Pool evaporation calculator
Evaporation is the dominant loss from any warm open water. Each litre that leaves takes 2.26 megajoules of heat with it.
%
m/s
Water lost per day
17 459.3L
- Drop in water level per day
- 545.60 mm
- Per month
- 531 418
- Per year
- 6 377 018
- Heat carried away per day
- 10 960.58 kWh
- Evaporation rate
- 42.6253 kg per m2 per hour
- Vapour pressure at the water surface
- 3.7735 kPa
- Vapour pressure in the air
- 1.4001 kPa
Show the working
- Surface area = 32
- Water temperature = 28
- Air temperature = 20
- Relative humidity = 60
- Wind speed at the surface = 1
- Hours covered per day = 16
- Vapour pressure at the water surface = 0.61094 * exp(17.625 * 28 / (243.04 + 28)) = 3.773 52
- pa_sat = 0.61094 * exp(17.625 * 20 / (243.04 + 20)) = 2.333 44
- Vapour pressure in the air = 2.333 44 * 60/100 = 1.400 06
- Evaporation rate = max((25 + 19 * 1) * (3.773 52 - 1.400 06) / 2.45, 0) = 42.6253
- uncovered_hours = 24 - 16 = 8
- Water lost per day = 42.6253 * 32 * (8 + 16 * 0.3) = 17 459.3
- Heat carried away per day = 17 459.3 * 2.26 / 3.6 = 10 960.6
- Per month = 17 459.3 * 30.4375 = 531 418
- Drop in water level per day = 32 > 0 ? 17 459.3 / 32 : 0 = 545.604
- Per year = 17 459.3 * 365.25 = 6 377 020
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- Carrier equation: evaporation rate is proportional to (25 + 19 v) x (pw - pa)
- latent heat of vaporisation of water is about 2.26 MJ per kg
A cover cuts evaporation by roughly 70%, which is what the covered-hours term assumes.
Evaporation continues in winter, and is often higher, because the vapour pressure difference is larger.