Engineering Guide
How to Calculate Relative Humidity: Wet Bulb, Dry Bulb, Dew Point & Psychrometrics for Engineers
Published June 18, 2026 · by Industrial Unit Converter Editorial Team
Why humidity calculations matter in HVAC and process engineering
Every air handling unit, every cooling tower, every spray dryer, and every cleanroom is governed by psychrometrics — the physics of moist air. A 1°F error in wet bulb temperature measurement can shift a relative humidity calculation by 3-5%, enough to push a pharmaceutical cleanroom outside its validated range or cause a data center humidification system to waste thousands of gallons of water annually.
The challenge: relative humidity cannot be directly measured with a simple sensor in most industrial environments. Instead, engineers measure dry bulb (ordinary air temperature) and wet bulb temperature (the temperature a wetted surface reaches under evaporative cooling), then calculate RH from the difference.
This guide covers every practical method: the psychrometric formula, the chart approach, dew point relationships, and the free calculation tools that eliminate manual computation.
The four humidity parameters every engineer must know
| Parameter | Symbol | Definition | Units |
|---|---|---|---|
| Dry bulb temperature | Tdb | Ordinary air temperature measured by a dry thermometer | °F or °C |
| Wet bulb temperature | Twb | Temperature reached by evaporative cooling of a wetted wick | °F or °C |
| Relative humidity | RH (φ) | Ratio of actual vapor pressure to saturation vapor pressure, as % | 0-100% |
| Dew point temperature | Tdp | Temperature at which air becomes saturated (100% RH) at constant pressure | °F or °C |
The relationship: Tdp ≤ Twb ≤ Tdb (always, with equality when RH = 100%)
When dry bulb and wet bulb are equal, the air is saturated (RH = 100%) and the dew point equals both.
How to calculate relative humidity from wet bulb and dry bulb
The psychrometric formula (Stull, 2011)
The standard method for converting wet bulb and dry bulb temperatures to relative humidity uses the saturation vapor pressure curve. The formula is:
e = 6.112 × exp[(17.67 × T) / (T + 243.5)] (for T in °C)
Or in US units:
e = 0.6113 × exp[(17.67 × T) / (T + 243.5)] (T in °C, e in kPa)
The actual vapor pressure is calculated from the wet bulb depression:
eactual = esat,wb − 0.00066 × P × (Tdb − Twb) × (1 + 0.00115 × Twb)
Where P is the barometric pressure (in the same units as e). Then:
RH = (eactual / esat,db) × 100%
Where esat,db is the saturation vapor pressure at the dry bulb temperature.
Simplified psychrometric approximation
For most HVAC applications at sea level, a good approximation is:
- Calculate the wet bulb depression: ΔT = Tdb − Twb
- Calculate saturation vapor pressure at Tdb
- Subtract the psychrometric correction: eactual ≈ esat,db − 0.00066 × 101.325 × ΔT × (1 + 0.00115 × Twb)
- RH = (eactual / esat,db) × 100%
Worked example
Given: Tdb = 30°C, Twb = 22°C, P = 101.325 kPa
- esat,wb = 0.6113 × exp[(17.67 × 22) / (22 + 243.5)] = 2.644 kPa
- esat,db = 0.6113 × exp[(17.67 × 30) / (30 + 243.5)] = 4.245 kPa
- eactual = 2.644 − 0.00066 × 101.325 × (30 − 22) × (1 + 0.00115 × 22) = 2.109 kPa
- RH = (2.109 / 4.245) × 100% = 49.7%
Use our Wet Bulb to Relative Humidity Calculator for instant results. The tool handles both °C and °F inputs and adjusts for barometric pressure.
The psychrometric chart: a visual reference
The psychrometric chart plots all moist air properties on a single diagram. Once you can read it, any humidity parameter can be found from any other two independent parameters.
How to read the chart
- X-axis: Dry bulb temperature
- Y-axis (right): Humidity ratio (grains of moisture per pound of dry air, or g/kg)
- Curved lines (sweeping up from left): Relative humidity lines (10%, 20%, ..., 100%)
- Diagonal lines (sweeping down from left to right): Wet bulb temperature lines
- Horizontal lines (right axis): Dew point temperature — follow a horizontal line to the saturation curve
Quick reference: psychrometric relationships
| Dry Bulb | Wet Bulb | RH (approx) | Dew Point | Condition |
|---|---|---|---|---|
| 75°F | 62.5°F | 50% | 55°F | Comfort cooling design |
| 75°F | 75°F | 100% | 75°F | Saturated air (fog, rain) |
| 95°F | 75°F | 40% | 67°F | Hot dry climate |
| 95°F | 85°F | 65% | 81°F | Hot humid climate |
| 40°F | 35°F | 70% | 31°F | Winter heating |
| 72°F | 60°F | 50% | 52°F | Data center (ASHRAE TC 9.9) |
Use our Wet Bulb to Relative Humidity calculator to fill in any cell of this table for your specific design conditions.
Calculating relative humidity from dew point
If dew point temperature is known instead of wet bulb, the calculation is simpler:
- Calculate saturation vapor pressure at dew point: esat,dp
- Calculate saturation vapor pressure at dry bulb: esat,db
- RH = (esat,dp / esat,db) × 100%
This works because at the dew point, the air is saturated — so the actual vapor pressure equals the saturation vapor pressure at Tdp.
Worked example: RH from dew point
Tdb = 25°C, Tdp = 15°C
- esat,dp = 0.6113 × exp[(17.67 × 15) / (15 + 243.5)] = 1.705 kPa
- esat,db = 0.6113 × exp[(17.67 × 25) / (25 + 243.5)] = 3.169 kPa
- RH = (1.705 / 3.169) × 100% = 53.8%
The humidity ratio (specific humidity) and its role
While relative humidity is the most commonly referenced parameter, humidity ratio (also called specific humidity or absolute humidity) is more useful for mass and energy balances:
ω = 0.622 × eactual / (P − eactual) (lb water / lb dry air, or kg water / kg dry air)
At standard atmospheric pressure (14.7 psia), the conversion factor 0.622 derives from the ratio of the gas constants for water vapor and dry air: Rdry air / Rwater vapor = 287 / 461.5 = 0.622.
Humidity ratio is constant through sensible heating and cooling (no moisture added or removed) but changes through latent processes (humidification, dehumidification, evaporative cooling).
Practical applications
Cooling coil performance
A cooling coil both cools and dehumidifies air. The sensible heat ratio (SHR) determines the split:
SHR = Sensible load / Total load = cp × ΔT / (cp × ΔT + hfg × Δω)
For typical comfort cooling at sea level: SHR ≈ 0.70 to 0.85
Use our Sensible Heat Calculator for HVAC load calculations requiring dry bulb temperature and CFM inputs.
Evaporative cooling
When water evaporates into air, the air temperature drops along a line of constant wet bulb temperature on the psychrometric chart. The theoretical limit of evaporative cooling is the wet bulb temperature — which is why cooling towers can deliver 85°F water when the ambient dry bulb is 105°F if the wet bulb is 75°F.
Data center environmental control
ASHRAE TC 9.9 recommends:
- 18-27°C (64-81°F) dry bulb
- 5.5-15°C (42-59°F) dew point (corresponds to roughly 20-80% RH at 25°C)
- Maximum dew point of 21°C (70°F) for Class A2 equipment
Common humidity measurement errors
1. Wet bulb wick drying out
A dry wick reads closer to dry bulb temperature, overestimating RH. Always ensure the wick is clean and saturated with distilled water. In HVAC, a dirty wet bulb sock can shift RH readings 5-10%.
2. Insufficient ventilation over the wet bulb
The psychrometric formula assumes adequate air velocity over the wet bulb (> 4.5 m/s or 900 FPM). In still air, the wet bulb reads higher (closer to dry bulb), overestimating RH.
3. Radiation errors
If the thermometer bulb "sees" a hot or cold surface, the reading is corrupted by radiant heat transfer. This is why sling psychrometers are whirled — the motion ensures convective equilibrium and minimizes radiation effects.
4. Barometric pressure neglect
The psychrometric formula depends on barometric pressure. At Denver (5,280 ft, P ≈ 12.2 psia), using the sea-level psychrometric constant introduces an RH error of approximately 1-2% at typical HVAC conditions.
Engineering standards and references
- ASHRAE Handbook — Fundamentals, Chapter 1 (Psychrometrics) — Complete moist air property formulations
- ASHRAE Standard 41.6 — Standard method for humidity measurement
- Stull, R. (2011) — "Wet-Bulb Temperature from Relative Humidity and Air Temperature," Journal of Applied Meteorology, 50(11), 2267-2269
- WMO Guide to Meteorological Instruments, Chapter 4 — Wet bulb psychrometer standards
- ISO 7726 — Ergonomics of the thermal environment — Instruments for measuring physical quantities
Summary: the four ways to find relative humidity
- From wet bulb and dry bulb — The standard psychrometric method, using a sling psychrometer or aspirated wet bulb. Use our Wet Bulb to RH calculator.
- From dew point and dry bulb — When a chilled mirror dew point sensor is available or when the weather report gives Tdb and Tdp.
- From the psychrometric chart — For graphical estimation when a quick visual reference is sufficient.
- From electronic sensors — Modern capacitive polymer sensors report RH directly, but require periodic calibration (drift of ±2-3% per year is typical).
For all other HVAC and process engineering calculations, explore our HVAC Conversions hub for the complete suite of tools: sensible heat, latent heat, air changes per hour, CFM per ton, and more.
Last reviewed: June 2026. Psychrometric formulations per ASHRAE Fundamentals 2021, Chapter 1. Wet bulb formula from Stull (2011).