Wet Bulb Calculator (Psychrometric Chart Screen)
Wet-bulb first: enter dry-bulb + RH (or dew point / wet bulb) for instant wet bulb, dew point, humidity ratio, and enthalpy. Example: 25 °C / 50% RH ≈ 18 °C wet bulb, ~14 °C dew point. Screening (Magnus + Stull)—not a stamped ASHRAE psych chart. Next: HVAC tonnage or chiller GPM×ΔT.
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Input parameters
Quick: dry-bulb + second state (RH / dew point / wet bulb). Advanced: elevation → barometric pressure. Defaults match a common office design point (25 °C, 50% RH).
Quick examples:
Advanced: elevation & barometric pressure
State-point chart (screening)
Dry-bulb vs humidity ratio with saturation curve at the current pressure. Blue dot = live state. Not a printable ASHRAE chart.
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 ≈ 18.0 °C · Humidity ratio ≈ 0.0099 kg/kg · Enthalpy ≈ 50 kJ/kg. Updates instantly when you change dry-bulb, second property, or elevation.
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 |
Worked Psychrometric Example (Cooling Coil Process)
A common HVAC calculation: sizing a cooling coil from entering and leaving air conditions. This walks through the psychrometric math step by step.
Given: Entering air: 90°F DB / 70% RH (summer design) · Leaving air: 55°F DB / 95% RH (cooling coil supply) · Airflow: 2,000 CFM · Sea level
Step 1 — Entering air properties: At 90°F / 70% RH: dew point ≈ 79°F, humidity ratio W ≈ 155 gr/lb (0.022 lb/lb), enthalpy h ≈ 44 Btu/lb
Step 2 — Leaving air properties: At 55°F / 95% RH: dew point ≈ 54°F, W ≈ 90 gr/lb (0.013 lb/lb), h ≈ 23 Btu/lb
Step 3 — Sensible cooling load: Q_s = 1.08 × CFM × (DB_in − DB_out) = 1.08 × 2,000 × (90−55) = 75,600 Btu/h ≈ 6.3 tons sensible
Step 4 — Latent cooling load (dehumidification): Q_l = 0.68 × CFM × (W_in − W_out) = 0.68 × 2,000 × (155−90) gr/lb ÷ 7,000 gr/lb × 1,060 Btu/lb ≈ 13,400 Btu/h ≈ 1.1 tons latent
Step 5 — Total cooling capacity: Q_total = 4.5 × CFM × (h_in − h_out) = 4.5 × 2,000 × (44−23) = 189,000 Btu/h ≈ 15.8 tons total
Step 6 — Sensible Heat Ratio (SHR): SHR = Q_s ÷ Q_total = 75,600 ÷ 189,000 = 0.40. Note: this is a very latent-heavy process (high humidity climate). Typical comfort cooling SHR = 0.65–0.75. Select a cooling coil with SHR ≤ 0.40 and total capacity ≥ 16 tons.
Constants: 1.08 = air density × specific heat (0.075 lb/ft³ × 0.24 Btu/lb°F × 60 min/h); 4.5 = 0.075 × 60; 0.68 = latent heat factor. Values rounded for screening — use manufacturer coil selection software for final design.
Next step: convert total coil load to equipment tons with the HVAC tonnage calculator, or size chilled-water plant tons with chiller GPM × ΔT. Tool picker: HVAC tools hub.
Frequently asked questions
Can I enter dew point or wet bulb instead of RH?
Yes. Choose Dew point or Wet bulb as the second independent property. The calculator solves RH (Magnus for dew point; Stull inverse for wet bulb), then returns the full property set and updates the state-point chart.
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.
How do you calculate a wet bulb temperature?
Enter dry-bulb and relative humidity (or enter wet bulb directly as the second property). This page returns approximate wet-bulb using a Stull-style correlation for screening—equal to dew point only at 100% RH. Example: 25 °C DB / 50% RH ≈ 18 °C WB.
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.