Discomfort Index (Thom) Calculator

Understanding the discomfort index calculator — concept and principle

The discomfort index (DI), also called the temperature-humidity index, combines air temperature and relative humidity into one number that describes how oppressive warm weather feels. It was proposed by Earl C. Thom in 1959 and is still used in East Asia for summer weather briefings and for deciding when cooling or dehumidifying is worth it.

30 °C with 70 % humidity gives a discomfort index of 81.38 — the very high band.

How it is calculated

DI = 0.81T + 0.01RH × (0.99T − 14.3) + 46.3

T is the dry-bulb air temperature in °C.
RH is relative humidity in percent (0–100).

The calculator then labels the value: 80 or more is "very high", 75 to below 80 "high", 68 to below 75 "moderate" and below 68 "low". This Celsius form is algebraically the same as the version written as 1.8T − 0.55(1 − RH/100)(1.8T − 26) + 32, which is the one often quoted by the Korea Meteorological Administration.

The principle

Thom originally defined the index from the average of dry-bulb and wet-bulb temperatures on the Fahrenheit scale. The wet-bulb reading falls below the air temperature when air is dry, because evaporation cools the bulb, and approaches it when air is saturated. Human sweat works the same way: in humid air sweat evaporates poorly, so the body loses heat more slowly. The formula above replaces the wet-bulb term with an approximation using relative humidity, so only two common measurements are needed. The result stays on a Fahrenheit-like scale, which is why values sit around 60–90 rather than near the Celsius temperature.

Worked example

Inputs: T = 30 °C, RH = 70%.
Temperature term: 0.81 × 30 = 24.3.
Humidity term: 0.99 × 30 − 14.3 = 15.4, and 0.01 × 70 × 15.4 = 10.78.
DI = 24.3 + 10.78 + 46.3 = 81.38.
Since 81.38 ≥ 80, the level is very high.

Each point is the calculator re-run with only “Temperature (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Temperature (°C)	Discomfort index
7.5	47.5625
15	58.835
22.5	70.1075
30	81.38
37.5	92.6525
45	103.925
60	126.47

How to read the result

Where does your result fall?

Below 68 most people feel comfortable. Between 68 and 75 some people start to feel uncomfortable, from 75 roughly half of people do, and above 80 nearly everyone feels discomfort. A humid 30 °C day can score higher than a dry 33 °C day, which matches everyday experience.

Common mistakes

Entering temperature in °F into a °C formula.
Typing humidity as a fraction (0.7) instead of a percentage (70).
Treating DI as a measure of heat-illness risk; it describes comfort, not physiological strain.

Keep in mind: The discomfort index ignores wind, sunshine, radiant heat, clothing and exertion. For outdoor work or sport in heat, use the wet-bulb globe temperature (WBGT) or official heat warnings, and remember that the index is not a medical assessment.

Frequently asked questions

Is the discomfort index the same as the heat index?

No. The US heat index estimates an apparent temperature in °F from a regression on temperature and humidity, while the DI is a comfort score on its own scale. Both rise with humidity, but the numbers are not interchangeable.

Why does the index rise so quickly with humidity on hot days?

The humidity term is multiplied by (0.99T − 14.3), which grows with temperature, so each extra percent of humidity matters more as it gets hotter.

Can the DI be used in winter?

It is designed for warm conditions. At low temperatures it says little about comfort, because wind chill and clothing dominate.

Sources: E. C. Thom (1959), "The Discomfort Index", Weatherwise 12(2), 57–61

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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