Skip to main content

Find your next tool

DashboardAndroid app
Language
Appearance

Everyday Planning Calculators

Heat Index Calculator — NWS Feels-Like Temperature

Calculate feels-like temperature with the full NWS heat-index decision procedure: simple screening formula, Rothfusz regression when applicable, and the official low/high-humidity adjustments. Results show °C and °F, method branch, adjustment and heat-category context.

Live calculationWorking shown

Editable live calculator

Change any input — the result updates immediately

No hidden assumptions. Inputs, units and the working stay visible so you can check the calculation.

Calculated result

37.1 °C / 98.7 °F

NWS method branch: NWS Rothfusz regression

Heat category screen: Extreme caution

Difference from air temperature: +9.1 °F (+5.1 °C)

No Rothfusz humidity adjustment applies.

Show the working
  1. 1. Convert/normalize air temperature: 32 °C = 89.6 °F; RH = 60%.
  2. 2. First NWS screen: 0.5 × [89.6 + 61 + (89.6 − 68) × 1.2 + 60 × 0.094] = 91.08 °F; average with air temperature = 90.34 °F.
  3. 3. Because the averaged screen is at least 80 °F, apply Rothfusz: HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541TRH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² − 0.00000199T²RH².
  4. 4. No Rothfusz humidity adjustment applies.
  5. 5. Final heat index = 98.73 °F = 37.07 °C.

NWS heat index assumes shade and light wind. Direct sunshine can make conditions feel substantially hotter; use official weather alerts and WBGT guidance when sun, exertion or occupational heat stress matters.

More actions

Shared links reopen these inputs. Recent and saved calculations stay in this browser unless you clear site data.

The calculation, without hidden assumptions

Heat index combines shade air temperature and relative humidity to estimate how hot conditions feel. MAXScanner follows the National Weather Service calculation sequence instead of applying the Rothfusz polynomial blindly: it runs the simple Steadman-consistent screen first, switches to the regression only when that screen reaches 80°F, then checks the NWS low- or high-humidity adjustment rules. The live working shows which branch your inputs actually used.

How to use this calculator

1

Choose Celsius or Fahrenheit, then enter the shade air temperature.

2

Enter relative humidity from 0% to 100%.

3

Read the heat index in both units, the NWS method branch and the difference from actual air temperature.

4

Open the working to verify the simple screening calculation, Rothfusz regression decision and any humidity adjustment applied to your exact inputs.

Where people use it

  • •Compare how humidity changes apparent heat at the same air temperature
  • •Check whether the NWS simple or Rothfusz branch applies to a measured condition
  • •Convert a heat-index result between °C and °F while keeping the original weather inputs visible
  • •Screen shade/light-wind conditions before consulting official forecasts or a sun-aware heat-stress measure such as WBGT

Worked example — 90°F and 70% RH

With T = 90°F and RH = 70%, the initial NWS screen exceeds 80°F, so the Rothfusz regression is used. No low- or high-humidity adjustment applies at these inputs. The calculated heat index is about 105.9°F (41.1°C), substantially hotter than the measured air temperature.

What the result does not assume

  • •The National Weather Service heat index is defined for shady, light-wind conditions. Direct sunshine can make conditions feel much hotter; NWS notes an increase of up to about 15°F is possible.
  • •The Rothfusz regression is not used automatically for every input. MAXScanner first applies the NWS simple-formula screen and uses the regression only when the averaged screen reaches at least 80°F.
  • •The regression has special low-humidity and high-humidity adjustments in defined temperature ranges; the live result reports whether one was applied.
  • •Heat index is not WBGT and does not model solar load, wind, cloud cover, clothing, workload, acclimatization or individual medical vulnerability. Use official alerts and appropriate occupational/sports heat guidance for safety decisions.

Frequently asked questions

Why does this calculator sometimes use a simple formula instead of the Rothfusz regression?+

That is the NWS procedure. A simpler Steadman-consistent expression is evaluated first and averaged with the air temperature. If that value is below 80°F, the Rothfusz regression is not appropriate and the simpler result is retained.

What humidity adjustments does the NWS heat-index method use?+

For very dry air below 13% RH between 80°F and 112°F, an adjustment is subtracted. For RH above 85% between 80°F and 87°F, a different adjustment is added. The calculator checks these conditions automatically.

Does heat index include direct sunlight?+

No. The standard NWS heat index assumes shade and light wind. Direct sunshine can increase apparent conditions substantially, so outdoor work or exercise in sun may require WBGT or other official heat-stress guidance.

Can I enter Celsius?+

Yes. Celsius is converted to Fahrenheit for the NWS equations, then the result is displayed in both Fahrenheit and Celsius.

Semantic next steps

Continue the calculation

These links move to a different input, formula or project stage rather than a keyword variation of this page.

View the full collection

Dew Point Calculator — °C/°F, RH & Reverse Solve

Calculate dew point, relative humidity or air temperature with the Alduchov–Eskridge Magnus approximation. Work in °C or °F and inspect dew-point depression, vapor pressure, reverse solving and humidity scenarios.

Open

Wind Chill Calculator — NWS Formula, °C/°F & Wind Units

Calculate NWS wind chill from air temperature and wind speed with °C/°F plus mph, km/h, m/s or knots. The calculator enforces the official weather-use range and shows substituted working and wind-speed scenarios.

Open

Density Calculator — Solve Density, Mass or Volume + Units

Solve density, mass or volume from any two known values with SI, metric and US customary units. See equivalent density units, specific gravity and substituted working.

Open

Molarity Calculator — Concentration, Moles, Mass or Volume

Solve amount concentration (molarity), moles, solute mass or final solution volume. Supports M, mM and µM plus mass-based calculations using molar mass.

Open

Running Pace Calculator — Pace, Time, Distance & Race Splits

Solve running pace, finish time or distance with km/mile units. See min/km, min/mile, km/h, mph and constant-pace splits for common race distances.

Open

Kinetic Energy Calculator — Solve Energy, Mass or Speed + Units

Solve classical translational kinetic energy, mass or speed from any two values. Convert mass, speed and energy units and see velocity-squared sensitivity.

Open

Momentum Calculator — Solve p, Mass or Velocity + Units

Solve linear momentum, mass or signed velocity using p = mv. Convert common mass, velocity and momentum units and inspect impulse and kinetic-energy context.

Open

Wavelength & Frequency Calculator — Solve λ, f or Wave Speed

Solve wavelength, frequency or propagation speed with unit conversion and optional medium presets for sound/light examples. Includes period and wave-number diagnostics.

Open

Free publisher widget

Put this calculator on your website

Give readers the live calculator without rebuilding the math. Copy the responsive embed in one click, or customize width, height, border and corner radius before publishing. The branded MAXScanner attribution stays visible.

Preview

Calculator embed HTML

Paste this code into your page. Focus the code box to select it manually.

Keep the result connected to the real job

MAXScanner keeps the formula and assumptions beside the result so you can verify the number before using it in a drawing, estimate, specification, worksheet or document. Where a supplier, manufacturer, drawing or applicable standard owns a requirement, that source remains authoritative.

Saved calculationsOpen My Tools