In May and June, a thermometer in Multan can read 40°C while the air feels like 55°C. That gap is the heat index — what temperature your body actually experiences when humidity blocks sweat from cooling you. It decides whether outdoor work is merely uncomfortable or genuinely dangerous: every year, Pakistan’s heatwaves hospitalise thousands with heatstroke, and the difference between caution and crisis is understanding the “feels like” number. Our free heat index calculator combines temperature and humidity using the Rothfusz regression (the formula behind US National Weather Service heat alerts) to give you the feels-like temperature and the danger level.
Heat Index Calculator
How hot does it really feel with the humidity?
The Rothfusz Regression — What the Tool Computes
The US National Weather Service’s heat index equation (Rothfusz, 1990) models how heat + humidity interact:
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R²
where T is temperature in °F and R is relative humidity %. The calculator converts your °C input automatically.
| Heat index | Danger level | What to do |
|---|---|---|
| Below 32°C (90°F) | Caution | Fatigue possible with prolonged activity |
| 32–41°C (90–105°F) | Extreme caution | Heat exhaustion likely; rest in shade |
| 41–54°C (105–130°F) | Danger | Heatstroke likely — limit outdoor work |
| Above 54°C (130°F) | Extreme danger | Heatstroke highly likely — avoid exposure |
How to Use the Heat Index Calculator
- Enter the air temperature (toggle °C or °F) and the relative humidity %.
- Click Calculate — the feels-like temperature and the danger level appear.
- Plan accordingly: shift outdoor labour, weddings, or sports to early morning or evening when the index is high.
Worked Example
Lahore, a June afternoon: 40°C with 50% humidity. The Rothfusz equation gives a heat index of about 54.8°C (130.6°F) — Extreme danger. Same 40°C at 20% humidity (dryer air) would feel like ~45°C — serious, but meaningfully less dangerous. Humidity is the hidden multiplier.
Survival Tips for Pakistani Summers
- Drink before you’re thirsty — add a pinch of salt or use ORS for heavy sweating (see our water intake calculator for daily targets).
- Shade + breeze beats air conditioning for survival; if AC fails during load-shedding, wet a cloth and use a fan.
- Watch vulnerable people: the elderly, infants, and outdoor workers are the first heatstroke victims.
- Heatstroke signs (confusion, hot dry skin, fainting) need emergency care — cool the person and call for help; it’s a medical emergency.
Frequently Asked Questions
What is the difference between temperature and heat index?
Temperature measures the air; heat index measures what your body feels when humidity slows sweat evaporation — your real thermal stress.
Why is Karachi’s 38°C harder than Quetta’s 38°C?
Humidity. Karachi’s coastal moisture raises the heat index far above the thermometer; Quetta’s dry air lets sweat work. The calculator shows exactly how much humidity matters.
At what heat index is it dangerous to work outside?
Above ~41°C (105°F) the danger zone begins; above 54°C (130°F) it’s extreme. Pakistan’s 2022 and 2024 heatwaves pushed many cities deep into the danger band.
Does the heat index work at night?
Yes — warm humid nights block recovery. If the overnight index stays above ~30°C, sleep and recovery suffer; that’s why heatwaves kill in waves over several days.
Heat index vs humidex vs “feels like” — same thing?
Roughly. Heat index (US NWS), humidex (Canada), and app “feels like” values all combine temperature and humidity with slightly different formulas. This calculator uses the NWS Rothfusz standard.
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