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TECHNICAL TOOL
Dew-point calculator
Calculate the temperature at which the entered air state reaches saturation. The result supports condensation checks, low-temperature reviews, and humidity-control discussions.
CALCULATED RESULTS
- Dew point (over water)
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- Relative humidity
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- Surface condensation / margin
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- Water-vapour partial pressure
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- Saturation vapour pressure
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- Dry-bulb to dew-point spread
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Dew point, formula and worked examples
Dew point is the temperature at which cooling air reaches saturation at unchanged water-vapour content. A surface at or below this temperature can collect condensation. A positive margin describes these inputs only; allow for sensor uncertainty and later cooling.
γ = ln(RH/100) + 17.62T/(243.12 + T); Td = 243.12γ/(17.62 − γ)
es(T) = 611.2 exp[17.62T/(243.12 + T)] Pa; RH = 100 es(Td)/es(T)
Temperatures are in °C. Coefficients and the −45 to 60°C water-equation range follow NPL: dew point and relative humidity. This is a screening approximation near atmospheric pressure, not a high-pressure gas or frost-point calculation. Dew points outside the range are rejected.
| Air °C | RH % | Dew point °C | Dew point °F |
|---|---|---|---|
| 10 | 80 | 6.71 | 44.07 |
| 20 | 50 | 9.26 | 48.66 |
| 25 | 60 | 16.69 | 62.05 |
| 30 | 80 | 26.17 | 79.10 |
| 35 | 40 | 19.38 | 66.89 |
Common mistakes
- RH changes with temperature even when water-vapour content stays constant.
- A subzero water dew point differs from an ice frost point. Check your sensor’s RH reference.
- Also check leaks, splashes and drain backflow: this calculation does not identify the source of visible water.
- Neither dew-point spread nor surface margin gives L/day. Equipment sizing needs moisture load and capacity at actual conditions.
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METHOD AND BOUNDARY
How this result is calculated
The NPL Magnus water equation uses 611.2 Pa, a = 17.62 and b = 243.12°C. The inverse returns dew point; the ratio of saturation pressures returns RH. Surface margin equals surface temperature minus dew point.
What this tool does not decide
A dew point alone does not provide moisture load, equipment capacity, transient surface temperature, leakage, air-change rate, or a safe project margin.
PROJECT REVIEW
A calculator defines a condition. Equipment selection needs the full operating context.
For a project review, include the enclosure or room, temperature and humidity range, target, moisture source, airflow, power, drainage, controls, quantity, and operating cycle.
Discuss the application