Heat Index Calculator
Adjust the inputs below. Results update as you type.
How it works
Heat index applies when temperature ≥ 80°F and humidity ≥ 40%. Uses the Rothfusz regression with humidity adjustments. At 95°F and 70% RH, it feels like 124°F. Heat index above 103°F: heat exhaustion possible. Above 125°F: heat stroke danger. Shade, low wind, and sun angle affect actual felt temperature—heat index assumes shaded, calm conditions.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
The heat index combines temperature and humidity into a feels-like value using the NWS Rothfusz regression, valid above about 80°F and 40% humidity.
Worked example
At 90°F with 70% humidity, the heat index is about 106°F — high enough to risk heat exhaustion with exertion.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
What is the heat index?+
A 'feels-like' temperature that accounts for humidity. High humidity slows sweat evaporation, so the body cools less and feels hotter.
Why does humidity make heat worse?+
Sweating cools you as it evaporates. In humid air, evaporation slows, so heat builds up and the perceived temperature rises.
When is the heat index dangerous?+
Above roughly 103°F, heat cramps, exhaustion, and heatstroke become likely with activity. Hydrate and seek shade or air conditioning.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.