Dew point and evaporation rate are both moisture calculators, but one describes the air and the other describes a water surface. Dew point reports the temperature to which the air would have to cool to become saturated — a direct measure of how much water vapor is actually present. Evaporation rate estimates how fast open water is lost, and its driving force is the gap between the vapor pressure the water surface can supply and the vapor pressure the air actually holds. The two calculators connect exactly at that gap, the vapor pressure deficit.
What each calculator does
The dew point calculator takes air temperature and relative humidity and applies the Magnus approximation with constants a = 17.27 and b = 237.7: it computes gamma = aT/(b + T) + ln(RH/100) and solves Td = b × gamma / (a − gamma). The result is rounded to two decimals and shown in the selected unit plus Celsius. The page pairs the number with condensation checks — a surface at or below the dew point can collect moisture — and comfort rules of thumb: dew points below about 50°F feel dry to many people, the low 60s are noticeable, and the low 70s are often described as oppressive.
The evaporation rate calculator estimates water loss from a pool, pond, tank, or reservoir using air temperature, water temperature, relative humidity, wind speed, and surface area. It computes saturation vapor pressure at the water temperature as 610.7 × e^(17.27Tw/(237.3+Tw)) and actual vapor pressure from air temperature and relative humidity, then multiplies the vapor pressure deficit by a wind term of 0.0018 × Ue × (1 + 0.15(Tw − Ta)) and the surface area, dividing by the latent heat of vaporization and water density. Outputs are liters per hour, gallons per day, and a depth loss, each rounded to two decimals and floored at zero. If the deficit is negative — saturated air, or water cooler than the air — the result displays zero.
Side-by-side
| Dew point calculator | Evaporation rate calculator | |
|---|---|---|
| Inputs | Air temperature and relative humidity | Air temperature, water temperature, relative humidity, wind speed, surface area |
| Primary output | Dew point in the selected unit plus Celsius | Liters per hour, gallons per day, depth loss |
| What the number means | Temperature at which the air saturates at constant moisture | Volume of open water lost per unit time |
| Formula basis | Magnus approximation (a = 17.27, b = 237.7) | Exponential saturation vapor pressure (610.7 × e^(17.27T/(237.3+T))), wind term, and vapor pressure deficit |
| Humidity role | Input to the moisture-content calculation | Input to the actual-vapor-pressure term that sets the deficit |
| Precision | Two decimal places | Two decimal places, floored at zero |
When to use which
Use the dew point calculator when your question is about the air: how much water vapor is actually present, whether a window or wall will condense, whether leaves will stay wet overnight, or why a warm afternoon feels dry or muggy. It is the number to pair with surface temperatures, because condensation depends on the surface being at or below the dew point.
Use the evaporation rate calculator when your question is about water loss: how fast a pool or pond is losing water on a given weather day, and whether that loss changes with humidity and wind. The two tools work well together — a low dew point explains why a hot dry day pulls water out of a pool quickly, while a humid day with a high dew point leaves the deficit small and the loss near zero. For a pool, treat the result as a weather-based screening estimate; if measured loss far exceeds it, leaks may be involved.
Limits and disclaimer
Both calculators are educational estimates, not engineering instruments. The dew point is a Magnus approximation, accurate for common weather and indoor conditions but not for precision psychrometrics, and the form accepts zero percent humidity even though the logarithm is undefined there — extremely dry air needs a small positive value. The evaporation calculator is a simplified vapor-pressure and wind model, not the FAO Penman-Monteith reference evapotranspiration method: it ignores solar radiation, cloud cover, salinity, waves, covers, and rainfall, and its depth-loss conversion has a known unit quirk that makes displayed millimeters much smaller than the usual liter-per-square-meter relationship. Comfort descriptions are rules of thumb, not official limits.