All calculators

LoadCairn / Moisture conditions

Dew point & humidity calculator

Estimate dew point, vapor pressure, humidity ratio, and condensation risk from indoor air conditions. The calculation runs in your browser and screens an ASHRAE recommended envelope for data-center planning.

Inputs

Illustrative example · editable

Temperatures convert together when you switch units.

Temperature unit

Indoor air temperature used for the dew-point and vapor-pressure estimate. Example: 75.

Enter 1% through 100%. A value of 0% is outside this calculation range. Example: 45.

Optional; blank uses 0 ft. Used for barometric pressure and humidity ratio only; valid from 0 to 15,000 ft. Example: 0.

Optional. Enter a coldest surface to screen its distance from the dew point.

Live estimate

Results update with every valid input and unit change.

Dew point
52.2°F
Humidity ratio
0.00828lb/lb
Moisture content
58.0grains/lb
Barometric pressure
101.325 kPa · 29.921 inHg

IN ENVELOPE

All recommended-envelope comparisons pass.

  • Dry-bulb temperature 75.0°F is within the recommended 64.4°F to 80.6°F range.
  • Dew point 52.2°F is within the recommended 15.8°F to 59.0°F range.
  • Relative humidity 45% is at or below the recommended 60% maximum.
Calculation details
  1. Step 1: Gamma: γ = 17.625 × 23.888889°C ÷ (23.888889°C + 243.04) + ln(45 ÷ 100) = 0.778847
  2. Step 2: Dew point (Td): Td = 243.04 × 0.778847 ÷ (17.625 − 0.778847) = 11.236458 °C
  3. Step 3: Saturation vapor pressure (Psat): Psat = 0.61094 × exp(17.625 × 23.888889°C ÷ (23.888889°C + 243.04)) = 2.958252 kPa
  4. Step 4: Partial vapor pressure (Pw): Pw = (45 ÷ 100) × 2.958252 kPa = 1.331214 kPa
  5. Step 5: Barometric pressure (P): P = 101.325 × (1 − 2.25577e−5 × 0 m)^5.2559 = 101.325 kPa; 101.325 × 0.295299830714 = 29.921255 inHg
  6. Step 6: Humidity ratio (W): W = 0.621945 × 1.331214 kPa ÷ (101.325 kPa − 1.331214 kPa) = 0.00828
  7. Step 7: Humidity ratio (grains/lb): grains/lb = 0.00828 × 7000 = 57.959514 grains/lb
  8. Step 8: Dry-bulb temperature: 75.0°F within 64.4°F–80.6°F = PASS
  9. Step 9: Dew point: 52.2°F within 15.8°F–59.0°F = PASS
  10. Step 10: Relative humidity: 45% ≤ 60% = PASS

Assumptions and limits

  • The Magnus approximation is accurate to about ±0.4°C between -40°C and 50°C.
  • ASHRAE Thermal Guidelines (recommended envelope): dry bulb 64.4°F–80.6°F, dew point 15.8°F–59.0°F, and RH no more than 60%.
  • Proposed TC 9.9 qualification — confirm with the owner: this recommended envelope is a planning reference for data-center IT equipment inlet conditions associated with ASHRAE TC 9.9 guidance; it does not replace equipment-manufacturer limits, project-specific psychrometric analysis, or a design review.
  • Altitude adjusts barometric pressure and humidity ratio in this estimate; it does not adjust the dew-point calculation.

Planning estimate only. This is not a final design, an equipment rating, or a code-compliance determination.