How to use this aquarium evaporation calculator
Measure how much water restored the same reference waterline over a known number of days. Exclude water changes, leaks, spills, and sampling. Enter that total and the observation period, then choose future planning days and an allowance. If comparing an ATO reservoir, enter only usable water above the pump's required minimum level. Leave the optional reservoir field empty if that quantity is unknown.
How the calculation works
The observed average is measured evaporative replacement divided by observation days. Multiplying that rate by planning days gives the base volume. The extra allowance increases this base by the percentage you enter. Its purpose is to make a planning assumption visible, not to certify a sufficient reserve. The example's 20% can be edited and is not derived from a safety standard or a forecast of your room conditions.
Evaporation varies with conditions. Room humidity, air movement, temperature, exposed water surface, covers, and changes in equipment operation can all make the next period differ from the observation period. This calculator does not ask for those variables or estimate a physical evaporation coefficient. A record collected under one setup should not be treated as a calibrated prediction after a major change to that setup.
A useful observation compares the same waterline at the start and end. Record additions specifically made to replace evaporative loss, including additions supplied automatically if you can measure them reliably. Do not count a reservoir refill twice or include a water change as evaporated water. If other removals or leaks cannot be separated, the data do not identify evaporation and the extrapolation will mislabel the missing water.
Usable reservoir capacity divided by the daily planning rate gives a theoretical interval. That is different from container brim capacity and different again from an unattended-operation guarantee. Pump minimum depth, sensor placement, tubing, controller limits, power availability, and failure modes remain outside the arithmetic. Follow equipment instructions and inspect operation independently. A larger calculated reservoir does not establish that its installation is safe.
Top-off and water changes serve different quantity balances. A top-off restores water lost through evaporation without first draining existing tank water. A water change removes tank water and replaces it. Do not use the top-off quantity as a water-change percentage or as permission to add salt, medication, or mineral products. Replacement-water composition and any dosing need a separate, appropriate method; this calculator gives only water volumes and a conditional time interval.
Formulas
Observed daily rate
Future quantity
Optional reservoir comparison
Worked examples
A ten-day quantity estimate
A record shows 7 L replaced specifically for evaporation over 7 days. The observed average is 1 L/day. Ten planning days need 10 L before allowance. An explicitly chosen 20% adds 2 L, giving 12 L. An 8 L usable reservoir corresponds to about 6.667 days at the increased planning rate, not a certified safe absence.
Planning in US gallons
A measured 2 US gallons over 5 days averages 0.4 US gallon/day. For 14 days, base need is 5.6 US gallons. A chosen 25% allowance gives 7 US gallons. A usable 3 US gallon reservoir divided by 0.5 US gallon/day gives a six-day theoretical interval under those assumptions.
Common uses
Budget top-off water
Estimate the amount to prepare over a chosen period using a recent, representative observation. Keep the allowance separate from the observed daily rate so you can distinguish what was measured from what was added as a planning assumption.
Compare usable reservoirs
Compare theoretical refill intervals for containers using their actual deliverable water, not their advertised total capacity. A blank optional input leaves this result unavailable while preserving the observed rate and quantity estimate. No pump or sensor reliability is inferred from the comparison.
Accuracy and limitations
Observation and planning days must be positive; measured loss can be zero. The allowance is limited to a visible numerical range and is never silently added. A zero observed rate yields no finite interval rather than an Infinity label or a claim of unlimited autonomy.
Short or unrepresentative observation periods can distort the average. Reassess after changes to covers, airflow, heating, or room conditions. The model does not detect leaks, account for equipment faults, or substitute for appropriate supervision and product instructions.
Sources and methodology
References support unit conversions, relevant material or energy calculations, and husbandry context where listed. NumUtility's geometric and arithmetic models are explained above; they do not establish safe stocking, maintenance requirements, or structural capacity.
Frequently asked questions
Can tank dimensions alone predict evaporation?
Not with this tool. It requires an actual observation record and does not model heat transfer, humidity, or airflow. Use a representative recent record and revise it when conditions change.
What if I observed no loss?
Zero observed replacement returns zero estimated quantities but no finite reservoir interval. A zero record does not prove that future evaporation will be zero or that a reservoir will last indefinitely.
Should I enter the full container capacity?
Use water the installed system can actually deliver above its required minimum, not the volume to the brim. Follow the equipment's operating requirements and consider installation risks separately.
Does the allowance guarantee enough water?
No. It is a user-chosen scenario. Unexpected evaporation, other water losses, or equipment faults can invalidate the projection. The calculator does not approve leaving animals or equipment without appropriate supervision.
Are my measurements saved?
No. Calculation happens in your browser without an account. NumUtility does not receive or store the values entered into these fields. Keep your own measurement log if you want to compare observations later.