Evaporation Rate Calculator
Calculate water evaporation rate from surface area, air speed, temperature, and relative humidity. Free online physics tool for pools, ponds, and open water bodies.
About This Calculator
The Evaporation Rate Calculator estimates how quickly water evaporates from an open surface based on environmental conditions. It uses the Dalton-type evaporation formula widely applied in hydrology, pool maintenance, agriculture, and civil engineering to compute water loss from ponds, reservoirs, swimming pools, and cooling towers.
The calculator requires four inputs: the surface area of the water body (m²), the air speed above the surface (m/s), the air temperature (°C), and the relative humidity (%). The maximum humidity ratio is derived from air temperature using a polynomial fit to psychrometric data (valid for 0–30 °C). The current humidity ratio is then computed from the relative humidity. The evaporation rate is calculated as gh = (25 + 19 × v) × A × (Xs − X), where v is air speed, A is surface area, Xs is the maximum humidity ratio, and X is the current humidity ratio.
The calculator outputs the evaporation rate in kilograms per hour and kilograms per day, along with the maximum and current humidity ratios. A breakdown table shows each parameter and intermediate value, and chart modes display the evaporation rates for quick visual comparison.
Regional Notes
India: Evaporation rates are high in most parts of India, especially during summer months (March–June). Typical residential swimming pools (8 m × 4 m = 32 m²) can lose 30–50 L/day during hot, dry conditions. The Central Water Commission uses similar evaporation models for reservoir management.
US: Pool owners in states like California, Texas, and Florida face significant evaporation losses. The National Oceanic and Atmospheric Administration (NOAA) provides local weather data for temperature, wind, and humidity readings. A typical 20 ft × 40 ft pool can lose 1–2 inches of water per week in summer.
UK: Due to the cooler and more humid climate, evaporation rates are lower than in tropical regions. Average summer temperatures of 18–22 °C with 70–80% humidity result in much slower evaporation. Garden ponds typically lose 1–3 cm of water per week depending on wind exposure.
Frequently Asked Questions
How does the Evaporation Rate Calculator work?
The calculator uses the Dalton-type evaporation formula gh = (25 + 19 × v) × A × (Xs − X), where v is air speed in m/s, A is surface area in m², Xs is the maximum humidity ratio at the air temperature, and X is the current humidity ratio. The maximum humidity ratio is computed using a polynomial fit to psychrometric data for temperatures between 0–30 °C.
Is the Evaporation Rate Calculator free?
Yes, it is completely free to use with no registration required. You can bookmark and share your calculations via the URL.
How do I use this evaporation rate calculator?
Enter your water body's surface area in m², the average air speed above the surface in m/s, the air temperature in °C, and the relative humidity in percent. Click Calculate to see the evaporation rate in kg per hour and kg per day along with a breakdown of all intermediate values.
What is the formula for evaporation rate of water?
The evaporation rate formula is gh = (25 + 19 × v) × A × (Xs − X), where gh is the evaporation rate in kg/h, v is air velocity in m/s, A is the surface area in m², Xs is the maximum humidity ratio of saturated air, and X is the current humidity ratio of the air.
How does wind speed affect evaporation rate?
Wind increases evaporation by continuously bringing fresh, dry air to the water surface, reducing the local humidity and allowing more water molecules to escape. Higher wind speeds result in a higher evaporation rate as shown by the 19 × v term in the evaporation formula.
How fast does pool water evaporate on a warm day?
A typical 30 ft × 15 ft pool (about 42 m²) under a light breeze of 3 mph (1.3 m/s), temperature of 28 °C, and 80% relative humidity loses roughly 10 kg of water per hour. Over a 6-hour afternoon, that amounts to about 60 litres (16 US gallons) of top-up water needed.
What is the maximum humidity ratio and how is it calculated?
The maximum humidity ratio Xs represents the maximum amount of water vapor air can hold at a given temperature. It is calculated using the formula Xs = 3.733×10⁻³ + 3.2×10⁻⁴×T + 3×10⁻⁶×T² + 4×10⁻⁷×T³, where T is temperature in °C (valid for 0–30 °C).
Can this calculator be used for swimming pools and ponds?
Yes, the evaporation rate calculator works for any open water body including swimming pools, garden ponds, reservoirs, and industrial water tanks. Simply enter the correct surface area, local wind speed, temperature, and humidity to estimate how much water you will lose to evaporation.