What the result means
Two measurements fix the state of moist air. From them you get the moisture content (humidity ratio), the heat content (enthalpy), and the temperature where condensation starts (dew point).
Techs use enthalpy across a coil to estimate total capacity, dew point to check condensation risk on ducts and surfaces, and wet bulb for charging and airflow checks. Altitude matters: thinner air holds more moisture per pound at the same relative humidity.
How it's calculated
- p (kPa) = 101.325 × (1 − 2.25577×10⁻⁵ × Z)^5.2559
- es (hPa) = 6.1094 × exp(17.625 × T ÷ (T + 243.04))
- W = 0.621945 × pw ÷ (p − pw)
- Td = 243.04 × g ÷ (17.625 − g), g = ln(pw ÷ 6.1094)
- h = 0.240 × t + W × (1061 + 0.444 × t) Btu/lb
- v = 0.370486 × (t + 459.67) × (1 + 1.607858 × W) ÷ p_psia
Z in meters, T in °C, t in °F. Wet bulb is solved by bisection with the ASHRAE psychrometric equation (SI, t* ≥ 0 °C).
Worked example
Return air at 75 °F and 50% RH at sea level.
- es at 23.9 °C = 29.4 hPa, so pw = 14.7 hPa
- W = 0.621945 × 1.471 ÷ (101.325 − 1.471) = 0.0092 lb/lb
- h = 0.240 × 75 + 0.0092 × (1061 + 33.3) = 28.0 Btu/lb
- Dew point ≈ 55 °F, wet bulb ≈ 62.5 °F
Any duct or pipe surface below about 55 °F in this space will sweat.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Dry bulb | Relative humidity | Interpretation |
|---|---|---|
| 75°F | 50% | Typical indoor comfort reference |
| 80°F | 50% | Warmer indoor air holds more moisture |
| 90°F | 40% | Hot outdoor air can have high moisture content |
| 95°F | 70% | Hot, humid outdoor design condition |
Illustrative conditions; use measured readings and local design data for equipment decisions.
Sources and assumptions
Z in meters, T in °C, t in °F. Wet bulb is solved by bisection with the ASHRAE psychrometric equation (SI, t* ≥ 0 °C). Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Which mode is most accurate?
Wet bulb or dew point readings from a good instrument are usually more reliable than a cheap RH sensor.
Why does altitude matter?
Lower pressure changes the humidity ratio and specific volume for the same temperature and RH. At 5,000 ft the difference is noticeable.
Why can't I enter a wet bulb below 32 °F?
Below freezing the wet-bulb wick ices over and a different equation applies. This version does not cover that range.
How do I get coil capacity from enthalpy?
A common field estimate is 4.5 × CFM × (h entering − h leaving) Btu/h at standard air. Airflow must be measured, not assumed.
What is grains per pound?
Humidity ratio × 7,000. It's the same moisture measure in the unit many dehumidifier specs use.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Confirm whether a value is nominal or measured, and whether readings were taken at design or actual operating conditions. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. Elevation, air temperature, equipment and installation details can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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