Wet bulb and relative humidity calculator

Enter a measured dry bulb with RH, wet bulb or dew point and get every other humidity value (RH, wet bulb, dew point, absolute humidity, vapor pressure, specific volume, enthalpy) with the same equations as the chart calculator.

Definitions

Relative humidity RH (%): Ratio of the actual vapor pressure to the saturation vapor pressure at the same temperature. With the same moisture, it falls as air warms and rises as it cools.
Absolute humidity (g/kg dry air · g/m³): HVAC uses the humidity ratio, grams of vapor per kg of dry air; meteorology uses vapor density, grams of vapor per m³ of air. The humidity ratio does not change with temperature alone.
Wet bulb temperature (°C): The temperature a thermometer wrapped in wet wick settles at as water evaporates. The drier the air, the larger the gap to the dry bulb; at saturation they are equal.
Dew point (°C): The temperature at which condensation starts when the air is cooled at constant moisture. It depends only on the vapor pressure, so it carries the same information as the humidity ratio.

Psychrometer RH table (101.325 kPa)

Rows are dry bulb, columns the dry bulb minus wet bulb depression (K), cells the relative humidity (%). Computed with the same engine at standard pressure (ventilated psychrometer basis; unventilated screen tables differ slightly). A dash means the combination cannot exist.

Dry bulb / depression °C	1 K	2 K	3 K	4 K	5 K	6 K	7 K	8 K	9 K	10 K
10	88	77	66	55	44	34	25	15	6	-
15	90	80	71	62	53	44	36	28	20	13
20	91	83	74	67	59	52	44	38	31	25
25	92	85	77	70	63	57	51	45	39	33
30	93	86	79	73	67	61	55	50	45	40
35	93	87	81	75	70	64	59	54	49	45
40	94	88	82	77	72	67	62	57	53	49

Saturation vapor pressure by temperature

Saturation vapor pressure from the Hyland-Wexler equations (ASHRAE 2017 chapter 1) and the saturated vapor density, the most water vapor one m³ of air can hold at 100 % RH. Values below 0 °C are over ice.

Temperature °C	Saturation pressure kPa	Saturated vapor density g/m³
−10	0.2599	2.14
−5	0.4018	3.25
0	0.6112	4.85
5	0.8725	6.80
10	1.2280	9.40
15	1.7054	12.82
20	2.3388	17.29
25	3.1692	23.03
30	4.2460	30.35
35	5.6278	39.57
40	7.3835	51.09
45	9.5932	65.34
50	12.3499	82.81

Frequently asked questions

What is the difference between relative and absolute humidity?

Relative humidity is the share (%) of the most vapor the air can hold at that temperature; absolute humidity is the actual amount (g/kg or g/m³). Cool 25 °C, 50 % air to 15 °C and the humidity ratio stays 9.88 g/kg while RH rises to about 93 %.

How is wet bulb temperature measured and calculated?

Wrap a thermometer in a wet wick, blow about 2~5 m/s of air over it (aspirated psychrometer) and read it once it settles. The calculator gets the humidity ratio from the ASHRAE wet bulb equation and then RH. Example: 30 °C dry bulb and 25 °C wet bulb is about 67 % RH.

How is the discomfort index calculated?

Put dry bulb t (°C) and RH (%) into the Korea Meteorological Administration formula DI = 0.81 t + 0.01 RH (0.99 t − 14.3) + 46.3. For 30 °C and 70 %, DI ≈ 81.4. See the KMA guidance for its comfort bands.

Equations and sources

Saturation pressure p_ws(t): Hyland-Wexler (ASHRAE 2017 ch. 1 eq. 5 and 6, ice below 0.01 °C)
Relative humidity RH = 100 p_w / p_ws(t), humidity ratio W = 0.621945 p_w / (p − p_w)
From wet bulb: W = ((2501 − 2.326 t*) W_s* − 1.006 (t − t*)) / (2501 + 1.86 t − 4.186 t*); dew point: p_ws(t_d) = p_w (iterative)
Vapor density ρ_v = p_w / (R_v T), R_v = 461.5 J/(kg·K); specific volume v = 0.287042 (t + 273.15)(1 + 1.607858 W) / p
Discomfort index DI = 0.81 t + 0.01 RH (0.99 t − 14.3) + 46.3

Sources

ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: eq. 5 and 6 (saturation pressure), 20 and 22 (humidity ratio), 26 (specific volume), 32 (enthalpy), 33 and 35 (wet bulb)
Korea Meteorological Administration, life weather index: discomfort index formula

Next calculation

The usual order in HVAC design: fix the air states on the chart, check condensation, get the supply airflow from the load, size the ducts and pipes, then find the pump head. At commissioning, check the coil capacity from two measured points at its inlet and outlet.

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3VPD calculatorGreenhouse humidity as VPD (kPa) and HD (g/m³)
4Cooling load calculatorSensible, latent, SHF, supply air
5Ventilation rate calculatorACH, CMH and ASHRAE 62.1
6Duct size calculatorAirflow to round and rectangular duct
7Fan static pressure calculatorDuct losses to fan pressure and power
8Pipe sizing calculatorWater flow to nominal pipe size
9Pump head calculatorSegment losses to total head and shaft power
10NPSH calculatorSuction-side cavitation check
11Steam pipe sizingRecommend a steam pipe size by velocity or pressure loss
12Condensate return line sizingRecommend a trap discharge line size from flash steam velocity
13Saturated steam tableLook up saturation temperature, latent heat and enthalpy by pressure or temperature
14Coil capacity calculatorTwo measured points to coil capacity kW
15Economizer calculatorBring in outdoor air or not, saving kW
16Refrigeration ton to kW · unit converterRT, kW, Btu/h, CFM, m³/h, Pa, inWG

Dry bulb / depression °C	1 K	2 K	3 K	4 K	5 K	6 K	7 K	8 K	9 K	10 K
10	88	77	66	55	44	34	25	15	6	-
15	90	80	71	62	53	44	36	28	20	13
20	91	83	74	67	59	52	44	38	31	25
25	92	85	77	70	63	57	51	45	39	33
30	93	86	79	73	67	61	55	50	45	40
35	93	87	81	75	70	64	59	54	49	45
40	94	88	82	77	72	67	62	57	53	49

Dry bulb / depression °C	1 K	2 K	3 K	4 K	5 K	6 K	7 K	8 K	9 K	10 K
10	88	77	66	55	44	34	25	15	6	-
15	90	80	71	62	53	44	36	28	20	13
20	91	83	74	67	59	52	44	38	31	25
25	92	85	77	70	63	57	51	45	39	33
30	93	86	79	73	67	61	55	50	45	40
35	93	87	81	75	70	64	59	54	49	45
40	94	88	82	77	72	67	62	57	53	49

Dry bulb / depression °C	1 K	2 K	3 K	4 K	5 K	6 K	7 K	8 K	9 K	10 K
10	88	77	66	55	44	34	25	15	6	-
15	90	80	71	62	53	44	36	28	20	13
20	91	83	74	67	59	52	44	38	31	25
25	92	85	77	70	63	57	51	45	39	33
30	93	86	79	73	67	61	55	50	45	40
35	93	87	81	75	70	64	59	54	49	45
40	94	88	82	77	72	67	62	57	53	49