Evaporation is a critical, yet frequently disregarded, element in swimming pool upkeep, particularly for natural pools and BioPonds. In these systems, maintaining water balance is paramount, as it directly impacts overall system efficiency and ecological health.
The calculation of evaporation loss in swimming pools, measured in millimetres per day (mm/day), is an essential metric for effective water management and sustainable design practices.
In this guide, we explain how to calculate evaporation in millimetres per day (mm/day)—a simple, practical unit widely used in water management and ecological design.
What Does Evaporation in mm Mean?
Evaporation loss measured in millimetres represents the depth of water lost from the pool surface.
A key principle:
1 mm of water loss over 1 m² = 1 litre of water
This makes calculations straightforward and highly useful for both design and maintenance.
Basic Formula for Evaporation (mm/day)
If you know how much water is lost: Evaporation (mm)=Surface area (m²)Water loss (litres)
Example
- Pool area = 50 m²
- Water loss = 200 litres/day
= 4 mm/day
Reverse Calculation (Used in Design)
When planning a pool, you typically start with an estimated evaporation rate: Water loss (litres/day)=Area (m²)×Evaporation (mm/day)
This helps determine:
- Daily make-up water requirement
- Tank sizing
- Overflow and drainage planning
Typical Evaporation Rates (Kerala Conditions)
Based on field data and models such as the ASHRAE evaporation model:
- Coastal humid climates (Kerala): 3–5 mm/day
- Peak summer exposure: up to 6 mm/day
- Indoor pools: 1–2 mm/day
For most BioPools, a working design value of 4–5 mm/day is recommended.
Simple Site Method to Measure Evaporation
For real-world verification:
- Mark the pool water level
- Ensure no rainfall and minimal usage
- Measure the level drop after 24 hours
👉 The drop in mm equals the evaporation rate.
Bucket Test (More Accurate Method)
To isolate true evaporation:
- Place a bucket filled with pool water inside the pool
- Mark both levels
- Compare after 24 hours
This eliminates errors from:
- Splash loss
- Leakage
- System circulation effects
💧 Pool Evaporation Calculator
BioPool Considerations
Natural pools behave differently from conventional pools:
- Larger surface areas increase exposure
- Aquatic plants contribute to transpiration
- Lower water temperatures slightly reduce evaporation
In practice:
BioPools may experience 10–20% higher overall water loss
Design Insight from Tony’s Bioponds
Evaporation should be treated as a core design parameter, not just a maintenance issue.
We recommend accounting for:
- Daily and seasonal evaporation variation
- Monsoon rainfall contribution
- Make-up water systems
- Overflow management
A properly designed system ensures long-term sustainability and minimal manual intervention.
Conclusion
Calculating evaporation in mm/day is simple, practical, and essential for maintaining water balance in any pool—especially BioPools. With the right approach, you can accurately estimate water loss, optimise design, and ensure efficient long-term performance.
Typically, outdoor pools lose 3–5 mm of water per day in humid coastal regions like Kerala. This can increase during hotter, windier conditions.
Multiply evaporation (mm) by pool area (m²):
1 mm over 1 m² = 1 litre
The simplest method is to mark the water level and measure the drop after 24 hours under no-use conditions.
Yes. BioPools may lose 10–20% more water due to larger surface areas and plant transpiration.
Yes. Rainfall must be excluded when measuring evaporation, or it will distort results.
High temperature, Wind exposure, Low humidity, Direct sunlight, Water movement
