SamCalculator

Heat Index Calculator

Calculate how hot it actually feels based on air temperature, humidity, and dew point using advanced meteorological heat index formulas.

Relative Humidity Method

The standard NWS approach — enter the air temperature and the relative humidity, and the calculator returns the heat index, feels-like temperature, and heat-stress category.

Quick scenarios

°C
0 – 100%
%

What Is the Heat Index?

The heat index — often called the “feels-like” or apparent temperature — is a measure of how hot it really feels to the human body when relative humidity is combined with the air temperature. On a humid day your body struggles to cool itself: sweat is the body’s air-conditioning, and it only works when perspiration can evaporate from the skin. When the air is already saturated with moisture, sweat lingers, evaporation slows, and your core temperature creeps upward. The heat index captures that physiological reality in a single number, which is why a 32 °C (90 °F) day at 70% humidity can feel like a punishing 41 °C (106 °F).

This calculator uses the same Rothfusz regression that the U.S. National Weather Service publishes, lets you work from either relative humidity or the dew point, and layers on apparent-temperature, heat-stress classification, and personalised safety guidance. Pair it with our calorie calculator for hot-weather fuelling, the temperature converter for unit conversions, or the target heart rate calculator when planning exercise in the heat.

How the Heat Index Works

Humidity blocks the body's cooling

Your body sheds heat mainly by evaporating sweat. The more water vapour already in the air, the slower that evaporation, and the hotter you feel. That is why humid heat is far more dangerous than the same temperature in dry desert air.

What is dew point?

The dew point is the temperature to which air must cool for moisture to condense. It is an absolute measure of how much water vapour the air holds, so unlike relative humidity it doesn't swing with temperature. A dew point above 21 °C (70 °F) feels oppressive; above 24 °C (75 °F) is sweltering.

Heat index vs air temperature

Air temperature is what a thermometer reads in the shade; heat index is what your body experiences once humidity is factored in. When humidity is high the heat index sits well above the air temperature; in very dry air it can actually fall slightly below it.

Why full sun makes it worse

The official heat index is measured in the shade. Standing in direct sunlight can add up to 8 °C (15 °F) to the value, which is why the NWS warns that sun exposure dramatically raises real-world heat stress.

How to Use This Heat Index Calculator

  1. 1

    Choose your input method

    Use the Relative Humidity tab if you know the humidity percentage (most weather apps show it). Use the Dew Point tab if your source reports dew point instead — the calculator derives humidity automatically.

  2. 2

    Enter temperature and moisture

    Type the air temperature and pick °C or °F, then enter relative humidity (0–100%) or the dew point. Switching units never loses your value — the maths runs in both systems at once.

  3. 3

    Add advanced conditions (optional)

    Open Advanced conditions to factor in sun exposure, wind speed, UV index, activity level, and clothing. These refine the apparent temperature, full-sun heat index, risk score, and the personalised safety insights.

  4. 4

    Compare two scenarios

    Turn on comparison mode to pit your current conditions against a custom temperature and humidity — a quick way to see how a forecast change shifts the heat-stress category.

  5. 5

    Read the results and stay safe

    Press Calculate to reveal the feels-like temperature, heat-stress category, severity meter, analytics dashboard, and tailored safety recommendations, then copy, share, or print the report.

Heat Stress Levels Explained

Safe

Below 80 °F · Below 27 °C

Conditions are comfortable for most people. Normal hydration is enough; no special heat precautions are needed.

Caution

80 – 90 °F · 27 – 32 °C

Drink water regularly and take breaks during sustained outdoor activity. Fatigue is possible with prolonged exposure.

Extreme Caution

90 – 103 °F · 32 – 39 °C

Heat cramps and heat exhaustion become likely with continued activity. Limit strenuous effort, rest in shade, and hydrate often.

Danger

103 – 124 °F · 39 – 51 °C

Heat cramps and exhaustion are likely, and heat stroke is possible with prolonged exposure. Avoid strenuous outdoor activity and stay in cool, shaded, or air-conditioned spaces.

Extreme Danger

125 °F and above · 52 °C and above

Heat stroke is highly likely. Stay indoors with cooling, avoid all non-essential outdoor exertion, and check on vulnerable people frequently.

Staying Safe in Extreme Heat

  • ✓Hydrate before you're thirsty. Thirst lags behind dehydration. Drink water steadily through hot days and add electrolytes when sweating heavily for long periods.
  • ✓Time outdoor work and exercise. Shift demanding activity to early morning or evening and avoid the 11 am–4 pm peak when the heat index and UV are highest.
  • ✓Seek shade and airflow. Shade can cut the effective heat index by several degrees, and a breeze speeds evaporative cooling — though above ~37 °C, wind can add heat rather than remove it.
  • ✓Dress for the heat. Light-coloured, loose, breathable fabrics let sweat evaporate. Heavy clothing and PPE trap heat and raise the effective load on the body.
  • ✓Know the warning signs. Learn the symptoms of heat exhaustion and heat stroke, and act at the first sign — move to a cool place, hydrate, and cool the skin.
  • ✓Protect the vulnerable. Check on children, older adults, outdoor workers, athletes, and pets often, and never leave anyone in a parked vehicle, even briefly.

Why the Heat Index Matters

Extreme heat is one of the deadliest weather hazards, quietly causing more deaths in many years than hurricanes, floods, or tornadoes. Because the danger comes from the combination of heat and humidity rather than temperature alone, the air temperature on its own routinely understates the real risk. A 35 °C (95 °F) afternoon feels manageable in the dry mountains but becomes life-threatening in a humid coastal city, where the heat index can soar past 46 °C (115 °F).

For athletes, outdoor workers, parents, caregivers, travellers, and anyone planning summer activity, the heat index turns an abstract forecast into an actionable risk level. Knowing whether conditions sit in the Caution, Danger, or Extreme Danger band tells you when to hydrate harder, when to move activity indoors, and when staying out is simply unsafe.

Heat Illness: Symptoms & Warning Signs

Heat exhaustion symptoms

Heavy sweating, cool and clammy skin, a fast weak pulse, nausea, muscle cramps, tiredness, dizziness, and headache. Move to a cool place, loosen clothing, sip water, and apply cool cloths. If symptoms last more than an hour or worsen, seek medical help.

Heat stroke warning signs

A body temperature above 40 °C (104 °F), hot red skin that may be dry or sweating, confusion, slurred speech, a throbbing headache, a rapid strong pulse, and possible loss of consciousness. Heat stroke is a medical emergency — call emergency services and cool the person immediately.

Dehydration

Dark urine, dry mouth, dizziness, and reduced sweating. Dehydration accelerates every other heat illness and is easy to miss in dry, breezy conditions where sweat evaporates before you notice it.

Heat cramps

Painful muscle spasms — usually in the legs, arms, or abdomen — caused by salt and fluid loss during heavy sweating. They are often the earliest sign of heat strain and a signal to rest, cool down, and rehydrate with electrolytes.

The Heat Index Formula Explained

The heat index comes from a multiple-regression analysis by Lans Rothfusz (1990) of Robert Steadman’s 1979 human-comfort model. T is air temperature in °F and R is relative humidity in percent.

Rothfusz regression

HI = −42.379 + 2.049 T + 10.143 R − 0.2248 T·R − …

The full nine-term equation used when the heat index is roughly 80 °F or higher, with low- and high-humidity adjustments applied at the edges.

Steadman fallback

HI = 0.5 [T + 61 + 1.2(T−68) + 0.094R]

A simpler formula the NWS uses when conditions produce a heat index below about 80 °F, where the full regression is not appropriate.

Dew point ⇄ humidity

RH = 100 · e^(aTd/(b+Td)) ÷ e^(aT/(b+T))

The Magnus formula (a = 17.625, b = 243.04 °C) converts between dew point and relative humidity so either input drives the same result.

Heat Safety Guidelines

🏃

Heat & exercise

Exertion can raise core temperature by 1 °C every 5 minutes if cooling fails. Reduce intensity as the heat index climbs, schedule sessions for cooler hours, pre-hydrate, and stop at the first sign of dizziness, cramping, or nausea.

👷

Heat safety for workers

OSHA-style guidance pairs work/rest cycles with hydration and shaded recovery areas. Above the Danger band, rotate strenuous tasks, use a buddy system to spot symptoms, and acclimatise new or returning workers gradually over a week or two.

🏟️

Heat safety for athletes

Many sports bodies use wet-bulb globe temperature alongside the heat index to set practice rules. As risk rises, add mandatory water breaks, lighten gear where allowed, and be ready to delay, shorten, or relocate events.

🌞

Safe outdoor activity

In the Caution band, normal activity with regular water is fine. By Extreme Caution, shift demanding tasks to morning or evening. In Danger and Extreme Danger, limit or postpone outdoor activity and stay in cool, shaded, or air-conditioned spaces.

💧

Hydration recommendations

A common guideline is roughly 200–300 ml of water every 15–20 minutes during sustained activity in the heat. Add electrolytes for prolonged heavy sweating, and avoid alcohol and excess caffeine, which worsen fluid loss.

🌡️

Heat wave survival

During multi-day heat waves, use air conditioning or public cooling centres, block direct sun with shades, avoid using the oven, take cool showers, and check on elderly neighbours, young children, and pets at least twice a day.

Common Heat Index Mistakes

  1. 1

    Reading air temperature alone

    A 33 °C reading at 30% humidity and at 80% humidity feel completely different. Always factor humidity in — that is the entire point of the heat index.

  2. 2

    Forgetting the sun

    The official heat index is a shade value. In direct sun, add up to 8 °C (15 °F). A 'Caution' shade reading can be a 'Danger' value in full sunlight.

  3. 3

    Assuming wind always cools

    A breeze helps below about 37 °C (98 °F). When the air is hotter than your skin, wind transfers heat to you instead — like a convection oven.

  4. 4

    Ignoring acclimatisation

    People used to cool climates feel heat stress sooner than locals. The same heat index is riskier for the unacclimatised, the very young, the elderly, and the unwell.

  5. 5

    Trusting indoor comfort

    Heat index applies indoors too. Homes without air conditioning can hold dangerous heat overnight during heat waves, raising the risk for sleeping residents.

  6. 6

    Overlooking the dew point

    Relative humidity can mislead because it changes with temperature. A high dew point is the clearest sign that humid heat will feel oppressive all day.

Built for athletes, outdoor workers, coaches, parents, caregivers, and anyone facing summer heat.

Heat index uses the U.S. National Weather Service Rothfusz regression and category chart; dew-point conversion uses the Magnus–Tetens approximation; apparent temperature uses the Australian Bureau of Meteorology formula. See our methodology and editorial policy. This tool is for educational use only and is not a substitute for official heat warnings or medical advice.

Frequently Asked Questions

The heat index is the 'feels-like' temperature — how hot the air actually feels to your body once relative humidity is combined with the measured air temperature. Because humidity slows the evaporation of sweat (your body's main cooling mechanism), a humid 32 °C (90 °F) day can feel like 41 °C (106 °F). It is the warm-weather counterpart to wind chill.

This calculator uses the U.S. National Weather Service Rothfusz regression — a nine-term equation in air temperature and relative humidity — together with the standard low- and high-humidity adjustments. Below about 80 °F it falls back to Steadman's simpler formula, exactly as the NWS does. If you enter a dew point instead of humidity, the Magnus formula converts it to relative humidity first.

Risk rises in bands: 80–90 °F (27–32 °C) is Caution, 90–103 °F (32–39 °C) is Extreme Caution where heat cramps and exhaustion become possible, 103–124 °F (39–51 °C) is Danger where heat exhaustion is likely and heat stroke is possible, and 125 °F (52 °C) or above is Extreme Danger where heat stroke is highly likely. Direct sun can push any of these one band higher.

The dew point is the temperature to which air must be cooled for water vapour to start condensing. Unlike relative humidity, it doesn't change as the air warms or cools during the day, so it is an absolute measure of moisture. A dew point above 21 °C (70 °F) feels humid and uncomfortable, and above 24 °C (75 °F) feels oppressive.

Humidity doesn't change the air temperature, but it changes how that temperature feels. Your body cools itself by evaporating sweat; when the air is already moist, sweat evaporates slowly, cooling is impaired, and you feel hotter. The higher the humidity, the larger the gap between the thermometer reading and the heat index.

Heat exhaustion develops when the body loses too much water and salt through heavy sweating and can no longer cool itself effectively, usually during prolonged exposure or exertion in hot, humid conditions. Symptoms include heavy sweating, cool clammy skin, weakness, dizziness, nausea, and headache. Left untreated it can progress to heat stroke.

Air temperature is what a thermometer measures in the shade and ignores humidity. The heat index combines that temperature with relative humidity to estimate what the conditions actually feel like to a human body. When humidity is high, the heat index is well above the air temperature; in very dry air it can sit slightly below it.

In hot weather, yes — 'feels-like temperature' and 'apparent temperature' usually refer to the heat index. In cold weather, the feels-like value instead comes from the wind chill formula. Some agencies use a broader apparent-temperature model that also accounts for wind and solar radiation, which this calculator reports separately.

Drink water before you feel thirsty, stay in shade or air conditioning during the hottest hours, wear light loose clothing, avoid strenuous outdoor activity at peak heat, and never leave children or pets in a parked vehicle. Learn the signs of heat exhaustion and heat stroke, check on vulnerable neighbours, and follow official heat warnings.

Because sweat is the body's air-conditioning, and it only cools you when it evaporates. In humid air the surrounding moisture slows evaporation dramatically, so sweat drips instead of cooling, your core temperature rises, and the same temperature feels much hotter and more exhausting than it would in dry air.

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