Mean Radiant Temperature Calculator
Mean radiant temperature from a globe thermometer reading, the air temperature and the air speed — the ISO 7726 method — with the operative temperature that comfort standards use.
A black globe thermometer settles at a temperature between the air and the surrounding surfaces, so the surfaces’ radiant temperature can be recovered from it once air movement is allowed for: MRT = [(T_g + 273)⁴ + 1.
How the mean radiant temperature calculator works
A black globe thermometer settles at a temperature between the air and the surrounding surfaces, so the surfaces’ radiant temperature can be recovered from it once air movement is allowed for: MRT = [(T_g + 273)⁴ + 1.1 × 10⁸ v^0.6 (T_g − T_a) ÷ (ε D^0.4)]^¼ − 273 for a globe of diameter D. Operative temperature — what people feel — is roughly the average of air and radiant temperatures at low air speed, which is why a cold window or a hot ceiling matters as much as the thermostat.
Formula: MRT = [(T_g + 273.15)⁴ + 1.1×10⁸ v^0.6 (T_g − T_a) ÷ (ε D^0.4)]^¼ − 273.15; T_op ≈ (T_a + MRT) ÷ 2
Worked examples
| Inputs | Mean radiant temperature | Note |
|---|---|---|
| Globe 26.5 °C, air 24 °C, still air | 28.3 °C | MRT about 28.5 °C |
| Cold window: globe 20, air 22 | 18.8 °C | MRT about 18.5 °C |
| Globe and air equal | 22 °C | MRT 22 °C |
FAQFrequently asked questions
What is a globe thermometer?
A thermometer inside a matt-black hollow sphere, usually 150 mm across. It absorbs radiation from the surroundings and exchanges heat with the air, so its reading combines the two; the formula separates them using the air speed.
Why does radiant temperature matter for comfort?
People exchange heat with surfaces by radiation as much as with the air by convection. Sitting by a cold window in a 22 °C room can feel like 19 °C; under a sunlit skylight like 26 °C. Comfort standards use the operative temperature for that reason.
How long does the globe take to settle?
About 15–20 minutes for a 150 mm globe, a few minutes for a 40 mm one. Read the air temperature and speed at the same spot and time.
What is the operative temperature target?
About 20–24 °C in winter and 23–26 °C in summer for sedentary office work in ordinary clothing (ASHRAE 55 / ISO 7730); the acceptable band shifts with clothing, activity and humidity.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- Building Regulations — Approved Documents — the UK building regulations
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.