Psychrometric Calculator (Wet Bulb, Dew Point & Enthalpy)
Instant wet-bulb & dew-point screen: enter dry-bulb + RH. Example: 25 °C / 50% RH ≈ 18 °C wet bulb, ~14 °C dew point. Facility HVAC air-state screening—not a stamped psych chart.
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Input parameters
Quick: dry-bulb + RH. Advanced: barometric pressure for altitude. Defaults match a common office design point (25 °C, 50% RH).
Quick examples:
Advanced: barometric pressure
About this calculator
Computes moist-air state points from dry-bulb and RH for condensation risk, coil leaving air, and dehumidification context. For cooling tons / BTU/h, use the HVAC Size Calculator. Browse all HVAC tools on the HVAC Calculators Hub.
Calculation results
Example (25 °C, 50% RH, sea level)
Dew point ≈ 13.9 °C · Wet bulb ≈ 17.9 °C · Humidity ratio ≈ 0.0099 kg/kg · Enthalpy ≈ 50 kJ/kg. Updates instantly when you change inputs.
What you are seeing: first-pass moist-air properties for planning conversations. Not modeled: full psych chart processes, mixed-air economizer paths, or ASHRAE stamped psychrometrics. Next: HVAC size (tons) or ACH to CFM.
Engineering disclaimer
This calculator provides preliminary moist-air estimates only (Magnus vapor pressure and an approximate wet-bulb correlation). For final HVAC design, process simulation, and compliance, consult a licensed mechanical engineer and use published psychrometric charts or validated software.
Common air-state checkpoints
Order-of-magnitude dew-point bands for discussion—not design temperatures. Pressure assumed near sea level.
| Scenario | Dry-bulb | RH | Approx. dew point | Approx. wet bulb |
|---|---|---|---|---|
| Comfort office | 25 °C | 50% | ≈ 14 °C | ≈ 18 °C |
| Humid outdoor peak | 30 °C | 70% | ≈ 24 °C | ≈ 26 °C |
| Cold coil leaving | 12 °C | 90% | ≈ 10 °C | ≈ 11 °C |
Formula & explanation
Saturation vapor pressure uses the Magnus form
es = 6.112 · exp(17.67·T/(T+243.5)) (hPa, T in °C).
Actual vapor pressure e = es · RH/100.
Dew point inverts Magnus on e. Humidity ratio
W = 0.62198 · e / (P − e)
with pressures in the same units. Approximate enthalpy
h ≈ 1.006·T + W·(2501 + 1.86·T) (kJ/kg dry air).
Wet-bulb uses a published empirical correlation suitable for screening (−20 to 50 °C dry-bulb, 5–99% RH)—not a full adiabatic saturation solver.
Related: HVAC size (tons & BTU/h) · ACH to CFM · HVAC Sizing Guide · HVAC Calculators Hub
Psychrometric properties explained
Dry-bulb (DB) is the ordinary air temperature from a thermometer. Relative humidity (RH) is the ratio of actual to saturation vapor pressure at DB, expressed as a percent. Dew point (DP) is the temperature at which moisture condenses at constant humidity ratio. Wet-bulb (WB) is the adiabatic saturation temperature approached by evaporative cooling. Enthalpy (h) combines sensible and latent energy per kg dry air—useful for coil load screening.
Example: at 25 °C DB and 50% RH near sea level, expect DP ≈ 14 °C, WB ≈ 18 °C, humidity ratio ≈ 0.010 kg/kg, and h ≈ 50 kJ/kg dry air (matches the Office quick example above).
Common psychrometric processes
- Sensible heating/cooling — horizontal path on a chart: DB changes, humidity ratio roughly constant (typical hot-water coil or dry DX cooling above dew point).
- Humidification — moisture added (steam, spray, or wetted media); WB and humidity ratio rise toward saturation.
- Dehumidification — air cooled below dew point on a coil; moisture condenses; leaving air is colder and drier along a saturation line segment.
- Evaporative cooling — adiabatic saturation: WB approaches DB as RH rises; useful for dry-climate economizer or media coolers.
- Mixing — two air streams blend along a straight line between their state points (return + outdoor air, or zone bypass).
References
- ASHRAE psychrometrics resources — Handbook fundamentals and chart standards.
- Engineering Toolbox — psychrometric chart — printable chart and property definitions.
When to use psychrometrics vs capacity tools
Pick the tool that matches the decision you need this hour.
| Decision | Use |
|---|---|
| Condensation / dew-point risk | This psychrometric calculator |
| Cooling tons / BTU/h from area | HVAC Size Calculator |
| Fan / duct CFM from ACH | ACH to CFM |
| Trunk diameter from CFM | HVAC duct size |
Frequently asked questions
What is a psychrometric calculator used for in HVAC?
It converts dry-bulb and relative humidity into dew point, wet bulb, humidity ratio, and enthalpy so you can screen coil leaving conditions, condensation risk, and dehumidification context before a full chart study.
How do you calculate dew point from dry-bulb and RH?
Compute saturation vapor pressure at the dry-bulb (Magnus), multiply by RH/100, then invert Magnus on the actual vapor pressure. The live calculator applies that chain when you change inputs.
How do I calculate wet-bulb temperature from dry-bulb and RH?
Enter dry-bulb and relative humidity above—the calculator returns an approximate wet-bulb with dew point and enthalpy. For screening, wet-bulb sits between dew point and dry-bulb (equal only at 100% RH). Example: 25 °C DB / 50% RH ≈ 18 °C WB. Not a full adiabatic saturation solver.
Is this also a free wet bulb calculator?
Yes. Searches for wet bulb calculator land here with the psychrometric tool—same inputs (dry-bulb + RH), same free instant result. No separate thin wet-bulb URL.
Is wet-bulb temperature the same as dew point?
No. Dew point is condensation temperature at constant humidity ratio. Wet-bulb is the adiabatic saturation temperature from evaporative cooling. They coincide only at 100% RH.
Does this replace an ASHRAE psychrometric chart?
No. Use published charts or validated software for stamped design. This page is a first-pass screening estimate.
Example: 25 °C dry-bulb at 50% RH—what are dew point and wet bulb?
Near sea level, expect about 13.9 °C dew point and about 18 °C wet bulb, with humidity ratio near 0.01 kg/kg. Click the Office quick example to load those inputs.
How does this relate to HVAC tonnage or CFM?
Psychrometrics describes the air state. Use the HVAC Size Calculator for tons/BTU/h and ACH to CFM for airflow. All three live on the HVAC Calculators Hub.
What are common psychrometric processes in HVAC?
Sensible heating/cooling changes dry-bulb with little moisture change. Dehumidification cools air below dew point on a coil. Humidification adds moisture (steam or spray). Evaporative cooling moves toward wet-bulb along an adiabatic path. Mixed-air blends two streams between their state points—see the processes section under Formula & explanation.