Estimate daily, weekly, and monthly water loss from your fish tank based on surface area, temperature, humidity, and lid coverage. See remaining volume and plan your top-off water.
Total system volume including sump if applicable.
A 55 gal tank ≈ 48"×13" = 624 in². Use the actual water surface, not tank footprint if lids/rims reduce it.
Typical: 0.3–0.8% open top, 0.1–0.3% glass lid.
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gallons per day
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gallons per week
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gallons per month (30d)
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gallons after 7 days
Estimated Refill Requirement
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gallons of fresh water needed for the selected period
Water level falls over time — plan top-offs before the level drops too far.
Estimate only. Real evaporation varies significantly with airflow, water temperature vs. room temperature, tank stocking, lid fit, and HVAC. Use this as a planning guide, not a precise measurement. Always top off with fresh RO/DI or dechlorinated water — never saltwater.
Water evaporation is a natural and constant process in every aquarium. Liquid water molecules at the surface gain enough energy to escape into the air as vapor. The rate depends on several physical factors, and understanding them helps you maintain stable water parameters.
Evaporation happens at the air–water interface. A wider, shallower tank with the same volume as a tall, narrow tank will evaporate far more water. This is why a 55-gallon long tank loses more per day than a 55-gallon tall tank. The calculator uses surface area in square inches as a primary input.
Warmer water holds more kinetic energy, increasing evaporation. A larger difference between water temperature and room temperature accelerates loss. Low humidity means the air can accept more moisture, so evaporation speeds up. In a dry, heated home (winter), evaporation can double compared to a humid summer room.
A tight glass lid traps vapor and can reduce evaporation by 70–90%. Mesh lids slow it moderately. Open-top tanks with fans blowing across the surface — common in reef tanks for cooling — can evaporate 1–2% of total volume per day. The calculator offers four lid/airflow presets to reflect these differences.
When water evaporates, it leaves dissolved salts, minerals, and trace elements behind. Only pure H₂O leaves the system. This means salinity rises as the water level drops. Topping off with saltwater would continuously increase salinity and harm livestock. Always top off with fresh RO/DI water (or dechlorinated tap for freshwater tanks). The calculator tells you how much fresh water to add.
Evaporation is a physical phase change that affects only water molecules. Dissolved ions like sodium, chloride, calcium, and magnesium are not volatile at aquarium temperatures — they stay in the tank. Over time, without top-off, salinity and mineral concentration rise. This is why an auto top-off (ATO) system is one of the best investments for reef and marine aquariums.
A 55-gallon freshwater tank with a glass lid, 72°F room, 45% humidity, and 624 in² surface area might lose about 0.25 gallons per day. Over a week that's 1.75 gallons — roughly 3% of total volume. Without a lid, the same tank could lose 0.5–0.7 gallons daily. A reef tank with fans could exceed 1 gallon per day. This calculator helps you quantify those differences.
More calculators to help you manage your tank.
Plan partial water changes by volume and percentage.
Calculate salt needed for brackish or treatment baths.
Size your heater for your tank volume and temperature rise.
Estimate fish stocking capacity for your tank size.