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
| Field | What to enter |
|---|---|
| Water surface area | Units: m², cm², mm², km², in², ft² |
| Air speed above the surface | Units: m/s, km/h, mph, ft/s, knots, km/s |
| Water temperature | Units: °C, °F, K |
| Air temperature | Units: °C, °F, K |
| Relative 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) |
|---|---|
| 25 | 12.2404 |
| 37.5 | 18.3607 |
| 50 | 24.4809 |
| 62.5 | 30.6011 |
| 75 | 36.7213 |
| 100 | 48.9618 |
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)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.
Frequently asked questions
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).