How to Use the Evaporation Rate Calculator
The Evaporation Rate Calculator lets you find out how fast water evaporates from any open surface — a pond, pool, lake, or industrial basin — using a proven physics formula. Enter a few easy-to-obtain measurements and get an instant result in metric (kg/h, L/h) or American (lb/h, US gal/h) units.
Step-by-step guide
- Choose your unit system — Metric (m², m/s, °C) or American/Imperial (ft², mph, °F).
- Enter the surface area of the water body. For a 20 ft² garden pond that is about 1.86 m².
- Enter the wind speed above the water surface. You can look this up in today's weather report.
- Select the input mode:
- Temperature & Humidity — easiest: just type the air temperature and the relative humidity percentage shown on any weather app.
- Humidity Ratios (Xs, X) — advanced: enter the maximum (saturation) and current specific humidity directly in kg/kg.
- Click Calculate. The result shows the hourly and daily evaporation rate plus the formula values used.
How to Calculate Evaporation Rate
The evaporation rate is calculated with the empirical formula published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE):
gh = (25 + 19 × v) × A × (Xs − X)
where:
- gh — evaporation rate in kg per hour;
- v — air velocity above the water surface in m/s;
- A — surface area of the water body in m²;
- Xs — maximum (saturation) humidity ratio of air at the water surface temperature (kg water / kg dry air);
- X — current specific humidity of the ambient air (kg water / kg dry air).
Daily Evaporation Rate Formula
The daily evaporation rate is simply the hourly rate multiplied by 24:
Gday = gh × 24
Because 1 litre of water weighs approximately 1 kilogram, the result in kg/h is numerically equal to L/h.
Maximum Humidity Ratio Formula (Xs)
When you enter temperature and relative humidity, the calculator uses this empirical polynomial (valid for 0–30 °C / 32–86 °F) to find Xs:
Xs = 3.733×10−3 + 3.2×10−4 T + 3×10−6 T² + 4×10−7 T³
where T is temperature in °C. The current humidity ratio is then X = Xs × (RH / 100).
Frequently Asked Questions
How fast does pool water evaporate?
A typical backyard swimming pool with a surface area of about 50 m² (540 ft²) loses roughly 1–3 kg (0.26–0.79 US gal) per hour on a warm, breezy summer day (25 °C / 77 °F, 50% RH, wind 3 m/s). Over 24 hours that amounts to 25–70 litres, which is why pool owners need to top up the water regularly in summer.
What factors affect the evaporation rate?
- Wind speed — the single biggest driver. Doubling the wind speed (from 2 to 4 m/s) nearly doubles the evaporation rate because it continuously replaces humid air above the surface with drier air.
- Temperature — higher temperatures raise Xs (the air can hold more moisture), so evaporation accelerates. A pond at 30 °C evaporates about 5× faster than the same pond at 5 °C (all else equal).
- Relative humidity — lower humidity means the difference (Xs − X) is larger, so more water escapes into the air. At 100% RH the evaporation rate drops to zero.
- Surface area — evaporation is directly proportional to area. A pond twice as large loses water twice as fast.
Is the formula accurate outside 0–30 °C?
The polynomial approximation for Xs is derived from psychrometric data and is most reliable between 0 °C and 30 °C (32–86 °F). Outside this range the formula may underestimate Xs (at high temperatures) or overestimate it (near freezing). For temperatures below 0 °C you should consider that ice sublimation, not liquid evaporation, dominates. For precision work at temperatures above 30 °C, switch to the Humidity Ratios input mode and enter Xs directly from a psychrometric chart or steam tables.
How do I convert the result to other units?
- kg/h → lb/h: multiply by 2.20462
- L/h → US gal/h: divide by 3.78541
- L/h → UK gal/h: divide by 4.54609
- kg/day → tonnes/month: multiply by 30 then divide by 1000
What is the specific humidity (humidity ratio)?
Specific humidity (or humidity ratio) X is the mass of water vapour per kilogram of dry air. It differs from relative humidity: relative humidity compares current vapour pressure to saturation pressure, while X gives the actual mass fraction. Meteorological stations typically report relative humidity, but Xs and X are what the evaporation formula needs. The calculator converts automatically when you choose the Temperature & Humidity mode.