Heat Index Calculator

Frequently Asked Questions About Heat Index and Workplace Heat Safety

What Is the Heat Index?

The heat index (or "apparent temperature") combines air temperature and relative humidity to indicate how hot it actually feels to the human body. When humidity is high, sweat evaporates more slowly, reducing the body's ability to cool itself. The NWS heat index is calculated for shady conditions only — direct sunlight can increase the felt temperature by up to 15°F.

How Is the Heat Index Calculated?

This calculator uses the Rothfusz regression equation developed for the National Weather Service, which combines air temperature (°F) and relative humidity (%) into an apparent temperature. The formula includes adjustments for low humidity (below 13%) and high humidity (above 85%) conditions. Results have an accuracy of approximately ±1.3°F.

What Are OSHA's Heat Index Risk Levels?

OSHA defines four heat risk levels based on the heat index:

• Lower Risk (Caution) — Below 91°F: Basic precautions. Provide water, plan for emergencies, monitor new workers during acclimatization.
• Moderate Risk — 91–103°F: Implement precautions and increase monitoring. Remind workers to drink water frequently, take regular breaks in shade.
• High Risk — 103–115°F: Additional precautions required. Alert workers of high risk, actively watch for symptoms, establish mandatory work/rest cycles.
• Very High to Extreme Risk — Above 115°F: Aggressive protections. Reschedule non-essential work, continuous monitoring, very frequent rest breaks, buddy system required.

What Is the Difference Between Heat Index and WBGT?

The heat index combines temperature and humidity and is calculated for shaded conditions at rest. WBGT (Wet Bulb Globe Temperature) accounts for four factors: temperature, humidity, radiant heat (sunlight), and wind speed. WBGT is the preferred occupational standard because it reflects actual working conditions, but it requires specialized equipment. The heat index is more widely available and is used by the OSHA-NIOSH Heat Safety Tool.

What Is the Proposed OSHA Heat Standard?

OSHA's proposed Heat Injury and Illness Prevention rule (Notice of Proposed Rulemaking, August 2024) introduces two trigger thresholds:

• Initial trigger — 80°F heat index: Requires a written Heat Injury and Illness Prevention Plan (HIIPP), accessible drinking water, acclimatization procedures for new and returning workers, and emergency response planning.
• High-heat trigger — 90°F heat index: Requires paid 15-minute rest breaks every two hours, active monitoring for heat illness symptoms, a buddy system, and designation of one or more individuals to call for emergency medical services.

Several states — including California, Washington, Oregon, and Colorado — already have their own heat standards. Check your state's requirements as they may be more protective than the proposed federal rule.

What Work/Rest Schedule Should I Follow?

Work/rest schedules depend on the heat index and workload intensity. OSHA and ACGIH guidance generally recommends longer rest periods as the heat index rises and as workload increases. This calculator provides specific work/rest recommendations for each workload level. Remember that workers wearing heavy PPE or non-breathable clothing need more frequent breaks, and unacclimatized workers should be given a graduated schedule over their first 7–14 days.

Does Direct Sunlight Affect the Heat Index?

Yes. The NWS heat index is calculated for shaded conditions. The OSHA-NIOSH Heat Safety Tool and NWS both note that working in full sunlight can increase heat index values by up to 15°F. This calculator includes a direct sunlight checkbox that adds 15°F to help account for this. For the most accurate outdoor reading, use an on-site WBGT meter.

What Are the Symptoms of Heat-Related Illness?

Symptoms range from mild to life-threatening:

• Heat rash — Red clusters of bumps on skin, often in areas covered by clothing.
• Heat cramps — Painful muscle spasms, usually in legs or abdomen. Caused by loss of electrolytes through sweating.
• Heat exhaustion — Heavy sweating, weakness, nausea, dizziness, headache, fast pulse. Skin may be cool and moist. Requires immediate rest in shade, hydration, and cooling.
• Heat stroke — Medical emergency. Call 911 immediately. Core temperature above 104°F, confusion, slurred speech, loss of consciousness, hot dry skin or profuse sweating, seizures. Can be fatal without immediate treatment.

Who Is Most at Risk for Heat-Related Illness?

Risk factors include:

• Lack of acclimatization — OSHA data shows 50–70% of outdoor heat fatalities occur in a worker's first few days on the job.
• Heavy PPE or non-breathable clothing — Reduces the body's ability to release heat.
• Strenuous physical labor — Generates significant internal body heat.
• Certain medications — Including diuretics, blood pressure medications, antihistamines, and stimulants.
• Pre-existing conditions — Heart disease, obesity, diabetes, and prior heat illness.

What Is Heat Acclimatization and Why Does It Matter?

Heat acclimatization is the body's gradual adaptation to working in hot conditions. OSHA recommends:

• New workers: Start at 50% of the normal workload and increase gradually over 7–14 days.
• Returning workers (absent 3+ days): Resume at a reduced workload and re-acclimatize over 3 consecutive days.
• Acclimatized workers sweat more efficiently, maintain lower core temperatures, and have significantly reduced risk of heat illness.

What Other Free Safety Calculators Do You Have?

Click on the links below to view our other calculators:

• TRIR Calculator — Calculate your Total Recordable Incident Rate
• DART Rate Calculator — Calculate your Days Away, Restricted, or Transfer rate
• Severity Rate Calculator — Calculate your lost workday rate and average severity per incident
• Injury Cost Calculator — Estimate the true financial cost of workplace injuries

Who Can I Ask for Help With Heat Safety Compliance?

In addition to offering EHS Software to help track heat-related incidents, manage inspections, and document your Heat Illness Prevention Plan, Ecesis provides compliance assistance and EHS expertise. Please call us at (720) 547-5102 to learn more.

How EHS Software Can Help Manage Heat Safety

With OSHA's proposed heat standard requiring written prevention plans, documented training, and systematic monitoring, EHS software is essential for managing heat safety compliance efficiently: