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Heat Index Calculator: OSHA Heat Risk Categories and Work-Rest Ratios

Calculate Apparent Temperature from Air Temperature and Humidity Using NWS Steadman Formula

Free heat index calculator for construction foremen and safety managers. Enter air temperature and relative humidity to get the apparent temperature using the NWS Steadman 9-coefficient formula. The result maps to OSHA heat risk categories (Caution, Extreme Caution, Danger, Extreme Danger) with ACGIH work-rest ratios for light, moderate, and heavy workloads.

Heat stroke kills about 40 workers per year in the U.S. and hospitalizes thousands more. The air temperature alone does not tell you the real risk. At 90 degrees F with 70% humidity, the heat index hits 106 degrees F, which is "Danger" territory. This calculator gives you the number you need to set work-rest schedules, plan water breaks, and decide if the crew needs to start the shift earlier to beat the afternoon heat.

Pro Tip: OSHA's heat illness prevention initiative kicks in at a heat index of 80 degrees F. At 91+ degrees F, you need to provide shade, mandatory water breaks every 15-20 minutes, and a buddy system. New workers are at highest risk: OSHA says unacclimatized workers should start at 20% workload on day one and increase by 20% per day over 5 days. Returning workers who have been off 2+ weeks should start at 50% and ramp 10% daily.

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Heat Index & Work/Rest Schedule Calculator
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How It Works

  1. Enter Air Temperature

    Input the ambient dry-bulb temperature in degrees F or degrees C. The heat index formula is valid for temperatures at or above 80 degrees F (27 degrees C).

  2. Enter Relative Humidity

    Input the relative humidity as a percentage. Higher humidity impairs the body's ability to cool through sweat evaporation, increasing the apparent temperature.

  3. Calculate Heat Index

    The calculator applies the NWS Steadman regression equation with 9 coefficients to compute the apparent temperature that accounts for heat and humidity combined.

  4. Review Risk Category and Work-Rest Guidance

    Results show the OSHA heat risk category and ACGIH-recommended work-rest ratios for light, moderate, and heavy workloads. Use these to set crew schedules and mandatory rest breaks.

Built For

  • Construction foremen checking morning and afternoon heat index to set crew work-rest schedules on outdoor projects
  • Safety officers documenting heat stress assessments for OSHA General Duty Clause compliance on hot days
  • Roofing crews evaluating whether to start at 5 AM to finish heavy work before afternoon heat spikes above Danger level
  • Agricultural supervisors monitoring field conditions for harvest crews and determining mandatory shade break intervals
  • Facility managers assessing heat risk for workers in non-air-conditioned warehouses and loading docks
  • EHS teams developing acclimatization schedules for new hires starting outdoor work during summer months
  • Event coordinators determining hydration station placement and medical standby requirements for outdoor events

Features & Capabilities

NWS Steadman Formula

Uses the official National Weather Service 9-coefficient polynomial regression for accurate heat index calculation from temperature and humidity.

OSHA Risk Categories

Maps the calculated heat index to OSHA risk levels: Caution (80-90 F), Extreme Caution (91-103 F), Danger (103-124 F), and Extreme Danger (125+ F).

ACGIH Work-Rest Ratios

Shows recommended work-rest cycles for light, moderate, and heavy workloads based on heat stress level. Helps set crew schedules.

F and C Toggle

Switch between Fahrenheit and Celsius for both input temperature and heat index output. Humidity is always entered as a percentage.

Acclimatization Guidance

Provides OSHA-recommended ramp-up schedules for new workers (5-day) and returning workers (3-day) during hot weather operations.

PDF Export

Export heat index results as a branded PDF for daily safety briefings, JHA documentation, and compliance records.

Assumptions

  • Uses the National Weather Service Steadman 9-coefficient polynomial regression for heat index calculation
  • Input temperature must be at or above 80 degrees F (27 degrees C) for the heat index formula to be valid
  • OSHA heat risk categories applied: Caution (80-90 F), Extreme Caution (91-103 F), Danger (103-124 F), Extreme Danger (125+ F)
  • ACGIH work-rest ratios assume acclimatized workers in standard work clothing — not PPE, chemical suits, or impermeable garments
  • Acclimatization schedule follows OSHA guidance: new workers start at 20% workload, returning workers at 50%, ramping up over 5 and 3 days respectively
  • Humidity is entered as relative humidity percentage — not wet bulb temperature or dew point

Limitations

  • Does not measure Wet Bulb Globe Temperature (WBGT), which is the ACGIH TLV basis and accounts for radiant heat and wind
  • Direct sun exposure can add 10-15 degrees F to the effective heat index — this adjustment is not included in the NWS formula
  • Does not account for workload metabolic rate, which significantly affects heat strain (light work vs heavy manual labor)
  • Clothing correction factors for PPE, FR clothing, and Tyvek suits are not applied — these can add 5-11 degrees F equivalent
  • Individual risk factors (medications, fitness, prior heat illness, age) are not assessed
  • Cannot replace a site-specific heat illness prevention plan with medical monitoring, emergency response, and acclimatization protocols
  • Indoor radiant heat sources (furnaces, ovens, molten metal) create conditions the heat index formula was not designed to evaluate

References

  • OSHA-NIOSH Heat Illness Prevention Campaign — Water, Rest, Shade guidance and heat index risk categories
  • OSHA Technical Manual Section III Chapter 4 — Heat Stress (evaluation and control methods)
  • ACGIH TLV for Heat Stress — Screening Criteria for Heat Stress Exposure (WBGT-based action limits)
  • Steadman, R.G. (1979) — The Assessment of Sultriness (heat index polynomial regression coefficients)
  • NIOSH Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (2016, Publication No. 2016-106)
  • NWS Heat Index Chart and Rothfusz Regression Equation (National Weather Service Technical Attachment SR 90-23)

Frequently Asked Questions

OSHA does not have a single heat index trigger but uses a heat illness prevention initiative. Generally, a heat index at or above 80 degrees F triggers basic precautions (water, rest, shade). At 91 degrees F and above, additional protective measures are expected. OSHA can cite employers under the General Duty Clause for failing to protect workers from recognized heat hazards.
Heat index uses only air temperature and humidity. Wet Bulb Globe Temperature (WBGT) also accounts for radiant heat (sun exposure) and wind speed, making it more accurate for outdoor work environments. ACGIH TLVs for heat stress are based on WBGT, not heat index. Heat index is a simpler screening tool when WBGT instruments are not available.
ACGIH recommends work-rest cycles based on heat stress level and workload intensity. For example, at moderate heat stress with heavy work, a 25% work / 75% rest ratio per hour may be recommended. These ratios allow the body to dissipate accumulated heat during rest periods and prevent core temperature from rising to dangerous levels.
Workers who are not acclimatized to heat are at much higher risk of heat illness. OSHA recommends a gradual acclimatization schedule: new workers should start at 20% of normal workload on day one and increase by 20% each subsequent day. Previously acclimatized workers returning after absence should start at 50% on day one and increase by 10% per day.
Heat exhaustion symptoms include heavy sweating, weakness, cool and clammy skin, nausea, and dizziness. Heat stroke is a medical emergency with symptoms including high body temperature (above 103 degrees F), hot and dry skin (no sweating), rapid pulse, confusion, and loss of consciousness. Heat stroke requires immediate 911 call and aggressive cooling.
Disclaimer: Heat index estimates are for field screening and planning only. Actual heat stress depends on radiant heat, wind, clothing, and individual factors. Not a substitute for WBGT measurement or medical evaluation of heat-related symptoms.

Learn More

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Heat Stress: Work/Rest Schedules, Heat Index, and OSHA Guidelines

Understanding heat index calculations, OSHA heat risk tiers, ACGIH work/rest ratios, acclimatization, and preventing heat-related illness on the job.

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