Humidity Ratio Calculator
Grains and pounds of moisture per pound of air.
Enter your values
Enter the dry bulb temperature in °F.
Enter the relative humidity in %.
Enter the barometric pressure in psia.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Humidity ratio
- 64.5 grains/lb
- In lb/lb
- 0.00921
- Vapour pressure
- 0.2145 psia
- Saturation pressure
- 0.4291 psia
- Dew point (approx)
- 55.1 °F
Quick answer
Humidity ratio is the mass of water vapour per pound of dry air, usually expressed in grains per pound.
W = 0.622 x Pw / (P - Pw)
At a glance
| What it does | Grains and pounds of moisture per pound of air. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Dry bulb temperature, Relative humidity, Barometric pressure |
| Main output | Humidity ratio |
| Formula | W = 0.622 x Pw / (P - Pw) |
| Cost | Free — no sign-up, no download |
How to use the Humidity Ratio Calculator
- 1Measure dry bulb temperature and relative humidity.
- 2Adjust barometric pressure if you are well above sea level.
- 3Use the grain difference across a coil to calculate latent load.
Inputs explained
- Dry bulb temperature(°F)
- Enter the dry bulb temperature in °F.
- Relative humidity(%)
- Enter the relative humidity in %.
- Barometric pressure(psia)
- Enter the barometric pressure in psia.
Worked example
Using the values the calculator loads with:
Inputs
- Dry bulb temperature75 °F
- Relative humidity50 %
- Barometric pressure14.696 psia
Results
- Humidity ratio64.5 grains/lb
- In lb/lb0.00921
- Vapour pressure0.2145 psia
- Saturation pressure0.4291 psia
- Dew point (approx)55.1 °F
Frequently asked questions
Why grains?
Grains per pound gives convenient whole numbers — there are 7,000 grains in a pound.
Does RH alone tell me moisture?
No — 50% RH at 90°F holds nearly three times the moisture of 50% RH at 60°F.
What do I need to enter into the Humidity Ratio Calculator?
Just 3 values: dry bulb temperature, relative humidity, barometric pressure. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Humidity Ratio Calculator work out the answer?
Humidity ratio is the mass of water vapour per pound of dry air, usually expressed in grains per pound. It applies the formula W = 0.622 x Pw / (P - Pw) and shows the working so you can check each step by hand.
Is the Humidity Ratio Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.