Understanding WBGT & UTCI Calculation Spreadsheet

Heat stress is a growing occupational and environmental concern. Two of the most widely accepted indices for assessing thermal stress are the WetBulb Globe Temperature (WBGT) and the Universal Thermal Climate Index (UTCI). Both indices combine meteorological variables to produce a single number that can be compared with healthbased guidelines. This page explains the science behind each index, outlines the data required for calculation, and provides a readytouse spreadsheet template.

1. What Is WBGT?

WBGT was originally developed by the U.S. Army for training and operations in hot climates. It is a weighted average of three temperature measurements:

• T_g Globe temperature (a blackglobe thermometer that captures radiant heat).
• T_wb Wetbulb temperature (reflects evaporative cooling potential).
• T_db Drybulb air temperature (standard ambient temperature).

The standard formulas are:

Indoor or shaded conditions:
WBGT = 0.7Twb + 0.3Tg

Outdoor solar exposure:
WBGT = 0.7Twb + 0.2Tg + 0.1Tdb

Values range from about 10C (cold) to >40C (extreme heat). Health and safety agencies link WBGT thresholds to workrest cycles, hydration strategies, and personal protective equipment.

2. What Is UTCI?

UTCI is a newer, physiologybased index created by the International Commission on Climate and Energy (ICCE). Unlike WBGT, UTCI integrates six meteorological variables into a single equivalent temperature that predicts the thermal strain on a standard person dressed in typical summer clothing.

The required inputs are:

• Air temperature (C)
• Mean radiant temperature (C)
• Wind speed at 10m height (ms)
• Relative humidity (%)
• Atmospheric pressure (hPa) usually set to 1013hPa
• Clothing insulation (clo) standard summer clothing is 0.9clo

UTCI is calculated with a complex polynomial (about 200 terms) derived from a thermophysiological model. The result is expressed in degrees Celsius and is interpreted as follows:

UTCI (C)	Thermal stress category
<-40	Extreme cold stress
-40to-28	Very strong cold stress
-28to-16	Strong cold stress
-16to-5	Moderate cold stress
-5to9	Slight cold stress
9to26	Neutral (no stress)
26to32	Slight heat stress
32to38	Strong heat stress
38to46	Very strong heat stress
>46	Extreme heat stress

3. Why Use a Spreadsheet?

Both indices can be calculated manually, but a spreadsheet offers several advantages:

• Batch processing: Run thousands of hourly records in seconds.
• Automation: Builtin formulas eliminate transcription errors.
• Visualization: Conditional formatting highlights unsafe periods.
• Customization: Adjust clothing, activity level, or safety thresholds without rewriting code.

4. Spreadsheet Overview

The attached spreadsheet (download link below) contains three worksheets:

1. Input Data Columns for datetime, T_db, T_wb, T_g, RH, wind speed, and pressure. Fill this sheet with measured or modeled data.
2. WBGT Calc Automatically calculates indoor and outdoor WBGT using the formulas above. A dropdown lets you select Indoor, OutdoorShade, or OutdoorSun.
3. UTCI Calc Applies the full UTCI polynomial (embedded as a VBA userdefined function for speed). Results are shown in UTCI C, with a column that maps the value to the stress category.

5. StepbyStep Guide

5.1 Populate the Input Sheet

Enter your data in the order shown below. The sheet includes datavalidation rules to keep units consistent.

| Date/Time          | Tdb (C) | Twb (C) | Tg (C) | RH (%) | Wind (m/s) | Pressure (hPa) ||--------------------|----------|----------|--------|--------|------------|----------------|| 20240701 08:00   | 29.5     | 24.2     | 31.0   | 55     | 1.2        | 1013           || 20240701 09:00   | 31.0     | 25.0     | 33.2   | 58     | 1.5        | 1012           ||                   |         |         |       |       |           |               |        

5.2 Choose the WBGT Scenario

In the WBGT Calc sheet, select the scenario from the dropdown in cell B2. The spreadsheet will automatically use the appropriate formula.

5.3 Run UTCI Calculation

The UTCI column uses the custom function =UTCI(Tdb,RH,Wind,Tg,Pressure,Clothing). The default clothing value (0.9clo) is stored in cell G2 but can be changed to reflect workwear.

5.4 Interpret Results

Both worksheets apply conditional formatting:

• WBGT≤25C green (low risk)
• 25C<WBGT≤28C yellow (moderate risk)
• WBGT>28C red (high risk)

UTCI uses the stresscategory table above to color cells from blue (cold) to red (heat).

6. Practical Applications

6.1 Workplace Safety

Employers can set automatic alerts when WBGT exceeds 28C or when UTCI reaches Strong heat stress. The spreadsheet can generate daily summary reports that feed into shiftplanning software.

6.2 Sports & Outdoor Events

Event organizers often publish realtime heatstress dashboards. By linking the spreadsheet to a live weather feed (via Power Query or Google Sheets import), the index values update automatically, allowing participants to adjust activity intensity.

6.3 ClimateResponsive Design

Architects use UTCI to evaluate outdoor thermal comfort around building facades, plazas, and streetscapes. The spreadsheet can be used during the schematic design stage to compare shading strategies or ventilation options.

7. Limitations & Considerations

• Instrumentation accuracy: WBGT is highly sensitive to globe temperature errors; use a calibrated globe thermometer.
• Spatial representativeness: Both indices assume a homogeneous environment. Microclimates (e.g., near a heat source) need local measurements.
• Population differences: UTCI is based on a reference adult male. Adjusting for age, fitness, or acclimatization requires additional modeling.
• Data gaps: Missing wind or humidity data will produce unreliable UTCI values. Impute cautiously or use climatological averages.

8. Download the Spreadsheet

WBGT & UTCI Calculation Spreadsheet (Excel)

9. Further Reading

• ISO 7243:2007 Determination of WBGT for heat stress assessment.
• Foster, K. & Anfuso, C. (2021). UTCI: A universal thermal climate index for assessing human heat stress. *International Journal of Biometeorology*.
• NIOSH Heat Stress Reference Guide (2020).

By integrating reliable meteorological data with the WBGT and UTCI spreadsheets, you can turn raw observations into actionable heatstress information, protect peoples health, and comply with occupational standards.