Dew Point Calculator
Calculate Dew Point
Result
Step-by-step
Relative humidity is a moving target—it jumps when temperature drops at night even though the actual moisture in the air has not changed. That is why "60% humidity" feels crisp at 55°F and miserable at 85°F. Dew point cuts through the confusion: one number that tracks how much water vapor is really present. Enter air temperature and RH, pick °F or °C, and this calculator returns dew point with a comfort label plus full step-by-step math you can paste into a lab report.
How to Use This Dew Point Calculator
- Select temperature units. Choose Fahrenheit (°F) or Celsius (°C) for the air temperature and dew-point result.
- Enter air temperature. Dry-bulb reading from a thermometer, weather station, or forecast.
- Enter relative humidity (%). Must be between 1 and 100. Zero percent is rejected because the underlying logarithm in the Magnus formula is undefined at perfectly dry air.
- Click Calculate. Dew point appears in the result panel with a human comfort summary. Open the step-by-step section to see γ computed and converted back to your chosen unit.
Need wet-bulb alongside dew point from the same two inputs? Use the Wet Bulb Calculator. For summer safety thresholds that combine heat and humidity into one apparent temperature, try the Heat Index Calculator.
Dew Point Formulas and Practical Applications
Dew point is the temperature you must cool air to (at constant pressure) before water vapor starts condensing into dew, fog, or frost. Think of a cold drink sweating on a humid patio—the glass surface hits the dew point of the surrounding air and droplets form. Unlike RH, dew point stays tied to actual moisture content, so forecasters and HVAC techs lean on it when comparing mornings to afternoons.
This calculator uses the Magnus-Tetens approximation with air temperature T in °C and relative humidity RH in percent:
γ = ln(RH / 100) + (17.625 × T) / (243.04 + T)
Tdp = (243.04 × γ) / (17.625 − γ)
Example: T = 75°F at RH = 50% gives a dew point near 55°F—comfortable for most people. Push that same 55°F dew point against a 90°F afternoon and the air feels thick because RH would be much higher at the warmer temperature.
Where dew point shows up in real work
- Weather forecasting: Rising dew points ahead of warm fronts signal muggy, storm-favorable air masses.
- Aviation and visibility: When air temperature drops to dew point, fog and low clouds form.
- HVAC and indoor air: Surface temperatures below dew point cause condensation on windows and ducts—mold risk follows.
- Sports and events: Athletic trainers watch dew point because high values limit evaporative cooling even before heat index advisories trigger.
Standard Units and Conversion Tables
Magnus-Tetens runs in Celsius internally. Fahrenheit inputs convert before the formula and convert back for display. Relative humidity is always percent.
Dew point comfort scale
| Dew point | °C range | How it feels |
|---|---|---|
| Very dry | Below 10°C (< 50°F) | Comfortable; desert or winter dryness |
| Comfortable | 10–15.6°C (50–60°F) | Pleasant for most outdoor activity |
| Slightly humid | 15.6–18.3°C (60–65°F) | Noticeable stickiness beginning |
| Humid | 18.3–21.1°C (65–70°F) | Muggy; uncomfortable for exertion |
| Very humid | 21.1–23.9°C (70–75°F) | Oppressive; limited evaporative cooling |
| Extremely oppressive | Above 23.9°C (> 75°F) | Dangerous heat-stress conditions possible |
Dew point can never exceed the current air temperature. When they meet, RH is 100% and condensation, fog, or precipitation is underway.
Frequently Asked Questions
Can dew point be higher than air temperature?
No. Dew point is bounded by the actual air temperature. When temperature falls to dew point, relative humidity reaches 100% and moisture condenses out as fog, dew, rain, or snow depending on conditions.
Why is dew point better than relative humidity for comfort?
Relative humidity is measured against how much moisture warm air could hold at the current temperature. Sixty percent RH at 90°F means far more water in the air than 60% at 50°F. Dew point stays stable when the moisture mass is stable, so it is a clearer stickiness gauge across the day.
Why must relative humidity be at least 1%?
The formula uses ln(RH/100). At 0% humidity that term is undefined. Real air always contains trace moisture; enter the lowest value your instrument reports.
How does dew point relate to fog?
Fog forms when air cools to its dew point and water vapor condenses into tiny droplets. Radiation fog on calm, clear nights is a textbook case: ground cooling pulls air temperature down to dew point by sunrise.
Is this the same formula as the wet-bulb calculator's dew point?
Both use Magnus-type coefficients; constants differ slightly between implementations (17.625/243.04 here versus 17.62/243.12 on the wet-bulb page). Results agree within a fraction of a degree for typical outdoor conditions—close enough for field checks, not for legal metrology.
Disclaimer. RapidRatio is informational only. Dew point estimations use standard atmospheric approximations. Do not use for safety-critical HVAC design, psychrometric engineering, or high-risk industrial processes without professional validation.