Absolute humidity inside and out, and the moisture a swap moves
Enter indoor and outdoor temperature and relative humidity, the room volume in cubic feet, and how many times over you swapped that volume of air. The page returns absolute humidity on both sides in grams per cubic meter, the difference between them, and the mass of water that moves when that much air is exchanged. If the outdoor air is the damper of the two, the direction line says the moisture is coming in rather than going out.
The formula and its units
Saturation vapour pressure is es = 6.112 × exp(17.67 × T ÷ (T + 243.5)) in hectopascals, and actual vapour pressure is that figure times relative humidity. Absolute humidity is 216.74 × e(hPa) ÷ T(K). The same relationship is often written as 2.1674 × e(Pa) ÷ T(K), and that coefficient only holds when vapour pressure goes in as pascals. Because the saturation formula here produces hectopascals, this page stays consistently with 216.74 and prints that choice beside the result. Other coefficient pairs exist, including the World Meteorological Organization figures, but the 17.67 and 243.5 pair used here is accurate to roughly half a percent from −45°C to 60°C. Fahrenheit inputs are converted to Celsius internally. Current as of October 2026.
Limits and disclaimer
This page does not calculate a dew point and does not judge condensation risk; it converts humidity figures and masses only. The air you swap is treated as perfectly mixed, so a real exchange will differ. No target indoor humidity range and no ventilation frequency are suggested, because no authoritative figure for either is offered here.
Frequently Asked Questions
They are the same relationship in different units. Use 2.1674 when vapour pressure goes in as pascals and 216.74 when it goes in as hectopascals. The saturation formula on this page produces hectopascals, so 216.74 is used.
It means the outdoor air holds more water per cubic meter than the indoor air, so exchanging air brings moisture in instead of taking it out. The direction line then reads that the moisture is entering the room.
No. It reports absolute humidity and the mass of water a ventilation swap moves. Dew point calculation and risk verdicts are not supported here.