# Evaporation Rate Calculator

URL: https://speedycalculator.com/calculators/physics/evaporation-rate/
Last updated: 2026-10-30

> The evaporation rate calculator estimates how much water evaporates per hour from an open water surface – a pool, pond or tank – from its area, the air speed above it, the water temperature and the air temperature and humidity.

## Inputs

- Water surface area (m², cm², mm², km², in², ft²)
- Air speed above the surface (m/s, km/h, mph, ft/s, knots, km/s)
- Water temperature (°C, °F, K)
- Air temperature (°C, °F, K)
- Relative humidity (%)

## Results

- Saturation humidity ratio at the water temperature
- Humidity ratio of the air
- Evaporation rate
- Per day
- Heat lost to evaporation

## Formula

```
g = Θ·A·(x_s − x)/3600 kg/s,  Θ = 25 + 19·v (kg/(m²·h)),  x = 0.622·p_v/(p − p_v)
```

## Explanation

Evaporation draws water and heat from pools and reservoirs. It grows with wind speed, warm water and dry air. Pool covers save water and heating energy by stopping it.

### How to use the Evaporation Rate Calculator

- **Water surface area**
- **Air speed above the surface**
- **Water temperature**
- **Air temperature**
- **Relative humidity (%)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the water surface area, the air speed above it, the water temperature and the air temperature and relative humidity.

### Tips

- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.

### Common mistakes to avoid

- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).

### Inputs explained

FieldWhat to enterWater surface areaUnits: m², cm², mm², km², in², ft²Air speed above the surfaceUnits: m/s, km/h, mph, ft/s, knots, km/sWater temperatureUnits: °C, °F, KAir temperatureUnits: °C, °F, KRelative humidity (%)A number

### What each result means

- **Saturation humidity ratio at the water temperature** = 0.622×6.112×exp(17.62×[Water temperature]÷(243.12 + [Water temperature]))÷(1013.25 - 6.112×exp(17.62×[Water temperature]÷(243.12 + [Water temperature])))
- **Evaporation rate** = (25 + 19×[Air speed above the surface])×[Water surface area]×([Saturation humidity ratio at the water temperature] - [Humidity ratio of the air])
- **Per day** = [Evaporation rate]×24
- **Heat lost to evaporation** = [Evaporation rate]×2.45e6÷3600÷1000

### Example values

How the main result changes when **Water surface area** changes and everything else stays at the example values above:

Water surface area (m²)Evaporation rate (kg/h)2512.240437.518.36075024.480962.530.60117536.721310048.9618

## Worked example

Inputs — Water surface area: 50 m²; Air speed above the surface: 0.5 m/s; Water temperature: 28 °C; Air temperature: 25 °C; Relative humidity (%): 50.

Results — Saturation humidity ratio at the water temperature = 0.024045; Humidity ratio of the air = 0.009853; **Evaporation rate = 24.4809 kg/h**; Per day = 587.5411 kg/day; Heat lost to evaporation = 16.6606 kW.

## Key terms

- **Humidity ratio**: Mass of water vapour per mass of dry air.

## FAQ

**Why does wind increase evaporation?**
It carries away moist air from the surface.

**How much water can a pool lose?**
Often 3–7 mm per day outdoors.

**Why does a pool cover save energy?**
Evaporation carries away a lot of heat.

**Can evaporation be negative?**
Yes – if the air is more humid than saturation at the water temperature, condensation occurs.

**What formula does the Evaporation Rate Calculator use?**
g = Θ·A·(x_s − x)/3600 kg/s,  Θ = 25 + 19·v (kg/(m²·h)),  x = 0.622·p_v/(p − p_v).


## Sources

- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)
- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)