Dew Point, Absolute Humidity & THI Calculator

Last updated: 2026-05-20

What Is the Dew Point?

Enter the air temperature and relative humidity to get three complementary moisture metrics: the dew point, the absolute humidity, and the Temperature–Humidity Index (THI). Together they tell you far more about air quality and comfort than relative humidity alone.

Formulas

Dew point (Magnus approximation, accurate to ±0.4 °C):

\gamma = \ln\!\left(\frac{RH}{100}\right) + \frac{17.625 \cdot T}{243.04 + T}

T_d = \frac{243.04 \cdot \gamma}{17.625 - \gamma}

Absolute humidity (g/m³):

AH = \frac{217 \cdot \frac{RH}{100} \cdot 6.112 \cdot e^{\frac{17.625 \cdot T}{243.04 + T}}}{T + 273.15}

Temperature–Humidity Index (Thom 1959):

THI = 0.81 \cdot T + 0.01 \cdot RH \cdot (0.99 \cdot T - 14.3) + 46.3

Understanding the Three Outputs

Dew point tells you the temperature at which condensation forms. Below 10 °C feels dry; 10–15 °C is comfortable; 16–20 °C is noticeable; above 24 °C is widely considered oppressive.

Absolute humidity tells you the actual mass of water vapour in each cubic metre of air. Unlike relative humidity, it doesn't change when you heat the room — useful for HVAC sizing, archiving sensitive materials, and incubation.

THI comfort bands (Thom 1959):

THI	Perception
< 65	Cool
65–75	Comfortable
75–80	Mild discomfort for most
80–85	Widespread discomfort
> 85	Risk of heat stress

Practical Applications

Condensation risk — If a window or wall surface is colder than the dew point, water will form on it. Typical single-glazed glass in winter can be 5–10 °C below room air temperature.
Agricultural frost warning — Frost forms when the dew point drops below 0 °C on a still, clear night. Growers monitor dew point, not RH.
Storage and archiving — Museums and archives typically target 45–55% RH at 18–20 °C. Absolute humidity keeps those limits meaningful regardless of temperature swings.
HVAC design — Engineers size cooling coils to remove enough moisture to hit a target dew point, not a target RH.

Frequently Asked Questions (FAQ)

What is dew point?

The temperature at which water vapour in the air starts to condense. Cool a glass of water below the dew point and droplets form on the outside. Unlike RH, the dew point is an absolute measure of how much water is in the air, so it tracks felt mugginess much more reliably.

Why use dew point instead of relative humidity?

RH depends on temperature: 60% RH at 5°C and 60% RH at 30°C describe completely different amounts of moisture. Dew point does not — it directly states "the air contains this much water". Meteorologists usually report both because RH is easier to measure, but dew point is the more interpretable number.

What do the THI numbers mean?

THI bands (Thom 1959, calibrated for a sedentary indoor adult): below 65 is cool, 65–75 comfortable, 75–80 mild discomfort for at least half of people, 80–85 widespread discomfort, above 85 risk of heat stress. Adjustments apply for clothing, activity level, and acclimatisation.

How do I know if my windows will fog up?

Condensation forms on any surface colder than the dew point. In winter, single-glazed window glass can be 5–10°C cooler than the room — if room dew point exceeds the glass temperature, water beads. Reduce risk by lowering room humidity (ventilate, use a dehumidifier) or by raising the surface temperature (double glazing, curtains drawn open).

Disclaimer

The Magnus formula used here is a well-validated approximation; results are accurate to about ±0.4°C for dew point over typical room conditions. THI is a coarse single-number index and does not account for clothing, activity, sun exposure, or air movement.

Dew Point, Absolute Humidity & THI Calculator

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