Enter the freshwater volume needed to restore your aquarium to its normal operating level over a measured period. Include any top-offs made during that period. Exclude water removed for maintenance, leaks, spills and other losses.
Use litres for every volume and days for every period; fractional days are allowed. Reserve allowance is an additional volume in litres, not a percentage. Reservoir water means the usable freshwater available above the pump’s minimum operating level, not the container’s advertised capacity.
The calculator uses numeric values in the labelled units. It does not convert gallons, millilitres or hours automatically.
The average-volume-rate relationship is volume divided by elapsed time, as described in OpenStax, Flow Rate and Its Relation to Velocity. Here it is applied to measured evaporative loss. Multiplying that rate by the planned interval gives expected replacement volume; adding your chosen reserve gives the planned supply. Dividing usable reservoir volume by the rate gives its estimated duration. These planning equations are direct arithmetic derivations, not an aquarium evaporation prediction model.
Duration includes all usable reservoir water: the reserve allowance is not subtracted from it. A negative supply margin means more water is needed to meet the planned interval plus reserve. The allowance is your chosen buffer, not a recommended universal amount.
These estimates assume the measured rate continues. Repeat measurements when conditions change. Refill before the estimated run-out time; a volume estimate does not establish that an automatic top-off system can operate unattended.
| Inputs | Daily evaporation | Expected top-off | Supply including reserve | Reservoir duration | Surplus or shortfall |
|---|---|---|---|---|---|
| 6 L lost over 3 days; plan 7 days; reserve 4 L; reservoir 20 L | 6 ÷ 3 = 2 L/day | 2 × 7 = 14 L | 14 + 4 = 18 L | 20 ÷ 2 = 10 days | 20 − 18 = +2 L |
| 1.5 L lost over 2 days; plan 10 days; reserve 2 L; reservoir 8 L | 1.5 ÷ 2 = 0.75 L/day | 0.75 × 10 = 7.5 L | 7.5 + 2 = 9.5 L | 8 ÷ 0.75 ≈ 10.67 days | 8 − 9.5 = −1.5 L |
| 0 L measured over 3 days; plan 7 days; reserve 4 L; reservoir 20 L | 0 L/day | 0 L | 4 L | Unavailable: division by zero | +16 L; zero measured loss does not establish indefinite duration |
Evaporation removes water while leaving dissolved salts behind, increasing salinity. NOAA explains this salt-concentration effect. For a saltwater aquarium, replace evaporative loss with suitable freshwater, not newly mixed saltwater. A saltwater change removes both water and dissolved salts and needs separately prepared replacement saltwater appropriate for the aquarium.
For a freshwater aquarium, use freshwater suitable for its water chemistry. Top-off replaces evaporated water; it does not remove accumulated dissolved substances or replace routine water changes. Do not count deliberately drained water as evaporation.
Use the Aquarium Water Change Calculator to plan water changes separately.
What input ranges are accepted?
Volumes accept 0–100,000 L in increments of 0.01 L. The measurement period accepts 0.01–365 days; the planned interval accepts 0–365 days, both in increments of 0.01 day. These are calculator input limits, not recommended aquarium operating ranges. Use a representative measurement period and equipment-appropriate reservoir volume.
What happens if the observed loss is zero?
Daily evaporation and expected top-off are zero, and planned supply equals the reserve allowance. Reservoir duration is unavailable because its calculation divides by zero. This does not mean the reservoir will last forever. Check measurement resolution and observe for longer.
Can I measure a drop in water level instead of a volume?
This calculator requires a volume in litres. A direct approach is to measure the freshwater needed to restore the same normal operating level, with equipment in the same operating state. Do not enter a height in millimetres or centimetres as litres.
Does a positive margin guarantee enough freshwater?
It means the entered usable volume covers the calculated demand plus your chosen allowance under a constant-rate assumption. It does not account for leaks, delivery failures or changes in evaporation. Unavailable results indicate a calculation failure and must not be interpreted as zero.
Formula and background sources
- OpenStax — College Physics 2e, section 12.1: volume-per-time rate relationship.
- NOAA — Sea Water: evaporation leaves salt behind and increases salinity.
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