Skip to content

Authorized dealer for 12+ brands · Real people on the phone, Mon to Fri

Shop Now
About Us Contact Us FAQs Return and Refund Policy Shipping Policy Privacy Policy Terms of Service Disclaimer

Psychrometric Calculator: Grains per Pound and Dew Point

Enter the air temperature and either relative humidity or dew point to get grains per pound (GPP), dew point and vapor pressure. Tick the second reading to compare two spots, such as the air going into a dehumidifier and the air coming out.

Reading 1
Second value
Change the air pressure (high altitude)

29.92 inHg is the standard atmosphere at sea level. Use the station pressure (not the sea-level-corrected number) where you are; thinner air holds more grains at the same RH.

Reading 1

54.5 grains per pound

Dew point
50.5 °F
Relative humidity
50.0%
Vapor pressure
0.370 inHg (12.52 hPa)
Humidity ratio
0.00778 lb/lb

Results come from the National Weather Service formulas below and can differ slightly from a printed psychrometric chart, which uses fuller equations. For daily drying logs, keep using the same method so the readings compare.

How it works

Relative humidity alone does not tell you how much water is in the air, because warm air holds more. Grains per pound does: it is the weight of water vapor in a pound of dry air, with 7,000 grains to the pound. The calculator works it out in these steps:

es = 6.11 × 10^(7.5 × T ÷ (237.3 + T)) saturation vapor pressure, hPa, T in °C
e = es × RH ÷ 100 actual vapor pressure
w = 621.97 × e ÷ (P - e) grams of water per kilogram of dry air
GPP = w × 7 (7,000 grains per pound ÷ 1,000)
Dew point = 237.3 × L ÷ (7.5 - L), where L = log10(e ÷ 6.11)

Example: 70°F and 50% RH at sea level comes to about 54.5 grains per pound, with a dew point of about 50.5°F.

Comparing two readings gives the difference in grains between them. Restorers call the difference between the air entering a dehumidifier and the air leaving it the grain depression; at the same airflow, a bigger difference means more water removed.

Grains per pound chart

Grains of moisture per pound of dry air at sea level (29.92 inHg). Rows: air temperature. Columns: relative humidity.
Temp20%30%40%50%60%70%80%90%
60°F1523313846546269
65°F1827374655647483
70°F2233435466778899
75°F263952657891104118
80°F3046617692108124140
85°F36547290109127146165
90°F426384106128150172195
95°F497499124150176202229
100°F5786116145176206237269

Sources

National Weather Service, Vapor Pressure and Mixing Ratio calculation sheets. NIST Special Publication 811, Appendix B.9: grain, pound, inch of mercury and standard atmosphere.

Meters and dehumidifiers

Frequently asked questions

What is grains per pound?

It is the weight of water vapor in one pound of dry air, counted in grains (7,000 grains make a pound). Unlike relative humidity, it does not change when the air warms up or cools down without gaining or losing water.

How do you calculate dew point from temperature and humidity?

Find the actual vapor pressure from the temperature and RH, then solve the National Weather Service vapor pressure formula for temperature. At 70°F and 50% RH the dew point is about 50.5°F.

Why does GPP matter when drying a building?

It shows whether the air is actually getting drier. Relative humidity changes whenever the temperature changes, but GPP only falls when water vapor leaves the air, so it is the steadier number to log day to day.

Does altitude change the result?

Yes, a little. Lower air pressure means more grains per pound at the same temperature and RH. Open the air pressure setting and enter your station pressure; 29.92 inHg is sea level.

More tools

Guides, how-to and research on Discover