Free heat index calculator · No sign-up · Runs entirely in your browser
Calculate the "feels like" temperature from air temperature and relative humidity, with National Weather Service risk bands. Assumes shade and light wind.
| Metric | Value |
|---|
Australia regularly experiences extreme heat events. The record Australian air temperature is 50.7°C (Oodnadatta, SA, 1960). Heatwaves cause more deaths in Australia than any other natural disaster.
| Feels like (°C) | Risk level |
|---|---|
| 27–32°C | Caution — fatigue possible with prolonged exposure |
| 32–41°C | Extreme caution — heat cramps and exhaustion possible |
| 41–54°C | Danger — heat cramps/exhaustion likely, heat stroke possible |
| >54°C | Extreme danger — heat stroke highly likely |
Your body cools itself mainly by evaporating sweat. Evaporation carries heat away from the skin, and the drier the air, the faster it happens. Raise the humidity and that mechanism slows down — the sweat sits on your skin instead of leaving it, and your core temperature climbs even though the thermometer has not moved.
The heat index expresses that effect as a temperature. It answers the question: in dry, comfortable conditions, what air temperature would feel the same as this combination of heat and humidity?
| Air temperature | Relative humidity | Heat index |
|---|---|---|
| 35°C | 20% | 33.0°C — feels cooler than the thermometer |
| 35°C | 40% | 37.2°C |
| 35°C | 60% | 45.1°C |
| 35°C | 80% | 56.5°C |
The mechanism is worth seeing, because it explains every number on this page. Same air temperature, same person — only the humidity differs:
The thermometer reads 35°C in both panels. The difference in what your body experiences is more than 23°C — and nothing about the air temperature changed. That gap is the whole reason a "feels like" figure exists.
The heat index used worldwide comes from a multiple regression by Lans Rothfusz for the US National Weather Service, fitted to Robert Steadman's earlier human heat-balance modelling. It is defined in Fahrenheit, so a Celsius calculator converts in and back out.
The official procedure has two steps that most online calculators skip. A simpler equation is computed first and averaged with the air temperature; only if that result reaches 80°F is the full regression applied. Two further adjustments handle very dry air and very humid air near the low end of the range.
| Condition | Adjustment |
|---|---|
| Humidity below 13% and 80–112°F | Subtract, because very dry air evaporates sweat efficiently |
| Humidity above 85% and 80–87°F | Add, because evaporation is nearly blocked |
| Below roughly 27°C (80°F) | Outside the regression's domain; use the air temperature |
| Published charts start at 40% humidity | A charting convention, not a limit — the dry-air adjustment covers lower humidity |
Brisbane in February: 30°C, 70% relative humidity.
| Step | Value |
|---|---|
| Air temperature | 30.0°C (86.0°F) |
| Relative humidity | 70% |
| Heat index | 35.0°C |
| Risk band | Extreme caution |
| In direct sun | Roughly 43°C apparent |
Thirty degrees sounds ordinary. At 70% humidity it loads the body like a dry 35°C — and standing in the sun rather than the shade takes it to around 43°C. This is the combination that catches people out, because the thermometer reading looks unremarkable.
Inland New South Wales in January: 40°C, 30% relative humidity.
| Step | Value |
|---|---|
| Air temperature | 40.0°C (104.0°F) |
| Relative humidity | 30% |
| Heat index | 43.1°C |
| Risk band | Danger |
| In direct sun | Roughly 51°C apparent |
Air temperature down the side, relative humidity across the top. Shading follows the National Weather Service risk bands.
| Air temp | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 27°C | 27 | 27 | 28 | 29 | 30 | 31 |
| 30°C | 30 | 31 | 33 | 35 | 38 | 41 |
| 32°C | 32 | 34 | 37 | 40 | 44 | 49 |
| 34°C | 35 | 38 | 42 | 47 | 52 | — |
| 36°C | 39 | 43 | 48 | 54 | — | — |
| 38°C | 43 | 49 | 55 | — | — | — |
| 40°C | 48 | 55 | — | — | — | — |
| 42°C | 54 | — | — | — | — | — |
Grey cells are combinations that do not occur in nature, or that fall outside the range the regression was fitted to. Charts published by weather agencies leave the same cells blank rather than printing a number that means nothing.
| Heat index | Band | What agencies associate with it |
|---|---|---|
| Below 27°C | No elevated risk | Heat index does not apply below this temperature |
| 27–32°C | Caution | Fatigue possible with prolonged exposure or activity |
| 32–39°C | Extreme caution | Heat cramps and heat exhaustion possible |
| 39–52°C | Danger | Heat exhaustion likely; heat stroke possible with prolonged exposure |
| Above 52°C | Extreme danger | Heat stroke highly likely |
If you compare this result against the "feels like" figure in the BoM app, they will usually disagree — sometimes substantially. That is not an error in either. Australia does not use the heat index.
| Heat index (US) | Apparent temperature (Australia) | |
|---|---|---|
| Used by | NWS, NOAA, most US media | Bureau of Meteorology |
| Based on | Rothfusz regression of Steadman's earlier model | Steadman 1994, Norms of apparent temperature in Australia |
| Inputs | Temperature, humidity | Temperature, humidity, wind speed |
| Can read below air temperature? | Effectively no | Yes |
| Includes direct sun? | No | No |
The Bureau's approximation is AT = T + 0.33e − 0.70ws − 4.00, where e is water vapour pressure and ws is wind speed in metres per second. Because wind carries a negative coefficient, a breeze pulls the figure down.
| Conditions | Heat index | BoM apparent temp, still air | BoM at 15 km/h | BoM at 25 km/h |
|---|---|---|---|---|
| 30°C, 70% RH | 35.0°C | 35.8°C | 32.9°C | 30.9°C |
| 40°C, 30% RH | 43.1°C | 43.3°C | 40.4°C | 38.4°C |
This is the limitation that matters most and the one most calculators omit entirely.
The heat index was derived for a person in the shade with light wind. The National Weather Service states that full sunshine can raise heat index values by up to 15°F. The Bureau of Meteorology says its apparent temperature model excludes direct sunlight and that midday Australian sun can add about 8°C. Two agencies, two models, the same order of magnitude.
The heat index describes the air, not the person standing in it. At the same reading, individual risk varies enormously — and knowing why a group is at higher risk is more useful than knowing that it is.
| Group | Why risk is higher |
|---|---|
| Babies and young children | Higher surface area relative to body mass, less efficient sweating, and dependent on an adult to notice and act |
| Older adults | Reduced thirst sensation, less efficient sweating, and a higher likelihood of conditions or medications that affect heat tolerance |
| Pregnant women | Higher baseline body temperature and greater cardiovascular load |
| People with heart, lung or kidney conditions | Cooling the body demands extra cardiovascular work, and fluid balance is harder to maintain |
| People on certain medications | Some affect sweating, fluid balance or the body's temperature regulation. Your pharmacist can tell you which |
| Outdoor workers | Sustained exposure, physical exertion adding internal heat, and protective clothing that blocks evaporation |
| Athletes and active people | Exertion generates significant internal heat on top of the environmental load |
| People living alone or socially isolated | Early symptoms may go unnoticed, and there may be nobody to intervene |
| Anyone not acclimatised | Adaptation to heat takes one to two weeks. Early-season heat and returning from leave both carry elevated risk |
These are different conditions with very different urgency. The information below is general and drawn from Australian and US public health guidance; it does not replace medical assessment.
| Heat exhaustion | Heat stroke | |
|---|---|---|
| Severity | Serious | Life-threatening medical emergency |
| Commonly reported signs | Heavy sweating, weakness, dizziness, nausea, headache, cool clammy skin, rapid weak pulse | Confusion or altered behaviour, collapse, seizures, very high body temperature, skin that may be hot and dry |
| First response | Move to a cool place, rest, sip water, loosen clothing, cool the skin | Call 000 immediately, then begin cooling while waiting |
Risk is not evenly distributed. Older adults, babies and young children, pregnant women, people with heart, lung or kidney conditions, people taking certain medications, and anyone not yet acclimatised to hot conditions all carry elevated risk at the same heat index.
Heat is a workplace hazard under Australian work health and safety law, and managing it is the employer's duty. Safe Work Australia publishes guidance on managing the risks of working in heat, and each state and territory regulator supplements it.
Physical activity adds internal heat to the environmental load, so the heat index alone understates risk for anyone exerting themselves.
| Heat index (shade) | General approach for sustained activity |
|---|---|
| Below 27°C | Normal activity; maintain usual hydration |
| 27–32°C | Increase fluid intake, add rest breaks, watch newcomers |
| 32–39°C | Reduce intensity and duration, move sessions to early morning |
| 39–52°C | Postpone or move indoors; strenuous outdoor activity is high risk |
| Above 52°C | Cancel outdoor activity |
Add roughly 8°C before applying this if the activity is in open sun. Many Australian sporting bodies publish their own extreme heat policies, and where one exists for your sport it takes precedence over any general table.
Fluid needs vary with body size, activity, acclimatisation, medication and health conditions, so no page can tell you how much to drink. What it can do is describe the principles.
| Heat index | General approach |
|---|---|
| 27–32°C | Drink regularly rather than waiting for thirst; take breaks in shade |
| 32–39°C | Increase fluid intake and rest frequency; watch for early symptoms in yourself and others |
| 39–52°C | Avoid strenuous outdoor activity where possible; seek cool or air-conditioned spaces |
| Above 52°C | Remain indoors or in air conditioning if you can |
Australian heatwaves are usually forecast several days out, which makes most of the useful work preparation rather than reaction.
| Before the heat arrives | During |
|---|---|
| Check the Bureau of Meteorology forecast and heatwave warnings | Shift outdoor activity to early morning |
| Identify a cool place — air-conditioned room, library, shopping centre | Carry water whenever you go out |
| Confirm fans and air conditioning are working | Wear light, loose, light-coloured clothing |
| Ask your pharmacist whether any medication affects heat tolerance | Stay in shade where you can; add a hat and sunscreen |
| Arrange to check on older or isolated neighbours and relatives | Check on those people, by phone or in person |
| Reschedule strenuous outdoor work or sport in advance | Close blinds and curtains on sun-facing windows during the day |
| Plan pet care — shade, water, and never in a vehicle | Never leave children, older people or pets in a parked car, even briefly |
Australian heatwave severity is assessed using both daytime and overnight temperatures, because the body recovers overnight. A forecast run of warm nights is a stronger warning sign than a single hot afternoon.
What is the heat index?
The heat index is a "feels like" temperature that combines air temperature and relative humidity. It estimates the dry-air temperature that would place the same load on your body as the current hot, humid conditions. High humidity slows sweat evaporation, which is how the body sheds heat, so humid air feels hotter than the thermometer suggests.
How is the heat index calculated?
It uses the Rothfusz regression developed for the US National Weather Service — a nine-term equation in air temperature and relative humidity, defined in Fahrenheit. A simpler equation is computed first and averaged with the air temperature; the full regression is applied only if that result reaches 80°F. Two further corrections adjust for very dry and very humid conditions.
Does humidity really make it feel hotter?
Yes, and the effect is large. Your body cools mainly by evaporating sweat, and evaporation slows as the air becomes more saturated. At 35°C, moving from 20% to 80% humidity raises the heat index from about 33°C to about 56°C — the same thermometer reading, a completely different physiological load.
What heat index is dangerous?
Under the current National Weather Service classification, 39–52°C is the "Danger" band, where heat exhaustion is likely and heat stroke becomes possible with prolonged exposure. Above 52°C is "Extreme danger". These assume shade — add roughly 8°C for direct sun — and individual risk varies considerably with age, health and acclimatisation.
Why does the Bureau of Meteorology show a different "feels like" temperature?
Because Australia does not use the heat index. The Bureau uses Steadman apparent temperature, which includes wind speed as well as temperature and humidity. Wind lowers the figure, so on a breezy day the Bureau's number can fall below the air temperature — something the heat index cannot show. For official Australian conditions, use the Bureau's figure.
Does the heat index account for direct sunlight?
No. It was derived for shaded conditions with light wind. The National Weather Service notes that full sunshine can raise heat index values by up to 15°F, and the Bureau of Meteorology says midday Australian sun can add about 8°C to apparent temperature. If you are in the open, add roughly that much before judging risk.
What is the difference between heat index and air temperature?
Air temperature is a physical measurement of the air, taken in shade and shelter. Heat index is a derived estimate of how that air, at a given humidity, loads the human body. When humidity is high the heat index sits well above air temperature; in very dry air it can sit slightly below.
What is the difference between heat exhaustion and heat stroke?
Heat exhaustion typically involves heavy sweating, weakness, dizziness, nausea and cool clammy skin, and usually improves with rest in a cool place and fluids. Heat stroke is a life-threatening emergency involving confusion or altered behaviour, collapse, seizures and very high body temperature. Heat stroke requires an immediate call to 000 — do not wait to see whether it improves.
How accurate is the heat index?
It is a population-level estimate, not a personal measurement. The underlying model assumes a particular body size, clothing level and activity, and it excludes sun and strong wind entirely. It is well suited to comparing conditions and planning activity, and poorly suited to judging what any specific individual can tolerate.
What is wet bulb globe temperature and how does it differ?
Wet Bulb Globe Temperature (WBGT) is the measure used in occupational and sports heat standards. It incorporates radiant heat from the sun and air movement in addition to humidity, so it captures conditions the heat index leaves out. Where a workplace or sporting body specifies WBGT thresholds, those take precedence over any heat index figure.
Who is most at risk in extreme heat?
Older adults, babies and young children, pregnant women, people with heart, lung or kidney conditions, people taking medications that affect fluid balance or sweating, outdoor workers, athletes, and anyone not yet acclimatised to hot conditions. Social isolation is also a significant risk factor, which is why checking on people living alone during a heatwave matters.
Why do hot nights matter during a heatwave?
The body recovers from heat load overnight when temperatures fall. When overnight minimums stay high for several consecutive nights, that recovery does not happen and cumulative strain builds. This is why heatwave severity is assessed using both daytime and overnight temperatures rather than the daily maximum alone.
The heat index is a good tool for one job: deciding whether today is a day to move the run to dawn, reschedule the outdoor work, or check on someone who lives alone. It is a description of the air, not a verdict on any individual, and it assumes shade you may not be standing in.
For official Australian conditions, use the Bureau of Meteorology's forecast. For the humidity side of the picture, our dew point calculator shows how much moisture the air is actually holding, and the wind chill calculator covers the opposite end of the year.