Heat Index Calculator

Last updated: Jul 22, 2026 | Reviewed by Calkulator Team | 2 min read

Weather

Check how dangerous the heat really is with humidity

At 35°C with 70% humidity, the apparent temperature is about 46°C — in the "Danger" zone where heat exhaustion is likely. Indian summers frequently reach these levels, making outdoor work risky between 11 AM and 4 PM.

Above 40°C heat index, limit outdoor exertion to 15-minute intervals with water breaks in shade.

Air Temperature

°C

Relative Humidity (%)

Formula Notes

The Rothfusz regression equation is valid when temperature ≥ 80°F (27°C) and relative humidity ≥ 40%. Below these thresholds, humidity has less impact on perceived temperature.

Heat Index

Feels Like (°F)

—

Feels Like (°C) —

Danger Level

—

Live Result Illustration

Visual summary — updates instantly as you enter values above

LIVE

Real-Life Guide to Using the Heat Index Calculator

Feels-like temp in humidity. Use the examples and checks below to turn the number into a practical decision.

When this calculator is useful

Important during India's pre-monsoon summer months (April-June) in humid coastal or northern plains cities like Chennai, Kolkata, or Delhi, where high humidity makes the actual air temperature feel far more dangerous than the thermometer suggests.

For most people, the best way to use the Heat Index Calculator is to try the real case first, then change one input at a time. That makes the trade-off visible. For example, with a loan calculator you can change tenure while keeping the same rate; with an investment calculator you can change return assumption while keeping the same monthly contribution; with a health, education or measurement calculator you can check how much one input changes the final category.

The result should answer a practical question: Can I afford this? How much should I save? Is this score enough? Is this measurement within range? What is the safer or cheaper option? If the output does not answer the decision clearly, adjust the inputs until the scenario matches your real situation.

Real-Life Example

Humid summer afternoon in Chennai: A construction worker in Chennai checks conditions showing 36°C (96.8°F) air temperature with 65% relative humidity during a June afternoon.

1Applying the Rothfusz regression formula to T=96.8°F, RH=65% gives a heat index of approximately 118°F (about 48°C), which falls in the "danger" category on the NWS heat index chart.

2Now change one input, such as rate, time, quantity, unit or score, and compare the new result with the first one.

A thermometer reading of 36°C can translate to a felt 48°C in high humidity, which is well past the point where sustained outdoor labor becomes genuinely hazardous.

Practical Advice

Use the Heat Index Calculator as a planning tool, not just a number generator. Write down the inputs you used, because the final answer is meaningful only when you remember the assumptions behind it.

If the decision affects money, health, tax, safety, academics or legal compliance, keep a second check ready. That second check may be a bank quote, payslip, official rule, prescription, site measurement, mark sheet or invoice.

Common Mistakes

Using heat index calculations for temperatures below about 27°C (80°F) — the Rothfusz regression formula behind heat index is only calibrated and reliable above this threshold.
Ignoring direct sunlight exposure — the standard heat index assumes shade; full sun exposure can add another 8°C (about 15°F) to the perceived heat, which the base formula doesn't capture.
Confusing relative humidity percentage with absolute humidity — a "70% humidity" reading means something very different at 25°C than at 40°C, since warm air holds far more moisture at the same relative percentage.
Underestimating risk during humid heat versus dry heat of the same temperature — 40°C with 60% humidity is significantly more dangerous for heatstroke than 40°C in dry desert conditions.
Not adjusting outdoor activity or labor schedules despite a "danger" or "extreme danger" heat index reading, which is when heat exhaustion and heatstroke risk rises sharply.

How to Interpret Results

The heat index tells you how hot it actually feels to your body when humidity is factored in — values above roughly 39-40°C fall into "danger" territory where heat cramps and heat exhaustion become likely with prolonged exposure or physical activity, and above 51°C is "extreme danger" for heatstroke.

A good interpretation looks at both the main result and the supporting values. If a page shows totals, ratios, categories, schedules or warnings, read those together instead of focusing only on the biggest number.

Heat Index Calculator FAQs

Useful answers for interpreting the output, avoiding mistakes and using the result responsibly.

What is heat index and what does this calculator compute?

Heat index is the "feels like" temperature that combines actual air temperature with relative humidity to reflect how hot the human body perceives it, since high humidity reduces the effectiveness of sweat evaporation, our main cooling mechanism. This calculator applies the standard meteorological formula to your entered temperature and humidity.

What formula does the calculator use?

It uses the Rothfusz regression, a polynomial equation developed by the US National Weather Service that combines temperature (T in °F) and relative humidity (RH in %) with several coefficients to output an estimated apparent temperature, with additional adjustment terms applied at very low or very high humidity extremes.

Why doesn't the calculator show a meaningful result for cooler days?

The Rothfusz formula is only valid and accurate above roughly 27°C (80°F) — below that, humidity doesn't meaningfully increase perceived heat, so the raw air temperature is already a good enough estimate of how conditions feel.

Why does the same temperature feel so different in Delhi versus Chennai in summer?

Delhi's summer heat is often drier (lower relative humidity), allowing sweat to evaporate and cool the body effectively, while Chennai's coastal humidity is much higher, trapping moisture near the skin — so a "dry" 42°C in Delhi can feel less oppressive than a "humid" 36°C in Chennai.

Does the calculator account for direct sunlight or shade?

No — the standard heat index formula assumes shaded conditions. The US National Weather Service notes that full direct sun exposure can add up to 8°C (15°F) to the effective heat index, so outdoor readings in direct sun should be treated as more dangerous than the calculated number suggests.

What heat index range is considered dangerous?

Roughly 32-39°C is "caution" (fatigue possible with prolonged exposure), 39-51°C is "danger" (heat cramps and heat exhaustion likely), and above 51°C is "extreme danger" with heatstroke highly likely — these bands come from the NWS heat index chart used globally as a reference.

How is heat index different from "real feel" temperatures shown on weather apps?

Many weather apps blend heat index (for hot conditions) with wind chill (for cold conditions) plus additional factors like solar radiation into a single "real feel" or "feels like" number, so app values may differ slightly from a pure heat index calculation depending on which formula and adjustments they use.

What should I actually do with a "danger" level heat index reading?

Limit strenuous outdoor activity to early morning or evening, stay hydrated well beyond normal thirst cues, wear light and breathable clothing, and watch for early heat exhaustion signs like dizziness or nausea — construction and agricultural workers in particular should get scheduled shade breaks during danger-level heat index periods.

What is the Heat Index?

The heat index (also called apparent temperature or "feels like") combines air temperature and relative humidity to measure perceived temperature. High humidity slows the evaporation of sweat from skin, reducing the body's ability to cool itself — making it feel hotter than the actual air temperature.

The Rothfusz equation used here is the formula adopted by the US National Weather Service. It's accurate for temperatures ≥ 80°F (27°C) and humidity ≥ 40%. A heat index above 40°C (104°F) is potentially dangerous; above 54°C (130°F) can cause heat stroke in minutes.

Example

35°C (95°F), 80% humidity:

1Using Rothfusz equation

2HI ≈ 124°F (51°C)

✓ Extreme Danger — heat stroke risk

Frequently Asked Questions

What is heat index and why does it differ from actual temperature?

The heat index (apparent temperature or "feels like") combines air temperature with relative humidity. High humidity prevents sweat from evaporating efficiently, reducing your body's ability to cool itself. At 38°C with 70% humidity, it can feel like 51°C. The NWS heat index formula applies when temperature exceeds 27°C and humidity exceeds 40%.

At what heat index level is it dangerous to be outdoors?

Heat index 32–41°C: caution, fatigue possible. 41–51°C: extreme caution, sunstroke possible with prolonged exposure. 51–54°C: danger, sunstroke likely. Above 54°C: extreme danger, heatstroke imminent. In India, many cities exceed 48°C apparent temperature during May–June, making outdoor work dangerous without hydration breaks every 30 minutes.

How does humidity accelerate heat exhaustion?

At low humidity (below 30%), the body can cool effectively via sweating even at high temperatures. At high humidity (above 70%), sweat cannot evaporate — body temperature rises rapidly. Heat exhaustion occurs when core temperature reaches 38–40°C; heat stroke (life-threatening) begins at 40°C. Stay hydrated and seek shade or air conditioning when the heat index is high.