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: September 2026
Formula below · 2 sources (weather.gov, CDC) · Updated Sep 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. This calculator follows the US National Weather Service procedure exactly. First it computes Steadman's simple formula, HI = 0.5 × [T + 61 + (T − 68) × 1.2 + RH × 0.094]. If the average of that value and T is below 80 °F, the simple value is the heat index (so 70 °F at 50% RH gives about 69 °F). Otherwise it uses the Rothfusz regression: 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, with two NWS adjustments: if RH < 13% and T is 80–112 °F, subtract [(13 − RH)/4] × √[(17 − |T − 95|)/17]; if RH > 85% and T is 80–87 °F, add [(RH − 85)/10] × [(87 − T)/5]. T is air temperature in °F and RH is relative humidity in percent (0–100). Edge cases: heat index is defined for shade; direct sun can add up to about 15 °F. It does not account for wind, and the Rothfusz model assumes a standardized adult in light clothing; children, the elderly and people on certain medications experience heat stress at lower thresholds. NWS categories: 80–90 °F caution, 90–103 °F extreme caution, 103–124 °F danger, 125 °F and above 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" (103 °F and above) — 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. At 80 °F and 40% RH the NWS procedure stays on the simple formula: 0.5 × (80 + 61 + 14.4 + 3.76) = 79.6 °F, slightly below the air temperature, because dry air 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.
What does the calculator do below 80 °F or at low humidity?
The Rothfusz regression was fit to conditions above about 80 °F, so the National Weather Service only uses it when the heat index is at least about 80 °F. Below that, the NWS uses Steadman's simpler formula, HI = 0.5 × [T + 61 + (T − 68) × 1.2 + RH × 0.094], which this calculator applies automatically; it keeps the heat index close to the air temperature in mild conditions (for example 70 °F at 50% RH gives about 69 °F). The NWS also applies small corrections for very dry air (RH below 13% at 80–112 °F) and very humid air (RH above 85% at 80–87 °F), which are included here.
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 90–103 °F (extreme caution), heat cramps and heat exhaustion can develop within 1–2 hours of prolonged exposure. At 103–124 °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. In mild weather (heat index below about 80 °F) the result is close to the air temperature and is not a heat-stress warning. 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.