Inputs
Example data table
Use this example to verify your setup and units.
| Dry-bulb | RH | Pressure | Humidity ratio | Dew point | Enthalpy |
|---|---|---|---|---|---|
| 25 °C | 50 % | 101.325 kPa | ~9.9 g/kg | ~13.9 °C | ~50.3 kJ/kg dry air |
| 30 °C | 70 % | 101.325 kPa | ~18.8 g/kg | ~23.9 °C | ~78.5 kJ/kg dry air |
Formula used
This calculator follows standard psychrometric relationships. Saturation vapor pressure uses a Buck-type equation (kPa) with separate water/ice regimes.
1) Saturation vapor pressure
Pws(T) is computed from temperature (°C). Actual vapor pressure is Pv = RH · Pws.
2) Humidity ratio
W = 0.621945 · Pv / (P − Pv), where P and Pv are in kPa.
3) Dew point
Dew point is found by inverting the vapor-pressure relation using a Magnus-style approximation.
4) Enthalpy (per kg dry air)
h = 1.006·T + W·(2501 + 1.86·T) in kJ/kg dry air, with T in °C.
5) Specific volume (per kg dry air)
v = Rda · Tk · (1 + 1.607858·W) / P, where Rda = 0.287042 kPa·m³/(kg·K).
6) Wet-bulb temperature
Wet-bulb is solved iteratively using a ventilated-psychrometer relation:
Pv = Pws(Twb) − A·P·(Tdb − Twb), with A = 0.00066·(1 + 0.00115·Twb).
Engineering note: approximations are accurate for typical HVAC ranges. For research-grade work, verify against a trusted psychrometric chart or ASHRAE formulations.
How to use this calculator
- Enter the dry-bulb temperature and choose °C or °F.
- Enter relative humidity in percent (0–100).
- Select pressure mode: direct pressure or altitude estimate.
- Set rounding if you want fewer or more decimals.
- Press Compute to show results above the form.
- Use Download CSV or Download PDF to export.
Professional article
1) Psychrometric state variables
Moist air is treated as dry air plus water vapor. Enter dry-bulb temperature, relative humidity, and pressure. The calculator reports vapor pressure, humidity ratio, dew point, wet-bulb temperature, enthalpy, specific volume, and density for comfort, drying, and HVAC load work.
2) Inputs and unit discipline
Temperature supports °C or °F, while RH is 0 to 100%. Choose direct pressure (kPa, Pa, bar, atm, psi) or an altitude-based estimate. Keep units consistent because pressure or RH errors can shift humidity ratio by several g/kg and change load results.
3) Pressure and altitude sensitivity
Pressure strongly affects density and specific volume, and it also nudges humidity ratio because W = 0.621945 Pv/(P − Pv). Example: at 25 °C and 50% RH, Pv is about 1.58 kPa. W is about 0.0099 kg/kg at 101.3 kPa, and about 0.012 kg/kg at 84 kPa (near 1500 m).
4) Saturation vapor pressure reference points
Saturation vapor pressure Pws(T) is computed using a Buck-type relation, switching to an ice form below 0 °C. Useful checkpoints are Pws ≈ 0.611 kPa at 0 °C, ≈ 3.17 kPa at 25 °C, and ≈ 4.24 kPa at 30 °C. Actual Pv = RH × Pws.
5) Humidity ratio and moisture scale
Humidity ratio W is mass water vapor per mass dry air. A practical scale is grams per kilogram: W(g/kg) = 1000W. Typical indoor targets often sit near 6–12 g/kg, while humid outdoor air can exceed 18 g/kg, increasing latent cooling demand and condensate rates.
6) Enthalpy for load estimates
Enthalpy per kg dry air is estimated as h = 1.006T + W(2501 + 1.86T) in kJ/kg with T in °C. At 25 °C and 50% RH, h is about 50 kJ/kg; at 30 °C and 70% RH it is about 78 kJ/kg. Specific volume rises as pressure drops, often from about 0.86 m³/kg at sea level to about 1.0 m³/kg near 1500 m.
7) Wet-bulb solving and sanity checks
Wet-bulb temperature is solved iteratively from a ventilated psychrometer relation and refined by bisection for stability. Expect Twb ≤ Tdb, and Twb approaches Tdb as RH approaches 100%. A large Tdb–Twb gap indicates strong evaporative cooling potential.
8) Interpretation, limits, and verification
Use dew point to assess condensation risk on coils, glazing, and chilled piping. Use W and h for latent and total load estimates, and use v for flow and fan calculations. For extremes or compliance work, verify against a trusted chart or standards-based library. For quick checks, compare your outputs against the example rows above first.
FAQs
1) What pressure should I use for my city?
Use local station pressure if available. Otherwise, use altitude mode as an estimate. Pressure slightly changes humidity ratio and noticeably changes specific volume, especially at higher elevations.
2) Why can dew point be close to dry-bulb?
When RH is high, air is near saturation, so the temperature where condensation begins approaches the current dry-bulb. At 100% RH, dew point equals dry-bulb temperature.
3) Is wet-bulb always lower than dry-bulb?
Yes for unsaturated air. Evaporation cools the wet sensor, making Twb lower than Tdb. As RH rises toward saturation, Twb increases and approaches Tdb.
4) What does humidity ratio represent physically?
It is the mass of water vapor carried per mass of dry air. Converting to g/kg is intuitive for HVAC and drying: higher values mean more moisture to remove or add.
5) Can I use this for subfreezing conditions?
Yes. The saturation vapor pressure switches to an ice regime below 0 °C. Accuracy is reasonable for engineering work, but verify critical cold-chain or icing analyses with specialized references.
6) Why do results change when I switch between °C and °F?
They should not, aside from rounding. If you see large differences, confirm you entered the same physical temperature and that RH and pressure settings stayed the same.
7) What is the quickest way to sanity-check outputs?
Compare W and dew point with the example table at similar conditions. Also check that Pv is less than P, Twb is not greater than Tdb, and degree of saturation stays between 0 and 1.