The heat index,
read like an instrument.
Live heat index from temperature and humidity, the NWS risk bands, and the 80°F / 90°F trigger points published in OSHA’s proposed heat rule. Works fully offline with manual entry.
One number, read correctly
- Heat index from live weather or manual entry
- NWS risk band, plain text — never color alone
- 80°F / 90°F trigger markers, labeled “OSHA-proposed”
The estimate, honestly labeled
- Computed from temperature and humidity
- Always shown as an estimate, never a measurement
- Not a substitute for an on-site WBGT meter
Estimate only — not a substitute for an on-site WBGT meter.
A private record, on-device
- Timestamped readings with an optional note
- Export to CSV whenever you need it
- Stored on your device — never on our servers
See for yourself
What does your heat index read?
Move the sliders — this is the same NWS formula the app runs offline.
The NWS heat-index formula, computed live — the same math the app runs offline from manual entry.
One purchase. Not a subscription.
The free gauge is complete on its own. Pro unlocks exactly four things:
Guides
Read the instrument, not just the number
Questions, answered
Is Swelter affiliated with OSHA, NIOSH, CDC, or the National Weather Service?+
No. Swelter is an independently developed app. It is not affiliated with, endorsed by, or connected to OSHA, NIOSH, CDC, or the National Weather Service. The heat index formula and the 80°F/90°F markers are based on their published public information.
Is the WBGT number a real measurement?+
No — it’s an estimate. A true WBGT reading needs a physical black-globe thermometer in direct sun and wind. Swelter’s Pro WBGT figure is a software estimate from available weather data, always labeled as an estimate, and it is not a substitute for an on-site WBGT meter.
Is Swelter a medical device?+
No. Swelter is an informational tool, not a medical device. It computes a public, published heat-index formula and shows the NWS risk bands — it does not diagnose, treat, or guarantee anyone’s safety.
Does Swelter work without a network connection?+
Yes. The free core works fully offline: type in a temperature and relative humidity and Swelter computes the heat index, risk band, and 80°F/90°F markers with zero network access. An optional weather lookup can fill temperature and humidity in automatically when you have a connection.
Not affiliated with, endorsed by, or connected to OSHA, NIOSH, CDC, or the National Weather Service. Threshold values and guidance are based on their published public information. Swelter is an informational tool, not a medical device. It does not diagnose or prevent heat illness.