Mumbai monsoon: 33°C with 85% humidity
HI ≈ 49°C (120°F). DANGER level. Heatstroke very possible with outdoor activity. Stay indoors, hydrate aggressively, avoid heavy work between 11 AM and 4 PM.
Heat Index Calculator - Free Online Calculator
Heat Index - the 'feels like' temperature when humidity is factored in. High humidity slows sweat evaporation, making your body feel hotter than the actual air temperature. Used by NWS for heat advisories.
- Formula shown on the page
- Runs in your browser
- Your inputs stay private
°C
%
Heat index (feels like)
45 °C
Danger
Danger
Heat cramps / exhaustion likely; heatstroke possible.
Understanding the Heat Index Calculator
Heat index is the 'feels-like' temperature when humidity is high - and few countries experience this more than India. Coastal cities like Mumbai, Chennai, Kochi, Visakhapatnam, and Kolkata routinely see 32-35°C air temperatures with 80-90% humidity in summer and monsoon, producing heat indices of 45-50°C - squarely in the Danger zone. The human body cools by sweating; when air is already saturated with moisture, sweat can't evaporate efficiently and body temperature rises. Heat index quantifies this physiological discomfort. The IMD (India Meteorological Department) and NDMA (National Disaster Management Authority) issue heat-wave warnings partly based on heat index thresholds.
This calculator uses the standard NWS Rothfusz regression formula - a polynomial fit calibrated to actual physiological response data. It accepts temperature in °C or °F and relative humidity in percent, returning the heat index and a risk category. Critical for: outdoor workers (construction, agriculture, delivery workers in Indian summer), athletes and gym-goers training outdoors, parents monitoring children's outdoor playtime, school administrators deciding whether to call off outdoor activities, and senior citizens whose heat tolerance is reduced. Heat-related illnesses kill hundreds in India every summer - knowing heat index helps prevent avoidable tragedies.
Using the Heat Index Calculator in 4 steps
Pick units: °C or °F.
Enter air temperature.
Enter relative humidity in %.
Read heat index and risk category (Caution / Extreme caution / Danger / Extreme danger).
How the Heat Index Calculator works
Uses the Rothfusz regression - calibrated polynomial of temperature and humidity. Valid for T ≥ 80°F (27°C) and RH ≥ 40%; below that, a simpler formula is used.
Formula
HI = −42.379 + 2.05T + 10.14RH − 0.225T·RH + … (Rothfusz regression)
- TTemperature in °F
- RHRelative humidity %
How the Heat Index Calculator gets its answer
Rothfusz regression: HI (°F) = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R². T is in °F, R is relative humidity in percent. Result is heat index in °F. Convert to °C using HI_C = (HI_F - 32) × 5/9.
The formula is valid for air temperatures above 27°C (80°F) and relative humidity above 40%. Below these thresholds, heat index essentially equals air temperature - humidity doesn't have a strong perceptual effect at lower temperatures. For very high temperatures with very low humidity, a small correction is applied (subtracts a 'dryness offset' that captures faster sweat evaporation in dry heat).
Risk thresholds (NWS standard): Caution (27-32°C HI / 80-90°F) - fatigue possible with prolonged exposure. Extreme caution (32-40°C HI / 90-105°F) - heatstroke, heat cramps, heat exhaustion possible. Danger (40-54°C HI / 105-130°F) - heatstroke likely with prolonged exposure or physical activity. Extreme danger (>54°C HI / >130°F) - heatstroke imminent. Indian coastal cities frequently reach 'Danger' levels during summer afternoons.
Worked examples
Delhi summer (dry heat): 42°C with 25% humidity
HI ≈ 41°C (106°F). Slight dryness correction since RH<40%. Still in DANGER zone. Heat dissipates faster than humid coastal heat, but high temperature alone is hazardous.
Chennai April afternoon: 36°C with 70% humidity
HI ≈ 49°C (120°F). DANGER. Combined with sea breeze that's still warm/humid, makes outdoor work very challenging. Construction sites in Tamil Nadu adopt early-morning/evening shift schedules in such conditions.
When to use the Heat Index Calculator
- Heat-wave warning planning by IMD and NDMA.
- Construction site scheduling - avoiding peak heat hours.
- School outdoor activity decisions.
- Agricultural fieldwork timing for farmers.
- Outdoor sports planning - cricket, football, athletics.
- Delivery worker route and break planning.
- Public health advisories - heat-related illness prevention.
- Tourism - visitors checking comfort levels for sightseeing.
Common mistakes and gotchas
- Confusing heat index with actual air temperature for medical purposes.
- Ignoring humidity in heat index calculation. Humidity is the key second variable.
- Using indoor air conditioning temperature for outdoor risk decisions.
- Comparing heat index across very different humidity environments without context.
- Applying formula outside its validity range (T < 27°C, RH < 40%).
Get more out of this tool
- Drink water proactively when HI exceeds 32°C, even if not thirsty.
- Schedule outdoor work for early morning (5-9 AM) or evening (after 5 PM) during summer.
- Wear loose, light-coloured cotton clothing in high heat index.
- Children, elderly and people on certain medications are more vulnerable; check on them in heat waves.
- Pair with the wind chill, dew point and humidity calculators on this site for related weather work.
Questions about the Heat Index Calculator
Heat index in Indian summer?
Common - Chennai, Mumbai and Kolkata routinely hit 32°C with 80% humidity, giving a heat index of ~42°C (108°F) - danger zone.
When does heat index become dangerous?
Caution starts at 80°F (27°C). Extreme caution at 91°F (33°C). Danger at 103°F (39°C) - heatstroke possible. Extreme danger above 125°F (52°C).
How is dew point related?
Both depend on humidity. Heat index focuses on perceived temp. Dew point indicates absolute moisture - a 75°F dew point feels oppressive regardless of air temp.
Why doesn't my AC display this?
Most consumer thermostats show only air temp. Some smart thermostats and weather services do compute heat index - useful in muggy climates.
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Many tools in the Daily Utilities category complement each other - date difference + day counter + day-of-the-week, or password generator + QR code + base64 encoder. The related-tool grid links them together so you can move between adjacent problems in a click.
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