Humidity Ratio Calculator

Convert between humidity ratio (gr/lb and kg/kg), relative humidity, dew point, and specific humidity. Live bidirectional from DB + any one humidity measure.

💧 Inputs

📊 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 Zone

📚 Reference Conditions

ConditionDB (°F)RH (%)W (gr/lb)
ASHRAE comfort — summer7550~65
ASHRAE comfort — winter7030~32
Cooling coil leaving air5590–95~55–60
Hot humid outdoor (Miami)91~74~163
Hot dry outdoor (Phoenix)108~13~60
Cold outdoor (Minneapolis)-16~80~3
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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 comfort-zone check.

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 pressurePws = 0.6112 · exp(17.67 · T ÷ (T + 243.5)) kPa (T in °C)
Humidity ratioW = 0.622 · Pw ÷ (P − Pw),   Pw = RH · Pws
Grains per poundWgr = W × 7000
Specific humidityq = W ÷ (1 + W)
Dew pointMagnus inverse of RH and T (a = 17.67, b = 243.5)

Saturation vapor pressure Pws uses a Magnus-type formula; vapor pressure is Pw = RH · Pws; and humidity ratio follows the mass-balance relation W = 0.622 · Pw ÷ (P − Pw) at standard sea-level barometric pressure (P ≈ 101.325 kPa, 14.696 psia). Grains per pound multiply W by 7000, specific humidity is W ÷ (1 + W), and dew point is the Magnus inverse. These relations follow ASHRAE Fundamentals Chapter 1 psychrometrics.

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 computes saturation pressure with a Magnus-type formula, vapor pressure as Pw = RH times Pws, and humidity ratio from W = 0.622 times Pw divided by (P minus Pw) at standard sea-level pressure. Dew point is the Magnus inverse of temperature and RH, and specific humidity is W divided by (1 plus W). These follow ASHRAE Fundamentals Chapter 1 psychrometrics.
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. The ASHRAE comfort zone generally falls between roughly 30 and 60 percent relative humidity at normal room temperatures; this tool flags whether your entered dry bulb and RH land inside that comfort band.

Related Tools

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