Evaporation Rate Calculator
Enter the water surface area, wind speed, air temperature, relative humidity, and water surface temperature to find how fast water evaporates. Results update instantly in kilograms, litres, or gallons per hour or per day. The step-by-step panel shows every part of the ASHRAE humidity-ratio formula, and the ACI concrete risk flag tells concrete crews whether evaporation is high enough to trigger plastic-shrinkage cracking.
Formula
Worked example
A 10 m² pool with 3 m/s wind, water at 28 °C, air at 25 °C, and 50% RH: Xs(28) = 0.02399 kg/kg, Xs_air(25) = 0.02024, X = 0.5 × 0.02024 = 0.01012, dX = 0.01387. Coefficient = 25 + 19×3 = 82. Rate = 82 × 10 × 0.01387 = 11.37 kg/h (273 L/day).
How the evaporation rate formula works
The evaporation rate from an open water surface is governed by the humidity ratio difference between the saturated air right at the surface and the ambient air above it. The formula used here is derived from ASHRAE heat and mass transfer principles and is widely applied in pool engineering and concrete practice: gh (kg/h) = (25 + 19 × v) × A × (Xs - X). The coefficient (25 + 19v) accounts for both natural and forced convection, where v is wind speed in m/s and 25 represents the quiescent (still-air) base. A is the exposed water surface area in m². Xs is the saturation humidity ratio of air at the water surface temperature, and X is the actual humidity ratio of the ambient air calculated from relative humidity. A higher wind speed, larger surface, hotter water, and drier air all increase the rate. If the air is fully saturated at the water temperature, the driving force Xs - X falls to zero and evaporation essentially stops.
Saturation humidity ratio and how it is calculated
The saturation humidity ratio Xs (kg of water vapour per kg of dry air) grows steeply with temperature. At 10 °C it is about 0.0077 kg/kg; at 20 °C it is about 0.0147 kg/kg; at 30 °C it is about 0.0273 kg/kg; at 40 °C it is about 0.0492 kg/kg. This is why hot water evaporates so much faster than cool water. The calculator uses a cubic polynomial fitted to psychrometric data valid over 0-40 °C: Xs = 3.733e-3 + 3.2e-4 × T + 3e-6 × T² + 4e-7 × T³. The ambient actual humidity ratio X is found by multiplying the saturation ratio at air temperature by the relative humidity fraction.
Pool evaporation: how much water you lose per day
A residential pool of 50 m² (about 540 ft²) at 28 °C water temperature, 25 °C air, 50% relative humidity, and a 2 m/s breeze loses roughly 28-35 litres per hour, or around 650-840 litres per day, if uncovered. That equals about 0.7-0.9 cm of depth per day. Adding a pool cover reduces evaporation by 90-95% because it eliminates the wind-driven driving force and keeps the air layer above the water saturated. The calculator lets you compare conditions by adjusting temperature, wind, and humidity to see how much a cover or windbreak saves.
ACI concrete evaporation risk
The American Concrete Institute (ACI 305R) recommends evaluating evaporation risk before placing concrete in hot, dry, or windy conditions. When the evaporation rate from the slab surface exceeds about 1.0 kg/m²/h (0.20 lb/ft²/h), there is a high risk of plastic-shrinkage cracking because water evaporates faster than bleed water can rise to replace it. Precautions include erecting windbreaks, applying evaporation retarder, pre-moistening the subgrade, shading the pour, or scheduling work for cooler, more humid periods. This calculator uses the same formula recommended in ACI 305R and flags the risk level automatically. The inputs needed are concrete (water) surface temperature, air temperature, relative humidity, and wind speed.
Typical evaporation rates by surface condition
| Surface / condition | Typical rate | Primary driver |
|---|---|---|
| Still indoor water (no wind) | 0.01-0.05 kg/m²/h | Vapor pressure gradient |
| Outdoor pool, calm day | 0.05-0.20 kg/m²/h | Wind + temperature |
| Outdoor pool, breezy (5 m/s) | 0.20-0.60 kg/m²/h | Wind speed |
| Fresh concrete, hot dry windy | 0.5-1.5 kg/m²/h | ACI 305 cracking risk |
| Open pond, summer afternoon | 0.1-0.5 kg/m²/h | Solar gain + humidity |
| Wet soil surface | 0.01-0.10 kg/m²/h | Soil saturation level |
| Cooling tower, industrial | 1.0-5.0 kg/m²/h | Forced air + heat load |
Order-of-magnitude reference values. Actual rates depend on area, wind, temperature, and humidity.
Frequently asked questions
What is the evaporation rate formula used here?
The calculator uses the ASHRAE/Hatch formula: gh = (25 + 19 × v) × A × (Xs - X), where gh is the evaporation rate in kg/h, v is wind speed in m/s, A is surface area in m², Xs is the saturation humidity ratio at the water surface temperature, and X is the actual humidity ratio of the ambient air. The humidity ratios are calculated from a polynomial fitted to psychrometric data.
Why does wind speed have such a large effect on evaporation?
Wind continuously replaces the humid air just above the water surface with drier ambient air, keeping the humidity ratio difference (Xs - X) large and the driving force strong. Without wind, the air layer directly above the water quickly becomes nearly saturated and evaporation slows to a trickle. The coefficient 25 + 19v grows linearly with wind speed, so going from calm (0 m/s) to a light breeze (3 m/s) more than triples the rate.
How do I calculate pool evaporation per day?
Enter your pool surface area, the typical wind speed, average air and water temperatures, and average relative humidity, then choose "litres per day" as the output unit. The calculator multiplies the hourly rate by 24 to give a daily total. For a more accurate annual estimate, run the calculation for each season with representative averages.
What does the ACI concrete risk flag mean?
ACI 305R (Guide to Hot Weather Concreting) identifies an evaporation rate of 1.0 kg/m²/h (0.20 lb/ft²/h) as the threshold above which fresh concrete is at high risk of plastic-shrinkage cracking. At rates above this level, the surface loses water faster than rising bleed water can replace it, leaving the surface in tension as it tries to shrink. The calculator automatically flags the risk as low, moderate, or high based on the area-normalised rate.
Is evaporation rate the same as evapotranspiration?
No. Evaporation refers to the direct conversion of liquid water to vapour from a free water surface. Evapotranspiration (ET) includes both evaporation from soil and transpiration from plants, so it accounts for root uptake and leaf stomata. Agricultural and hydrology models use ET (often via the Penman-Monteith or Hargreaves methods), while this calculator focuses on free-surface evaporation relevant to pools, ponds, tanks, and concrete.
Does salinity or water chemistry affect the evaporation rate?
Yes, slightly. Salt water has a lower vapour pressure than fresh water by an amount proportional to salt concentration. For typical seawater (about 3.5% salinity), this reduces the saturation humidity ratio Xs by roughly 2%, and for a typical residential saltwater pool (0.3-0.5% salt) the effect is less than 0.5%. For most practical purposes this is negligible and the fresh-water formula applies.