Swimming Pool Evaporation Calculator
A practical guide to VDI 2089 and ASHRAE evaporation calculations for indoor pools — with a free engineering-grade calculator.
Every indoor swimming pool continuously loses water to the surrounding air. That evaporation is the single largest driver of the HVAC load: it sets the dehumidifier capacity, the ventilation air flow, the pool-water heating demand, and the make-up water consumption. Getting the evaporation rate wrong — even by 20% — is the fastest way to end up with condensation on the ceiling, corrosion on the steelwork, and unhappy owners.
The physics in one equation
Evaporation from a free water surface is a mass-transfer process driven by the difference in water-vapor pressure between the pool surface and the room air:
E [kg/h] = β · A · (pw − pa) / R
- A — pool water surface area [m²]
- pw — saturation vapor pressure at pool water temperature [Pa]
- pa — partial vapor pressure of the room air [Pa]
- β — activity-dependent evaporation coefficient
- R — gas constant for water vapor
VDI 2089 activity factors
The German VDI 2089 standard is the most widely adopted method in Europe. It classifies pools by usage intensity and assigns an empirical activity factor:
- Unoccupied pool (covered): ~0.7
- Private / therapy pool: ~7
- Hotel / public pool: ~21
- Public recreational / wave pool: ~28–40
Higher activity means more surface disturbance, higher local air velocity, and therefore significantly more evaporation. A hotel pool can evaporate 4–5 times as much water as the same pool covered overnight.
ASHRAE approach
The ASHRAE Applications Handbook (Chapter 6) uses the Shah correlation, which has the same structure but derives β from empirical fits to instrumented pools. For typical 25–30 °C water and 26–30 °C / 55–60% RH room conditions, ASHRAE and VDI 2089 predict evaporation rates within 10–15% of each other. ASHRAE is the reference for US projects; VDI is the reference for European projects and for LEED-style envelope calculations.
Design conditions that matter
- Room air: 2–4 °C above water temperature, 50–60% RH — keeps evaporation manageable without discomfort.
- Water temperature: 26–28 °C typical, up to 32 °C for therapy pools — each +1 °C raises evaporation by ~7%.
- Air changes: minimum 4 ACH for the pool hall; below this, stratification and local condensation become likely.
Run the calculation
Our free calculator implements both VDI 2089 and ASHRAE, returns dehumidifier capacity, ventilation air flow, pool heater load, and make-up water, and generates an engineering report you can attach to a tender.
Frequently asked questions
How do you calculate swimming pool evaporation?
Use E = β · A · (pw − pa) / R with an activity factor β from VDI 2089 or ASHRAE, the pool surface area A, and the vapor-pressure difference between the water surface and the room air.
What is the difference between VDI 2089 and ASHRAE?
VDI 2089 is the German standard used across Europe; ASHRAE uses the Shah correlation and is the US reference. Both give comparable results for typical indoor pools.
Why does pool evaporation matter for HVAC design?
Evaporation sets the dehumidifier capacity, ventilation flow, and pool-water heater load. Undersizing leads to condensation, mold, and corrosion.