follow OSHA-recommended water-rest-shade cycles. At 125+ °F (extreme danger), heat stroke is imminent; outdoor activity should be suspended. Children, the elderly, people on diuretics, antihistamines, or psychiatric medications, and those acclimating to heat have lower thresholds. Heat stroke (core body temperature >104 °F) is a medical emergency — rapid cooling and 911 calls are critical because mortality rises sharply after 30 minutes.
What are common mistakes when interpreting heat index?
The most common mistake is treating heat index as the actual outdoor temperature for sun-exposed activities — direct sun adds 10–15 °F that the formula does not capture, so the real perceived load is higher. Another error is using heat index as a guide for cool-down planning indoors where AC is running; HI applies to the outdoor air, not your air-conditioned house. People also miscalculate by using temperature in Celsius without converting to Fahrenheit first — the Rothfusz coefficients only work in °F. Inputting RH as a decimal (0.70 instead of 70) produces wildly wrong outputs. Finally, ignoring acclimatization is a danger: a person used to 70 °F dry-air conditions experiences 90 °F with 60% humidity much more severely than a long-term resident of Florida or the Gulf Coast — the formula does not adjust for individual heat tolerance.
When should I NOT use this calculator?
Skip heat index for outdoor athletic events in direct sun — use WBGT instead, which accounts for solar radiation and wind. Do not use it for air temperatures below 80 °F or humidity below 40%; the NWS itself does not report heat index in those ranges, and the formula can return unphysical values. Avoid it for medical or occupational decisions for vulnerable populations (infants, elderly, pregnant women, people on heat-sensitive medications) without supplementary guidance — the formula assumes a healthy adult. Do not apply it in altitudes above ~5,000 ft without adjustment; lower air density changes evaporative dynamics. It is also not appropriate for industrial or kitchen environments where radiant heat from machinery or fires dominates — those need direct WBGT measurement. Finally, for non-US users, confirm that your national weather service uses the same NWS-Rothfusz model; some agencies (e.g., the Australian Bureau of Meteorology) use a humidex variant with slightly different coefficients.