Aquarium Evaporation Top-Off Calculator
Estimate how much fresh water to add back to your tank to replace what's evaporated, based on your tank's footprint and an observed evaporation rate (measured as inches of water level drop per week).
Depth lost, times footprint
Evaporation takes water off the top, so the volume is the surface area multiplied by how far the level fell:
drop (in) = (rate in/week / 7) × days
volume (gal) = length × width × drop / 231231 is the number of cubic inches in a US gallon. On a 48 × 13 in tank losing 0.25 in a week, seven days is a 0.25 in drop over 624 sq in of surface - 156 cubic inches, or 0.68 gallons. Leave it a fortnight and you owe the tank 1.35 gallons.
Measuring the rate, and replacing it correctly
- Find your own rate. Fill to a marked line, wait a week without topping off, and measure the drop with a ruler. Rates run from under 0.25 in a week under a tight lid to well over an inch on an open tank with a fan or skimmer.
- Use the water surface, not the glass. If you run a sump, its surface evaporates too and the level drops there, so include it.
- Top off with fresh water only - RO/DI, distilled, or dechlorinated tap. Salts and minerals do not evaporate, so replacing losses with salt water or unmixed conditioner drives salinity and hardness up week after week.
- Warm rooms, dry winter air and open tops all raise the rate; a glass lid roughly halves it.
Frequently asked questions
How much water evaporates from a 75 gallon tank in a week?
On the standard 48 × 13 in footprint at 0.25 in a week, about 0.68 gallons - under 1% of the volume. Uncovered and warm, the same tank can lose three or four times that.
Should I top off a saltwater tank with salt water?
No. Only the water evaporates and the salt stays behind, so topping off with salt water raises salinity every time. Use fresh RO/DI water for top-off and save mixed salt water for water changes.
How often should I top off?
Little and often. Daily, or an automatic top-off float, keeps the level and salinity steady; a large weekly refill swings both, which is harder on livestock than the evaporation itself.