Use this calculator to estimate how hot it feels in the shade from air temperature plus either relative humidity or dew point.
How to use our Heat Index Calculator
- Choose Humidity input as either Relative humidity or Dew point, then choose your Temperature unit.
- Enter Air temperature, then fill in either Relative humidity (%) or Dew point using the same temperature unit you selected.
- Pick Sun exposure. Use Shade for the official heat index, or Direct sun estimate to see a possible hotter range.
- Click Calculate, then read Heat index first, Heat caution level second, and Converted humidity value used to check the matching dew point or relative humidity.
- Sanity-check the result with Trust note. If conditions are cooler than about 80 deg F, very dry, or outside the usual NWS range, treat the number as a rough guide instead of a precise answer.
Definitions
Heat index: How hot it feels in the shade when air temperature and humidity are combined.
Relative humidity (%): The percent of water vapor in the air compared with the most the air could hold at that temperature.
Dew point: The temperature where air would start to form dew. A higher dew point usually means stickier air.
Heat caution level: A plain-language risk band based on the heat index. Higher levels mean more chance of heat stress, especially with longer time outside or hard activity.
Direct sun estimate: A possible hotter feel-like range for full sun. It is not the official heat index value.
Trust note: A short message that tells you whether the result is in the usual heat-index range and whether shade-only rules matter.
Common mistakes and quick fixes
Mistake: Entering both Relative humidity (%) and Dew point mentally and expecting both to be used at once.
Fix: The calculator uses only the field that matches Humidity input . Choose the right mode, then fill in that one humidity field.
Mistake: Typing 0, 105, or a blank value in Relative humidity (%) .
Fix: Enter a number greater than 0 and up to 100 for Relative humidity (%) .
Mistake: Using different units for Air temperature and Dew point .
Fix: Set Temperature unit first, then enter both temperatures in that same unit.
Mistake: Entering a Dew point higher than Air temperature .
Fix: Lower Dew point so it is not above Air temperature for this calculator.
Mistake: Reading Direct sun estimate as the official Heat index .
Fix: Use Heat index for the standard shade value, and use Direct sun estimate only as a possible hotter range in sun.
Mistake: Ignoring the Trust note when the air is not very hot or humid.
Fix: Read Trust note every time. It tells you when the formula is outside its usual range and should be treated carefully.
Limitations & Key Assumptions / Boundary Conditions
- The standard heat index is meant for shady places with light wind, so strong sun or unusual wind can make real conditions feel different.
- The calculator still computes outside the usual NWS heat-index range, but the result may be less reliable or less meaningful there.
- For cooler conditions, especially below about 80 deg F, the heat index is often close to the air temperature and may not add much useful information.
- If you use Dew point, the calculator first converts it to relative humidity with a Magnus-type approximation, so small differences from other weather tools are normal.
- Dew point cannot be higher than Air temperature here because that would be physically inconsistent for this use.
- Direct sun estimate is shown as a range because full sun can add up to 15 deg F, not one exact extra amount.
Methodology
How the calculator works
The calculator always computes the heat index in Fahrenheit first, then converts the final answer back to Celsius if needed. If you enter Relative humidity (%), that value goes straight into the heat-index formula. If you enter Dew point, the calculator first converts dew point and air temperature into relative humidity, then uses that relative humidity in the heat-index formula.
Core formulas
HI_F = -42.379 + 2.04901523*T_F + 10.14333127*RH - 0.22475541*T_F*RH - 0.00683783*T_F^2 - 0.05481717*RH^2 + 0.00122874*T_F^2*RH + 0.00085282*T_F*RH^2 - 0.00000199*T_F^2*RH^2
adj_low = ((13 - RH) / 4) * sqrt((17 - abs(T_F - 95)) / 17)
If RH < 13 and 80 <= T_F <= 112, then HI_F = HI_F - adj_low
adj_high = ((RH - 85) / 10) * ((87 - T_F) / 5)
If RH > 85 and 80 <= T_F <= 87, then HI_F = HI_F + adj_high
T_F = T_C * 9 / 5 + 32
T_C = (T_F - 32) * 5 / 9
RH = 100 * exp((17.625*Td_C)/(243.04 + Td_C) - (17.625*T_C)/(243.04 + T_C))
gamma = ln(RH/100) + (17.625*T_C)/(243.04 + T_C)
Td_C = (243.04*gamma)/(17.625 - gamma)
sun_low = HI_F
sun_high = HI_F + 15
What each result means
Heat index is the main answer and represents how hot it feels in the shade. Heat caution level places that number into a simple risk band. Converted humidity value used helps you check the humidity math by showing the matching relative humidity or dew point. If you choose Direct sun estimate, the calculator shows a range from the shade heat index up to 15 deg F higher, following NOAA guidance that direct sun can make it feel hotter [1].
Mini example
If Air temperature is 90 deg F and Relative humidity (%) is 60, the Rothfusz equation gives a Heat index of about 99.7 deg F. That means the air is 90 deg F, but it can feel close to 100 deg F in the shade because of humidity. If you switch Sun exposure to direct sun, the estimate range becomes about 99.7 to 114.7 deg F.
Assumptions used in the result
The heat index is mainly intended for warm, humid conditions, so cooler or very unusual inputs can make the number less useful. The dew point conversion uses a standard Magnus-type approximation, which is close but not perfect. If the calculation lands outside the usual NWS guidance range, the calculator still shows a result but adds a Trust note so you know to treat it as a rough estimate rather than a precise field measurement [2].