Free GPP calculator Grains per pound, dew point and grain depression
Enter the temperature and relative humidity from your thermo-hygrometer to get grains per pound (GPP) and dew point. Add a dehumidifier outlet reading for the grain depression. Set your altitude for readings above sea level.
What the numbers mean
GPP is the weight of water vapor in the air, in grains per pound of dry air (7,000 grains make a pound). Relative humidity changes whenever the air warms or cools; GPP does not. That makes GPP the number to compare across the drying chamber, the unaffected area, outside and the dehumidifier outlet.
Dew point is the temperature at which that air would be saturated. Any surface colder than the dew point collects condensation.
Grain depression is the chamber GPP minus the dehumidifier outlet GPP: how many grains the unit takes out of each pound of air that passes through it. Logging it on every visit shows whether each dehumidifier is still pulling water out of the air.
GPP chart
Whole grains per pound at sea level. Above sea level the values are higher; use the calculator with your altitude.
| °F \ RH | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|
| 60°F | 15 | 23 | 31 | 38 | 46 | 54 | 62 | 69 |
| 65°F | 18 | 27 | 37 | 46 | 55 | 64 | 74 | 83 |
| 70°F | 22 | 33 | 43 | 54 | 66 | 77 | 88 | 99 |
| 75°F | 26 | 39 | 52 | 65 | 78 | 91 | 104 | 118 |
| 80°F | 30 | 46 | 61 | 76 | 92 | 108 | 124 | 140 |
| 85°F | 36 | 54 | 72 | 90 | 109 | 127 | 146 | 165 |
| 90°F | 42 | 63 | 84 | 106 | 128 | 150 | 172 | 195 |
| 95°F | 49 | 74 | 99 | 124 | 150 | 176 | 202 | 229 |
Dew point chart
| °F \ RH | 30% | 40% | 50% | 60% | 70% | 80% |
|---|---|---|---|---|---|---|
| 60°F | 29 | 36 | 41 | 46 | 50 | 54 |
| 70°F | 37 | 45 | 51 | 55 | 60 | 64 |
| 80°F | 46 | 54 | 60 | 65 | 69 | 73 |
| 90°F | 54 | 62 | 69 | 74 | 79 | 83 |
How it is calculated
- Saturation vapor pressure at the air temperature, from the Hyland-Wexler equations (ASHRAE Handbook, Fundamentals).
- Water vapor pressure = saturation pressure × RH.
- Air pressure at your altitude: 101.325 kPa × (1 − 2.25577 × 10⁻⁵ × altitude in meters)5.2559.
- Humidity ratio = 0.621945 × vapor pressure ÷ (air pressure − vapor pressure); GPP = humidity ratio × 7,000.
- Dew point: the temperature whose saturation pressure equals the vapor pressure, found numerically.
These are the same calculations the DryLog app uses, so the numbers match.
Questions
What does GPP mean in water damage restoration?
GPP is grains per pound: the weight of water vapor carried by one pound of dry air, measured in grains (7,000 grains make a pound). Unlike relative humidity, it does not change when the same air warms up or cools down, so you can compare the drying chamber, the unaffected area, outside air and the dehumidifier outlet directly.
How do I calculate grain depression?
Take a temperature and RH reading in the chamber and another at the dehumidifier outlet, work out the GPP of each, and subtract: chamber GPP minus outlet GPP. The difference is how many grains of water the unit removes from each pound of air passing through it.
Why does altitude change the GPP?
Air pressure falls with altitude. The same temperature and RH give the same water vapor pressure, but at lower total pressure that vapor is a larger share of the air, so each pound of dry air carries more grains. At 5,000 feet the GPP is roughly a fifth higher than at sea level for the same reading.
What GPP should a drying chamber reach?
There is no single number. Compare the chamber with the unaffected area and with outside air on the same visit, and follow the drying standard and procedures your company works to. The calculator gives you the numbers; the drying decisions stay yours.
Is GPP the same as humidity ratio or specific humidity?
GPP is the humidity ratio (mass of water vapor per mass of dry air) multiplied by 7,000 to express it in grains per pound. Specific humidity divides by the mass of moist air instead; at indoor conditions the two differ by about one percent.