Evaporation Loss Estimator
Estimate how much water evaporates from an open storage — a dam, tailings pond, pit lake or process water pond — over a chosen period, from the pan evaporation rate, surface area and a pan factor. Results in m³, kL and ML.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the water-surface area either in hectares or in square metres (fill exactly one).
- Enter the pan evaporation rate in mm/day (from the nearest Class-A pan or climate data) and the number of days.
- Optionally adjust the pan factor (default 0.7); read off the loss in m³, kL and ML and the effective evaporation depth.
How it works
Evaporation is first expressed as a depth of water removed from the surface: effective depth (mm) = pan rate (mm/day) × days × pan factor. The pan factor scales a small evaporation pan's reading down to real open-water evaporation, because the pan heats faster than a large body of water; it usually sits between 0.6 and 0.8. That depth is then multiplied by the surface area to get a volume: volume (m³) = depth/1000 (mm to m) × area (m²). Hectares are converted with 1 ha = 10,000 m². Because 1 m³ = 1 kL, the kilolitre figure equals the cubic-metre figure, and 1 ML = 1,000 m³.
Worked example
Worked example. A tailings pond has a 10 ha surface, the pan evaporation rate is 8 mm/day, over 30 days, with a pan factor of 0.7. Effective depth = 8 × 30 × 0.7 = 168 mm. Area = 10 ha = 100,000 m². Volume = 0.168 m × 100,000 m² = 16,800 m³ = 16.8 ML lost to evaporation over the month.
Common mistakes
- Entering the area in both hectares and m² — enter exactly one; the tool converts hectares for you.
- Using the raw pan reading with no pan factor, which overestimates loss from a large water body by roughly 20–40%.
- Applying one average rate over a long period spanning seasons — evaporation is far higher in summer, so use monthly data for annual balances.
Frequently asked questions
What is a pan factor and why isn't it 1.0?
A Class-A evaporation pan is small and metal, so it warms and evaporates faster than a large water body. The pan factor (about 0.6–0.8) scales the pan reading down to the actual open-water rate. Large, deep storages sit at the lower end.
Does this account for rainfall or inflow?
No — it estimates gross evaporation loss only. For a net water balance you subtract evaporation from rainfall, runoff and inflows and add seepage separately.
Related tools
- Contaminant Plume Travel Time Estimator
- Streeter-Phelps DO Sag Calculator
- Turbidity to TSS Estimator
- Erosion Risk (RUSLE) Calculator
- Acid Mine Drainage Neutralisation Estimator
- Farm Dam Volume Calculator
Explore more in Marine, Outdoor, Agriculture, Land & Environment.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.