Humidity and evaporation are two sides of the same moisture balance. The relative humidity calculator reports how close the air is to saturation, using air temperature and dew point. The evaporation rate calculator estimates how fast an open water surface loses volume, using temperature, humidity, wind, and area — and relative humidity is one of its inputs, because humid air slows water loss. The two pages cross-link: the evaporation page compares humidity with the relative humidity calculator, and the relative humidity page points to the evaporation rate calculator for water loss from pools and ponds.
What each calculator does
The relative humidity calculator takes air temperature and dew point in Celsius or Fahrenheit. Fahrenheit values are converted to Celsius, and the calculator applies the Magnus-Tetens exponential approximation with constants 17.625 and 243.04: relative humidity is 100 times the ratio of the dew-point saturation term to the air-temperature saturation term. The display is clamped between 0 and 100 percent, the form rejects a dew point above the air temperature, and the result is labeled positive only inside the page’s 30-to-60 percent comfort band. The same moisture state is described from different angles: dew point is the saturation temperature, while relative humidity is the percent saturation at the current air temperature.
The evaporation rate calculator takes air temperature, water temperature, relative humidity, wind speed, and surface area. It estimates saturation vapor pressure at the water temperature with the exponential relation 610.7 times e to the power 17.27Tw over (237.3 + Tw), and actual vapor pressure from air temperature and relative humidity. The vapor pressure deficit is the difference, a wind term is 0.0018 times the effective wind speed times (1 + 0.15 times the water-minus-air temperature difference), and the volume rate combines that wind term, the deficit, and the surface area with water density and latent-heat constants. Liters per hour is that rate times 3,600, displayed with two decimals and floored at zero; gallons per day and millimeters per day are derived from it. It is a simplified open-water model, not the FAO Penman-Monteith reference evapotranspiration method.
Side-by-side
| Evaporation rate calculator | Relative humidity calculator | |
|---|---|---|
| Primary output | Water-loss rate: L/hour, gallons/day, mm/day depth loss | Saturation percentage with a comfort note |
| Key inputs | Air temp, water temp, RH, wind speed, surface area | Air temperature, dew point |
| Formula basis | Saturation/actual vapor pressure, VPD, wind term | Magnus-Tetens vapor-pressure ratio (17.625, 243.04) |
| What the number means | How fast open water is leaving a pool, pond, or tank | How close the air is to saturation at that temperature |
| Driving relationship | Relative humidity shrinks the vapor-pressure deficit | Dew point near air temperature pushes the ratio toward 100% |
| Typical uses | Pool and pond water-loss checks, watering context | Indoor comfort, condensation, mold risk, weather logs |
When to use which
Use the relative humidity calculator when your question is about the air: how close a basement, greenhouse, crawl space, or outdoor observation is to saturation, whether a surface will condense, or whether the room sits inside the page’s comfortable 30-to-60 percent band. It needs temperature and dew point for the same parcel of air.
Use the evaporation rate calculator when your question is about water: how much a pool, pond, or tank is likely to lose in a hot, windy, dry spell versus a humid, calm one, and which weather variables move the loss the most. The two tools combine naturally — check the air’s moisture state with the relative humidity calculator, then enter that humidity into the evaporation rate calculator to see its effect on loss.
Limits and disclaimer
The relative humidity result is only as meaningful as its inputs: temperatures and dew points must describe the same air, and the 30-to-60 percent comfort band is a page-defined range, not an official threshold. Mold risk depends on surface temperature, wetting duration, leaks, ventilation, and materials, not the room-air percentage alone. The evaporation estimate ignores solar radiation, covers, salinity, splash, and inflows, produces very small rates for ordinary backyard-pool inputs because of its unit structure, and is not a leak test — if measured loss far exceeds the estimate, leaks or plumbing losses may need investigation. Negative or tiny computed rates are clamped to zero.