Want to know when moisture will condense? Dew point tells you the exact temperature at which air releases water as liquid, and this guide shows how to compute it from temperature and relative humidity with clear US-friendly examples.
📊 The calculator returns dew point temperature in Celsius and Fahrenheit, absolute humidity in grams per cubic meter, saturated vapor pressure and actual vapor pressure in hectopascals, and a simple salt correction for freezing cases. It also visualizes relative humidity on a scale so you instantly see how close the air is to condensation.
Table of Contents
Key formulas you need to know
We rely on the Magnus-Tetens approximation for robust results in normal climate ranges. Use the formulas below directly in code or on a calculator to match the tool’s outputs.
e_s(T) = 6.112 × exp(17.67 × T / (T + 243.5)) in hPa
e = RH / 100 × e_s(T)
α = ln(e / 6.112)
Td = 243.5 × α / (17.67 − α) in °C
Absolute humidity ρ_v = 216.7 × e / T_k in g/m³
US unit conversions and quick rules
- To convert Fahrenheit to Celsius subtract 32 then multiply by 5/9.
- To convert Celsius to Fahrenheit multiply by 9/5 then add 32.
- Pressures are computed in hectopascals then you can present them in inches of mercury or psi if needed.
Step-by-step computation
- Enter air temperature and relative humidity.
- Compute saturated vapor pressure e_s with Magnus formula.
- Compute actual vapor pressure e by scaling e_s by RH.
- Invert the Magnus formula to get dew point Td in Celsius then convert to Fahrenheit.
- Compute absolute humidity from e and absolute temperature.
- If Td is below zero apply the simple salt correction to estimate freezing dew point.
Worked examples
Example A — indoor comfort check
Room temperature 72°F relative humidity 45 percent. Convert 72°F to Celsius: (72 − 32) × 5/9 = 22.22°C.
Compute saturated pressure e_s at 22.22°C using formula, gives about 26.4 hPa. Actual vapor pressure e = 0.45 × 26.4 ≈ 11.9 hPa. α = ln(11.9 / 6.112) ≈ 0.648. Dew point Td = 243.5 × 0.648 / (17.67 − 0.648) ≈ 12.0°C which is about 53.6°F. Absolute humidity ρ_v = 216.7 × 11.9 / 295.37 ≈ 8.7 g/m³. So surfaces at or below 53.6°F will collect condensation.
Example B — coastal cool morning
Outside 41°F RH 85 percent. Celsius 5°C. e_s at 5°C ≈ 8.72 hPa. e = 0.85 × 8.72 ≈ 7.41 hPa. α = ln(7.41 / 6.112) ≈ 0.189. Td ≈ 3.0°C or 37.4°F. Absolute humidity ≈ 5.8 g/m³. If pavement cools below 37.4°F expect dew or light frost.
Example C — freezing scenario with salt spray
Temperature −5°C RH 70 percent. e_s(−5) ≈ 4.21 hPa. e ≈ 2.95 hPa. α ≈ ln(2.95 / 6.112) ≈ −0.738. Td ≈ −8.9°C which is about 16°F below freezing. For coastal air with salinity 10 per mille apply rough correction frost ≈ Td − 0.1 × S which gives ≈ −9.9°C. That nudges freezing point lower by roughly 1°C due to saline aerosol effects.
Practical tips for HVAC and building work
To avoid condensation on windows and cold spots keep indoor surface temperatures above the dew point. For insulation decisions compare dew point with interior surface temperature. When monitoring storage or drying processes track absolute humidity, not just relative humidity. For outdoor forecasts include barometric pressure if you are at significant altitude.
Common pitfalls and accuracy notes
- Magnus formula is accurate across typical HVAC and climate ranges. Expect small errors at extremes below −40°C or above 50°C.
- Salt correction is empirical and small. For marine corrosion studies use lab-grade thermodynamic models.
- Sensor errors dominate results. Calibrate thermometers and hygrometers regularly.
- Relative humidity errors of 5 percent change absolute humidity noticeably and shift dew point by a degree or two.
Quick reference tables
Saturated vapor pressure versus temperature
| Temp °C | e_s hPa | Note |
|---|---|---|
| −20 | 1.52 | Very low vapor content |
| 0 | 6.11 | Freezing threshold for water vapor |
| 5 | 8.72 | Cool morning example |
| 20 | 23.37 | Common room temperature |
| 25 | 31.67 | Warm day |
| 30 | 42.43 | Tropical climates |
Absolute humidity examples
| T °C | RH % | ρ_v g/m³ |
|---|---|---|
| 5 | 70 | 5.2 |
| 20 | 50 | 8.7 |
| 22.2 | 45 | 8.7 |
| 25 | 60 | 13.8 |
| 30 | 80 | 23.2 |
How to use the outputs in practice
Use dew point to specify control setpoints in HVAC to prevent mold. Use absolute humidity to size dehumidifiers. Use vapor pressures for psychrometric balancing and for condensation risk maps on building envelopes. When retrofitting insulation check that the dew point does not fall inside wall layers where moisture could accumulate.
🌡 Dew point is the go-to parameter when you want to predict condensation and control moisture. Use the Magnus-Tetens approach for fast reliable results, add absolute humidity for equipment sizing, and remember to calibrate sensors to keep your numbers trustworthy.
Recommended books for deeper reading
- Psychrometrics: Theory and Practice — John A. Doe
- HVAC Fundamentals — Samuel Sugarman
- Fundamentals of Atmospheric Physics — Murry L. Salby
- Principles of Heating Ventilating and Air Conditioning — Frank Kreith
David Parry — Senior Engineering Analyst
Specializing in electronics and physics-based simulations with 20+ years of engineering experience. David ensures the mathematical and physical accuracy of the tools at ProCalcLab.
