Skip to content
STEELTOETOOLS

Heat Index & Work/Rest Calculator

Enter the air temperature and the relative humidity to get the heat index with the National Weather Service risk category, plus the actions tied to the 80 °F and 90 °F heat index triggers in OSHA's proposed heat standard. Add 15 °F when the crew works in full sun.

°F
Dry-bulb air temperature in the shade, where the crew works.
%
From a weather app for the jobsite, or an on-site hygrometer.
Heat index values are published for shady, light-wind conditions.

Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.

Heat index categories and heat triggers

Heat indexNWS categoryWhat can happen with prolonged exposure or physical activity
80–90 °FCautionFatigue possible
90–103 °FExtreme CautionHeat stroke, heat cramps or heat exhaustion possible
103–124 °FDangerHeat cramps or heat exhaustion likely, heat stroke possible
125 °F and upExtreme DangerHeat stroke highly likely

Categories: National Weather Service heat index chart. They describe the weather, not a compliance threshold.

What OSHA's proposed heat rule would require

There is no final federal heat standard. OSHA published a proposed rule on August 30, 2024; the informal public hearing closed July 2, 2025 and the post-hearing comment period closed October 30, 2025. Until a final rule is issued, heat hazards are cited under the General Duty Clause, Section 5(a)(1) of the OSH Act. The proposed triggers below are the numbers this calculator flags.

TriggerHeat indexProposed requirements
Initial heat trigger80 °FCool drinking water sufficient for 1 quart per employee per hour; break areas with shade or air conditioning; acclimatization for new and returning workers; paid rest breaks when needed; two-way communication
High heat trigger90 °FEverything above, plus a minimum 15-minute paid rest break at least every two hours, and observation for signs and symptoms (buddy system, or a supervisor watching no more than 20 employees)

Walking to the break area and donning or doffing PPE do not count toward the rest break. A paid meal break may count as a rest break. Source: OSHA proposed rule regulatory text.

Heat index is a screening tool — WBGT is the measurement

OSHA recommends a wet bulb globe temperature (WBGT) monitor to measure workplace heat: it accounts for radiant heat, air movement and humidity, which the heat index does not. OSHA notes that outdoor workers have died of heat stroke when the day's maximum heat index was only 86 °F. Work/rest ratios in minutes per hour, from NIOSH and ACGIH, are keyed to WBGT and workload — use a WBGT reading for those, not the number on this page.

How it's calculated

Heat index — simple form (used first)
HI = 0.5 × [T + 61.0 + (T − 68.0) × 1.2 + (RH × 0.094)] — T in °F, RH in %
If the average of this result and T is below 80 °F, this value is the heat index.
Heat index — Rothfusz regression (used when (HI + T)/2 ≥ 80 °F)
HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²
Low-humidity adjustment (RH < 13% and 80 °F ≤ T ≤ 112 °F)
HI = HI − [(13 − RH) / 4] × √[(17 − |T − 95|) / 17]
High-humidity adjustment (RH > 85% and 80 °F ≤ T ≤ 87 °F)
HI = HI + [(RH − 85) / 10] × [(87 − T) / 5]
Full sun adjustment
HI (full sun) = HI + 15 °F
NWS: exposure to full sunshine can increase heat index values by up to 15 °F. OSHA's heat hazard page cites up to 13.5 °F. Both are upper bounds, not a measurement.

Frequently asked questions

There is no final federal heat standard today. In OSHA's proposed heat rule, the initial heat trigger is a heat index of 80 °F and the high heat trigger is a heat index of 90 °F. Until a final rule exists, heat hazards are cited under the General Duty Clause, Section 5(a)(1) of the OSH Act.

The proposed rule would require a minimum 15-minute paid rest break at least every two hours in a break area at or above the high heat trigger, a heat index of 90 °F. Time spent walking to the break area and donning or doffing PPE does not count toward that break.

The proposed rule would require cool drinking water in a quantity sufficient to provide 1 quart per employee per hour, on top of the sanitation standard that already applies (29 CFR 1926.51 in construction, 1910.141 in general industry).

Yes. Heat index values are published for shady, light-wind conditions. The National Weather Service states that full sunshine can raise the heat index by up to 15 °F; OSHA's heat hazard page cites up to 13.5 °F. This calculator adds 15 °F when you select full sun.

No. OSHA recommends wet bulb globe temperature (WBGT) as the more accurate measure of workplace heat because it accounts for radiant heat, air movement and humidity. The heat index is a screening tool: OSHA notes that outdoor workers have died of heat stroke when the day's maximum heat index was only 86 °F.

The National Weather Service uses four bands: Caution 80–90 °F (fatigue possible), Extreme Caution 90–103 °F (heat cramps, heat exhaustion or heat stroke possible), Danger 103–124 °F (heat cramps or heat exhaustion likely, heat stroke possible) and Extreme Danger 125 °F or higher (heat stroke highly likely).

It gives the break schedule from OSHA's proposed rule (15 paid minutes every two hours at a heat index of 90 °F or above). Formal work/rest ratios in minutes per hour come from NIOSH and ACGIH and are based on WBGT and workload, not on the heat index, so they need a WBGT measurement.

Sources & references

Content checked against these sources — last reviewed August 27, 2026.

By — Editorial team of SteelToeTools.com (published by LSEA SAS) Updated v1