Dew Point Calculator
Calculate dew point from temperature and relative humidity, or find relative humidity from temperature and dew point. Includes comfort rating and condensation risk.
°C
%
Enter your values above to see the results.
Tips & Notes
- ✓Dew point is a better comfort indicator than relative humidity because it measures absolute moisture content. RH changes with temperature; dew point stays constant as long as moisture content does not change. A 60% RH at 50°F feels dry; the same 60% at 90°F feels oppressive.
- ✓Condensation forms on any surface cooler than the dew point temperature. In summer air conditioning, supply air must be cooled below the dew point to remove humidity — this is why AC units drip water. Cold pipes sweat when surface temperature is below surrounding dew point.
- ✓Frost point is the equivalent of dew point when the temperature is below 0°C (32°F) — the temperature at which water vapor deposits directly as ice without going through liquid phase. This is why frost forms on cold clear nights even when air temperature stays above 32°F if the radiative surface cools below 32°F.
- ✓Dew point depression (T − T_d) indicates atmospheric stability for aviation and convective weather. A dew point depression of 2-4°C (4-7°F) or less suggests cloud base at very low altitudes (fog risk). A 20+ °C depression indicates dry, stable air with high cloud bases.
- ✓Psychrometric chart: engineers use temperature and dew point (or wet bulb temperature) to determine all other air properties (enthalpy, humidity ratio, specific volume) for HVAC calculations. The dew point and dry bulb temperature define a unique state on the psychrometric chart.
Common Mistakes
- ✗Confusing relative humidity and dew point as equivalent comfort measures — a 70% RH morning in spring (55°F air temperature, dew point 45°F) is comfortable; 70% RH on a summer afternoon (90°F, dew point 77°F) is dangerously humid. Always check the dew point for an absolute moisture assessment.
- ✗Expecting RH to stay constant as temperature rises — as air temperature increases, RH decreases for the same moisture content. Air at 60% RH at 60°F warms to 80°F — its RH drops to about 35% without any moisture change. Only dew point stays constant.
- ✗Calculating condensation risk using air RH rather than surface temperature vs. dew point — condensation occurs when the surface temperature drops below the dew point, regardless of air RH at that moment.
- ✗Using the approximation formula outside its valid range — the simplified dew point formula T_d ≈ T − (100-RH)/5 is only accurate within a few degrees for RH above 50% and temperatures between 0-60°C. Use the full Magnus formula for precision.
- ✗Ignoring the dew point when designing insulation for cold storage or refrigeration — pipe and duct insulation must be thick enough to keep the outer surface above the ambient dew point. Insufficient insulation leads to surface condensation, mold growth, and structural damage.
Dew Point Calculator Overview
Dew point temperature measures the absolute moisture content of air — the temperature at which air becomes saturated and water begins to condense. Unlike relative humidity, which changes with temperature, dew point is a stable indicator of the actual amount of water vapor present.
Dew point formula (Magnus equation):
T_d = 243.04 × [ln(RH/100) + 17.625T/(243.04+T)] / [17.625 − ln(RH/100) − 17.625T/(243.04+T)]
EX: T = 28°C, RH = 75% → T_d ≈ 23.4°C. This means any surface cooled below 23.4°C will accumulate condensation — including cold drinks, AC vents, and insufficiently insulated pipes.Cloud base altitude from temperature-dew point spread:
Cloud base (ft AGL) ≈ (T_dry − T_dew) × 400 (°F) | Cloud base (m AGL) ≈ (T_dry − T_dew) × 122 (°C)
EX: T = 30°C, T_d = 18°C → spread = 12°C → Cloud base ≈ 12 × 122 = 1,464 m (4,800 ft) AGL. When spread is 2°C or less, fog or low cloud is imminent.Dew point comfort scale:
| Dew Point (°C) | Dew Point (°F) | Comfort Level | Description |
|---|---|---|---|
| Below 10°C | Below 50°F | Very Dry | Comfortable, possibly dry skin |
| 10–15°C | 50–59°F | Comfortable | Pleasant conditions |
| 15–18°C | 59–65°F | Slightly Humid | Comfortable for most |
| 18–21°C | 65–70°F | Uncomfortable | Feels noticeably humid |
| 21–24°C | 70–75°F | Oppressive | Muggy, most people uncomfortable |
| Above 24°C | Above 75°F | Dangerous | Extremely oppressive, health risk |
| Air Temperature | Dew Point | Relative Humidity | Comfort Perception |
|---|---|---|---|
| 15°C (59°F) | 10°C (50°F) | 72% | Comfortable — dew point low |
| 25°C (77°F) | 10°C (50°F) | 38% | Dry and comfortable |
| 35°C (95°F) | 10°C (50°F) | 21% | Hot but bearably dry |
| 25°C (77°F) | 22°C (72°F) | 84% | Oppressive — high dew point |
Frequently Asked Questions
Using the Magnus formula: T_d = 243.04 × [ln(RH/100) + 17.625T/(243.04+T)] / [17.625 − ln(RH/100) − 17.625T/(243.04+T)], where T is temperature in °C and RH is relative humidity in %. Example: T = 25°C, RH = 65% → T_d = 243.04 × [ln(0.65) + 17.625×25/(268.04)] / [17.625 − ln(0.65) − 17.625×25/268.04] = 243.04 × [−0.431 + 1.643] / [17.625 + 0.431 − 1.643] = 243.04 × 1.212 / 16.413 ≈ 17.9°C dew point.
Human comfort by dew point: below 10°C (50°F) — very dry, comfortable but skin may feel dry. 10-15°C (50-59°F) — comfortable and pleasant. 15-18°C (59-64°F) — slightly humid, comfortable for most. 18-21°C (64-70°F) — noticeably humid, uncomfortable for some especially with physical activity. 21-24°C (70-75°F) — oppressive, uncomfortable for most. Above 24°C (75°F) — extremely uncomfortable, potentially dangerous with exertion. The Gulf Coast of the United States and tropical regions regularly reach dew points of 24-27°C (75-80°F) in summer.
Dew forms when a surface cools below the ambient dew point temperature. At night, clear skies allow surfaces (grass, car roofs, leaves) to radiate heat to space, cooling below air temperature. When the surface temperature reaches the dew point, water vapor begins condensing on the surface. The dew point of the air determines at what temperature this condensation starts. Higher atmospheric moisture (higher dew point) means dew forms at a higher temperature, making it more likely on calm, clear nights. Cloudy nights suppress dew formation because clouds reflect surface radiation back down, keeping surfaces warmer.
Dew point is critical to flight safety. Cloud base height: clouds form when rising air cools to the dew point — cloud base altitude (ft) ≈ (T − T_d) × 400 (using °F) or ≈ (T − T_d) × 220 (using °C). Example: T = 25°C, T_d = 15°C → cloud base ≈ (25−15) × 220 = 2,200 ft AGL. Fog risk: when dew point depression is 2°C or less, fog is likely as surface cooling reaches the dew point. Icing risk: visible moisture (cloud, rain) at temperatures between 0°C and −20°C creates icing conditions. Pilots check both temperature and dew point at all altitudes to evaluate these hazards.
RH (%) = 100 × exp(17.625 × T_d / (243.04 + T_d)) / exp(17.625 × T / (243.04 + T)), where T and T_d are in °C. Simplified: RH ≈ 100 − 5 × (T − T_d) (approximate, valid for T_d > 0°C and temperature difference < 30°C). Example: T = 30°C, T_d = 20°C → RH ≈ 100 − 5 × (30−20) = 100 − 50 = 50%. Exact: RH = 100 × exp(17.625×20/263.04) / exp(17.625×30/273.04) = 100 × exp(1.339) / exp(1.936) = 100 × 3.815 / 6.927 = 55.1%.
Wet bulb temperature (T_wb) is the temperature a thermometer reads when its bulb is covered in wet cloth and air flows over it. It represents the cooling limit of evaporation — the temperature to which air can be cooled by evaporating water into it. Wet bulb temperature is always between dew point and dry bulb temperature: T_d ≤ T_wb ≤ T. Approximate: T_wb ≈ T × atan(0.151977 × √(RH + 8.313659)) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 × RH^1.5 × atan(0.023101 × RH) − 4.686035. Wet bulb temperature is used in WBGT heat stress calculations and in determining evaporative cooler effectiveness.