Air Density Calculator
Calculate air density from pressure, temperature, and humidity using the ideal gas law. Includes altitude reference table and moist air corrections.
Evaporation Rate Calculator
Calculate water evaporation rate from surface area, temperature, humidity, and wind speed with daily loss estimates and cooling power analysis.
Evaporation Rate⧉
0.1028 kg/m²·hr
Mass evaporation rate per unit area
Total Evaporation⧉
5.14 L/hr
Over 50 m2 surface area
Daily Loss⧉
123.3 L/day
2.47 mm depth per day
Depth Loss⧉
2.47 mm/day
Rate of water level drop due to evaporation
Cooling Power⧉
3,484 W
Evaporative cooling effect on the water body
Saturation Pressure⧉
3.168 kPa
At 25°C water temperature
Vapor Deficit⧉
1.584 kPa
Driving force for evaporation (sat − actual vapor pressure)
Evaporation vs Temperature
10°C
0.039 kg/m²h
15°C
0.055 kg/m²h
20°C
0.076 kg/m²h
25°C
0.103 kg/m²h
30°C
0.138 kg/m²h
35°C
0.184 kg/m²h
40°C
0.243 kg/m²h
Temperature Reference Table
| Temp (°C) | Sat P (kPa) | Deficit (kPa) | Rate (kg/m²h) | mm/day |
|---|---|---|---|---|
| 10 | 1.23 | 0.614 | 0.0393 | 0.94 |
| 15 | 1.71 | 0.853 | 0.0548 | 1.32 |
| 20 | 2.34 | 1.169 | 0.0755 | 1.81 |
| 25 | 3.17 | 1.584 | 0.1028 | 2.47 |
| 30 | 4.24 | 2.121 | 0.1384 | 3.32 |
| 35 | 5.62 | 2.811 | 0.1842 | 4.42 |
| 40 | 7.38 | 3.688 | 0.2429 | 5.83 |
| 45 | 9.58 | 4.791 | 0.3171 | 7.61 |
| 50 | 12.34 | 6.168 | 0.4103 | 9.85 |
Planning notes, formulas, and examples
About the Evaporation Rate Calculator
The evaporation rate calculator estimates how quickly water evaporates from an open surface based on temperature, humidity, wind speed, and surface area. Evaporation is driven by the vapor pressure deficit — the difference between the saturation vapor pressure at the water surface temperature and the actual vapor pressure of the surrounding air.
This process is critical in water resource management, swimming pool maintenance, cooling tower design, and climate science. A typical outdoor pool can lose 5–10 mm of water per day through evaporation during summer, while industrial cooling towers rely on controlled evaporation to reject heat. Understanding evaporation rates helps predict reservoir levels, design irrigation systems, and estimate water makeup rates for industrial processes.
The calculator uses a mass transfer approach based on the Penman-type equation, incorporating wind speed effects through the empirical relation for the convective mass transfer coefficient. It computes evaporation rate per unit area, total water loss, depth loss per day, and the evaporative cooling power — the energy removed from the water body as latent heat during phase change.
When This Page Helps
Evaporation rate estimation is critical for pool and spa maintenance, cooling tower makeup water calculations, reservoir water budgets, and agricultural irrigation planning. This calculator gives quick estimates based on easily measured parameters, helping pool owners estimate water costs and engineers size water supply systems.
How to Use the Inputs
- Enter the surface area exposed to air in m², ft², or cm²
- Enter the water surface temperature in °C
- Enter the ambient relative humidity as a percentage
- Enter the wind speed above the surface in m/s
- Review the evaporation rate, daily loss, and cooling power outputs
- Compare evaporation at different temperatures using the reference table
Formula used
Saturation vapor pressure: P_s = 610.78 × exp(17.27T/(T+237.3)) Pa. Vapor pressure deficit: VPD = (P_s − RH×P_s/100). Evaporation flux: E = (25 + 19v) × VPD / L where v = wind speed (m/s), L = latent heat ≈ 2.5 MJ/kg. Depth loss: mm/day = E × 24 × 3600 / ρ.Example Calculation
Result: ~109 L/day (2.2 mm/day depth loss)
A 50 m² pool at 25°C with 50% humidity and 1 m/s wind loses about 109 L/day to evaporation, equivalent to 2.2 mm of water level drop.
Tips & Best Practices
- Pool covers are the single most effective way to reduce evaporation — reducing it by 90%+
- Warm water evaporates much faster — a 35°C hot tub loses water several times faster than a 25°C pool
- Indoor pools still evaporate significantly due to low relative humidity in conditioned spaces
- In arid climates (RH < 20%), evaporation rates are 2–3× higher than humid climates
- Night evaporation is typically 50–70% of daytime rates due to lower temperature and no solar heating
Practical Guidance
Evaporation depends heavily on the air-water temperature difference, humidity, and wind speed. A small change in any of those inputs can meaningfully change the loss rate, especially for open pools and reservoirs.
Common Pitfalls
Do not treat a sheltered indoor pool the same as an exposed outdoor surface. Wind and air circulation can dominate the result, and covers reduce evaporation far more than small temperature adjustments.
Interpreting the Result
The output is a mass-transfer estimate, not a full weather model. It is useful for planning makeup water and cooling losses, but solar radiation, shading, and detailed boundary-layer effects can shift the real-world value.
Sources & Methodology
Last updated:
Frequently Asked Questions
A typical residential pool (40–60 m²) loses 100–300 liters per day depending on temperature, humidity, and wind. Pool covers can reduce this by 90%+.
Significantly. Wind removes the humid boundary layer above the water surface, maintaining a higher vapor pressure deficit. Doubling wind speed can increase evaporation by 50% or more.
VPD is the difference between the maximum water vapor the air can hold (saturation pressure) and what it actually holds. Higher VPD drives faster evaporation.
Higher humidity reduces the vapor pressure deficit, slowing evaporation. At 100% humidity, the air is saturated and net evaporation stops (though microscopic exchange continues).
Each kg of water that evaporates absorbs about 2.45 MJ of latent heat from the surface, cooling it. This is the principle behind swamp coolers, cooling towers, and perspiration.
This uses a simplified mass transfer model. For precise estimates, full Penman-Monteith or CFD models account for solar radiation, long-wave radiation, and detailed boundary layer effects.
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