Humidex Calculator

Humidex combines air temperature and moisture to reflect perceived heat stress.

• Humidex: H = T + 0.5555 × (e − 10)
• Vapour pressure from dew point: e = 6.11 × exp(17.27×Td / (237.7 + Td))
• Dew point from temperature and humidity (Magnus): Td = (b×γ)/(a−γ), where γ = ln(RH/100) + aT/(b+T)

Here, T and Td are in °C, and e is in hPa.

1. Select an input method: humidity or dew point.
2. Enter the air temperature and choose units.
3. Provide relative humidity or dew point, as selected.
4. Click Calculate Humidex to see results.
5. Download CSV or PDF for documentation and sharing.

For safety decisions, combine with local forecasts and hydration planning.

Values are rounded and meant for demonstration only.

1) What humidex represents

Humidex is a thermal comfort index that blends air temperature with moisture to describe how hot conditions feel. It was designed for warm-season messaging where humidity amplifies heat stress. A humidex value close to the air temperature indicates dry air, while higher values signal that sweating becomes less effective.

2) Why moisture changes perceived heat

When humidity is high, sweat evaporates slowly, reducing the body’s main cooling pathway. In practice, a day at 32 °C with moderate moisture can feel like the upper 30s, while the same temperature with very moist air can feel above the mid‑40s. This is why the calculator also reports dew point and vapour pressure.

3) Inputs and derived values

You can enter temperature with either relative humidity or dew point. If you supply humidity, dew point is estimated using a Magnus approximation. Vapour pressure e is then computed from dew point and used in the humidex equation. These intermediate values help you understand what is driving the final comfort number.

4) Interpreting the result

Use humidex alongside activity level. Many guidance tables label 20–29 as generally comfortable, 30–39 as some discomfort, 40–45 as great discomfort, and above 45 as hazardous for prolonged exertion. Above 54, heat stroke becomes a serious possibility, especially with dehydration or poor airflow.

5) Dew point data you should know

Dew point is a direct moisture measure: around 10 °C feels dry, 16–18 °C feels humid, and 21 °C or higher often feels oppressive. Because humidex depends on vapour pressure, a rising dew point can increase humidex even if the air temperature stays constant. Tracking dew point is useful for planning outdoor work.

6) Planning work and sport safely

For humidex above 35, schedule breaks and prioritize shade. Above 40, reduce intensity, shorten sessions, and increase hydration frequency. Teams and workplaces often add controls such as buddy checks, cooling towels, and early-morning shifts. Consider acclimatization: new exposure to heat typically raises risk during the first 7–14 days.

7) Limits and best practice

Humidex is an estimate and does not fully capture wind, sun load, clothing insulation, or medical factors. Direct sun can increase perceived heat beyond the index, while strong airflow can reduce stress. For critical decisions, combine humidex with local forecasts, shade conditions, and occupational heat guidelines.

8) Practical example

Suppose air temperature is 33 °C and relative humidity is 70%. Dew point is roughly 26.8 °C, vapour pressure is about 35.1 hPa, and humidex is near 48.4. That falls in a dangerous range for heavy activity, so plan cooling, reduce effort, and limit exposure time.

1) Is humidex the same as heat index?
No. Both combine temperature and moisture, but they use different formulas and were developed for different regions. Humidex relies on vapour pressure derived from dew point, while heat index is based on temperature and relative humidity.

2) Which input method is better: humidity or dew point?
Dew point is more direct, because it measures moisture without depending on temperature scaling. If you have accurate dew point from a weather station, use it. If not, relative humidity works well for general estimates.

3) Why does the calculator show vapour pressure?
Vapour pressure drives the humidity term in the humidex equation. Seeing it helps explain why humidex rises quickly on very moist days. Higher vapour pressure means evaporation is less efficient, increasing perceived heat stress.

4) What humidex value is considered dangerous?
Guidance commonly flags 40–45 as great discomfort and above 45 as hazardous for sustained exertion. Above 54 indicates very high heat‑illness risk. Individual tolerance varies with health, acclimatization, and airflow.

5) Can wind or shade change how it feels?
Yes. Wind improves sweat evaporation and can reduce strain, while direct sun can increase heat load substantially. Humidex does not fully include wind or radiation effects, so treat the result as a baseline and adjust for conditions.

6) Why can humidex increase even when temperature stays steady?
If moisture rises, dew point and vapour pressure rise too. That increases the humidity term in the equation, pushing humidex upward. This is common during late afternoon or before storms when air becomes more humid.

7) Is this suitable for medical or workplace compliance decisions?
It is a helpful screening tool, not a compliance instrument. For regulated environments, follow official occupational heat standards and local advisories. Use humidex to support planning, hydration, rest scheduling, and early warnings.