Method
The NWS simple estimate screens for the full Rothfusz temperature-humidity regression and its low/high-humidity adjustments.
Everyday worksheet
Heat Index Calculator
Estimate apparent heat with the National Weather Service simple method and Rothfusz regression branch.
The calculation runs locally. Results include the intermediate values, unit conversions and assumptions needed to audit the answer.
Method, assumptions and practical use
How the Heat Index Calculator works
The NWS simple estimate screens for the full Rothfusz temperature-humidity regression and its low/high-humidity adjustments. Inputs stay in this browser, and the result table exposes conversions and assumptions for independent checking.
Interpret the result
Heat index combines shade temperature and humidity; direct sun, exertion and health can increase risk.
Check the assumptions
Official heat-index values assume shade and light wind; direct sunlight can substantially increase apparent heat.
Local calculation
Everything runs in this browser. Inputs are not sent to an external calculator.
Rounding
Displayed values may be rounded for readability while calculations use full floating-point precision unless the tool explicitly uses exact integers.
Scope
Official heat-index values assume shade and light wind; direct sunlight can substantially increase apparent heat.
Simple by design
Three steps to a clearer number.
Enter your details
Use realistic values in each field. You can change them anytime.
Calculate instantly
The formula runs locally, so there is no account or waiting time.
Use the result
Treat the answer as a practical estimate for your next decision.