Enter the air temperature and the relative humidity to get the heat index (how hot it actually feels) and the associated heat-related illness risk level.
Below 27°C (80°F) the heat index adds little to the air temperature, so the simple air temperature is shown as the best estimate of how it feels.
| Heat index range | Heat-related illness risk |
|---|---|
| 27°C to 32°C (80°F to 90°F) | Caution: fatigue possible with prolonged exposure or activity |
| 32°C to 39°C (90°F to 103°F) | Extreme caution: heat exhaustion possible with prolonged exposure or activity |
| 39°C to 51°C (103°F to 124°F) | Danger: heat exhaustion likely, heatstroke possible with prolonged exposure or activity |
| Above 51°C (124°F) | Extreme danger: heatstroke highly likely |
The heat index is a comfort and risk index calculated for shaded conditions, not a direct physical measurement of temperature in direct sunlight, which can add up to 8°C (14°F) to the perceived heat. It does not replace official weather warnings. Seek medical advice if you or someone nearby feels unwell in the heat.
Why humidity makes hot air feel hotter
Air temperature only tells part of the story on a hot day. The body cools itself mainly by sweating: as sweat evaporates from the skin it carries heat away. High humidity slows that evaporation because the surrounding air is already close to saturated with water vapour, so sweat lingers on the skin instead of evaporating and the body struggles to shed heat. This is why a warm, humid day can feel far more oppressive than a hotter but drier one. The heat index combines air temperature and relative humidity into a single "feels like" figure using the Rothfusz regression adopted by the US National Weather Service, the standard used internationally for this kind of calculation, including by the UK Met Office in its own heat-health guidance.
Heat-related illness risk by heat index
| Heat index range | Heat-related illness risk |
|---|---|
| 27°C to 32°C (80°F to 90°F) | Caution: fatigue possible with prolonged exposure or activity |
| 32°C to 39°C (90°F to 103°F) | Extreme caution: heat exhaustion possible with prolonged exposure or activity |
| 39°C to 51°C (103°F to 124°F) | Danger: heat exhaustion likely, heatstroke possible with prolonged exposure or activity |
| Above 51°C (124°F) | Extreme danger: heatstroke highly likely |
What this calculator cannot do
The heat index is calculated for shaded conditions and does not account for wind, direct sunshine, or an individual's level of acclimatisation to heat. Direct sunlight can add as much as 8°C (14°F) to the perceived heat compared with the shade figure shown here, and someone who has recently arrived in a hot climate will tolerate the same heat index far less well than someone used to it. Treat the result as a general guide to heat exposure risk, not a precise medical or physical measurement. For the opposite problem, the wind chill calculator works out how much colder the air feels once wind is added to the mix.
Met Office heat-health alerts
The Met Office and UK Health Security Agency issue heat-health alerts when a spell of hot weather is expected to affect health, a system introduced after the summer of 2022, when a Met Office station at Coningsby recorded a UK record temperature of 40.3°C. Older people, young children, and people with heart or breathing conditions are most at risk during a heatwave. If a heat-health alert is in force, stay in the shade during the hottest part of the day, keep hydrated, and check on vulnerable neighbours and relatives rather than relying on this calculator alone as a safety guide.