Humidity Ratio Calculator

Inputs

Dry Bulb Temp (°F)

Relative Humidity (%)

Elevation (ft)

All Humidity Measures

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Humidity Ratio (gr/lb)

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Humidity Ratio (lb/lb)

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Humidity Ratio (g/kg)

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Relative Humidity (%)

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Dew Point (°F)

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Specific Humidity (lb/lb)

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Partial Pressure H₂O (kPa)

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Comfort Check (rule of thumb)

Reference Conditions

Condition	DB (°F)	RH (%)	W (gr/lb, sea level)
Typical indoor design — summer	75	50	64.6
Typical indoor design — winter	70	30	32.5
Cooling coil leaving air	55	90–95	57.8–61.1
Hot-humid outdoor (Gulf Coast type)	91	60	132.1
Hot-dry outdoor (desert)	108	13	46.9
Cold outdoor (northern US winter)	-16	80	1.8
ASHRAE 55 upper humidity limit	—	—	84.0 (W = 0.012)

Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This humidity ratio calculator converts dry-bulb temperature and relative humidity into the moisture content of the air, expressed as humidity ratio in grains per pound, pounds per pound, and grams per kilogram. It also returns dew point, specific humidity, water-vapor partial pressure, and a typical comfort-band check, corrected for your site elevation.

Humidity ratio is the absolute moisture measure engineers track through a system, because it stays fixed during sensible heating and cooling and only changes when moisture is added or removed. That makes it the right variable for coil latent load, dehumidification, and verifying outdoor-air moisture against indoor design targets.

Formula & Method

Saturation pressure	P_ws: Hyland–Wexler equations (over ice below 32 °F, over liquid water above)
Humidity ratio	W = 0.621945 · P_w ÷ (P − P_w), P_w = RH · P_ws, P = 14.696 · (1 − 6.8754×10⁻⁶ · Z)^5.2559 psia
Grains per pound	W_gr = W × 7000
Specific humidity	q = W ÷ (1 + W)
Dew point	P_ws(T_dp) = P_w, solved numerically (frost point below 32 °F)

These are the ASHRAE Handbook—Fundamentals (2021) Chapter 1 moist-air equations in IP units — the same set as the AIM Works psychrometric calculator, so the same state reads the same on every page. Barometric pressure P comes from your elevation through the standard atmosphere (14.696 psia at sea level): at a fixed dry bulb and RH, humidity ratio rises as pressure falls. Grains per pound multiply W by 7000, g/kg by 1000, and specific humidity is W ÷ (1 + W). The comfort check is a rule of thumb (30–60 % RH at 68–76 °F) plus the ASHRAE Standard 55 upper limit W ≤ 0.012; it is not a full Standard 55 thermal-comfort analysis.

Frequently Asked Questions

What is humidity ratio?

Humidity ratio, also written as W, is the mass of water vapor per unit mass of dry air. It is reported in pounds of water per pound of dry air, in grains per pound (1 pound equals 7000 grains), or in grams per kilogram. Unlike relative humidity, it does not change when you heat or cool the air sensibly, which makes it the natural variable for tracking moisture through a system.

How do I convert grains per pound to pounds per pound?

Divide grains per pound by 7000, because there are 7000 grains in one pound. So 65 grains per pound equals about 0.00929 pounds of water per pound of dry air. To go the other way, multiply the lb/lb value by 7000. The calculator shows gr/lb, lb/lb, and g/kg side by side so no manual conversion is needed.

What formula does this calculator use?

It uses the ASHRAE Handbook—Fundamentals (2021) Chapter 1 moist-air equations: Hyland–Wexler saturation pressure, vapor pressure P_w = RH times P_ws, and humidity ratio W = 0.621945 times P_w divided by (P minus P_w), where P is the standard-atmosphere barometric pressure for your elevation. Dew point is the exact inverse of the saturation curve, and specific humidity is W divided by (1 plus W).

What humidity ratio is comfortable indoors?

For typical office comfort near 75 degrees Fahrenheit and 50 percent relative humidity, humidity ratio is about 65 grains per pound. ASHRAE Standard 55 sets no lower humidity limit; its graphic comfort-zone method limits humidity ratio to 0.012 lb/lb (84 grains per pound, about a 62 °F dew point). Many designers also aim for roughly 30 to 60 percent relative humidity at 68 to 76 °F as a rule of thumb. This tool flags air above the 0.012 limit and shows whether your entry falls inside that typical band.

Related Tools

Psychrometric Calculator Dew Point Calculator Wet Bulb / RH Enthalpy Calculator Coil Leaving Air

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.