A heat index calculator converts temperature and humidity into what the body actually fights — — then maps it to OSHA-style risk bands with water, rest and shade actions. Cooling equipment lives on cooling gear and hydration-adjacent PPE on the breathable hi-vis shelf.
How does the heat index calculator band the risk?
Short answer: enter temperature and relative humidity; the tool computes the heat index and maps it to the caution-through-extreme-danger bands with the work/rest and hydration actions each band expects.
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Heat illness is a leading cause of weather-related worker deaths. Enter the temperature and humidity to get the heat index (what it feels like) and the corresponding OSHA risk level with protective-measure guidance.
OSHA heat-index risk levels
| Heat index | Risk | Protective measures |
|---|---|---|
| Below 91°F | Lower (Caution) | Water, rest, shade; acclimatize new workers |
| 91–103°F | Moderate | Active rest breaks, hydration schedule, buddy system |
| 103–115°F | High | Frequent breaks in shade/AC, adjust work, monitor closely |
| Above 115°F | Very High to Extreme | Reschedule non-essential work; strict work/rest cycles |
Gear for hot, high-visibility work
FAQ
Does the heat index account for sun and exertion?
No. The heat index assumes shade and light wind. Direct sun can add up to ~15°F, and heavy exertion or PPE raises effective heat load further — treat the result as a minimum.
What is acclimatization?
Gradually building heat tolerance over 1–2 weeks. New and returning workers are at highest risk; OSHA recommends ramping workload (the “20% rule”) for the first days.
Estimate using the NWS heat-index (Rothfusz) equation; risk bands per the OSHA-NIOSH Heat Safety Tool. Add ~15°F for full-sun exposure. Not a substitute for a heat-illness prevention program, WBGT monitoring, or medical guidance.
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Built and reviewed by Steven Eaton, WC Safety. We curate and review industrial PPE against ANSI, NIOSH and OSHA standards.
Our methodology
The calculator implements the NWS heat index regression and maps results to the risk bands used in OSHA heat guidance, with the caveat that direct sun adds up to 15°F the index does not see. Authoritative duties live at OSHA regulations and exposure science at NIOSH. We hold no safety certification, and this tool is not a substitute for professional judgment.
Why trust this page
WC Safety is an independent review site with no products of its own — nothing here is stocked, warehoused or shipped by us. Purchase links go to Amazon and earn an affiliate commission at no extra cost to you. This free tool is informational — it supports, and never replaces, a written program, a hazard assessment, or evaluation by a qualified safety professional.
Frequently Asked Questions
Heat index vs WBGT — which is better?
WBGT is the industrial-hygiene standard (sun, wind, radiant load); heat index is the accessible screen. Serious programs graduate to WBGT monitoring. The full comparison - what each measures and when each applies - is WBGT vs heat index.
What does the danger band require?
Aggressive work/rest cycles, shaded recovery, a quart of water per hour and buddy observation — acclimatized crews included.
How much does direct sun change it?
Up to 15°F on top of the shaded index — treat full-sun work a band worse than the calculator shows.
What is acclimatization?
The 1–2 week physiological ramp new and returning workers need — most heat deaths hit workers in their first days. Start them at reduced load.
What are the first heat exhaustion signs?
Heavy sweat, weakness, nausea, headache — move to shade, cool and hydrate. Confusion or dry skin is heat stroke: 911, cool aggressively.
Does OSHA have a heat standard?
Enforcement runs through the general duty clause plus state rules while federal rulemaking proceeds — the risk bands here mirror OSHA-NIOSH guidance. The current-law detail lives in OSHA workplace heat requirements.
Researched and written by Steven Eaton, editor of WC Safety. Steven holds no safety certification and does not test products; this page compares what manufacturers publish, with the gaps in that record marked. Last reviewed July 2026.