Every open aquarium loses water to the air, day and night. You notice it as a falling water line, a salty tidemark on the glass, or a filter that starts splashing because the level dropped. This calculator estimates how many litres evaporate per day from your surface area, water temperature, and room humidity, so you can plan top-ups instead of reacting to a low tank.
Evaporation is fastest from a large surface. A long, shallow tank loses more water than a tall, narrow tank of the same volume, because more water touches the air. That is why surface area, not volume, is the main input here. Measure the length and width of the water surface in centimetres and multiply them. A tank one hundred centimetres long and fifty wide has a surface of five thousand square centimetres.
Temperature is the second driver. Warm water gives off vapour faster than cool water, so a tropical tank near twenty-six degrees evaporates much more than a coldwater tank. Heaters, lights, and warm rooms all push the rate up. Room humidity pulls the other way: dry air accepts vapour eagerly, while humid air is already near saturation and slows evaporation. A tank in a dry, heated winter room can lose water surprisingly fast.
To use the calculator, enter the surface area, the water temperature, and the room humidity as a percentage. If you do not know the humidity, fifty percent is a fair guess for a normal home, thirty for a dry heated room, and seventy for a humid room. The result gives a daily figure in litres and a weekly total. Use the weekly total to plan how much treated top-up water you need on hand.
Here is a worked example. A surface of five thousand square centimetres at twenty-five degrees and fifty percent humidity loses about zero point six eight litres per day in this calculator, or about four point seven litres per week. Raise the temperature to twenty-eight degrees and the daily loss rises to about zero point eight litres. Drop the room humidity to thirty percent and it rises further. Small changes in the room make a visible difference over a week.
Top-up water is not the same as a water change. Evaporation removes pure water and leaves minerals and salts behind, so topping up with tap water slowly raises hardness over time. For most freshwater tanks, topping up with treated tap water is fine if you also do regular partial water changes, which reset mineral levels. For soft-water or sensitive tanks, many keepers top up with reverse-osmosis or distilled water to avoid mineral build-up.
A falling water line causes practical problems beyond appearance. Hang-on filters splash and lose flow when the level drops, heaters can become exposed and overheat, and surface skimmers stop working. Fish lose swimming space, and the concentration of waste rises as water leaves. Checking the level daily and topping up little and often keeps equipment safe and water chemistry steadier than one large top-up per week.
Covers and lids cut evaporation sharply. A glass lid or tight-fitting cover can halve the daily loss, because vapour condenses on the lid and drips back. Open-top tanks look natural and suit plants that grow above the water, but they pay for it in evaporation. If you run an open-top tank in a dry room, expect the highest losses and plan top-ups accordingly. A lid also reduces dust, jumping fish losses, and heat loss.
Airflow matters as much as temperature. A tank near a radiator, a fan, an air vent, or a draughty window evaporates faster than the same tank in still air. Moving air carries vapour away and keeps the surface dry, which speeds further evaporation. If your measured loss is much higher than this calculator suggests, look for airflow first. Moving the tank or shielding it from a direct draught often fixes the gap.
Seasonal patterns are worth tracking. Winter brings heated, dry indoor air, so evaporation peaks even though the tank temperature is steady. Summer brings humid air but warmer rooms, so the result is mixed. Keep a simple note of the water level each Sunday, and you will learn your tank seasonal rhythm within a month. That habit also catches slow leaks early, because a leak drops the level faster than evaporation ever should.
A leak and evaporation look similar at first, so learn to tell them apart. Evaporation leaves a mineral tidemark and drops the level evenly, with dry floors and dry stand wood. A leak leaves damp wood, wet carpet, or water under the stand. If the level falls much faster than this calculator predicts and the floor is damp, check seams and equipment seals at once. Do not keep topping up a leaking tank and hoping it stops.
Finally, build top-ups into your weekly routine. Keep a dedicated container of treated water ready, mark the normal water level on the glass with a small sticker, and top up to that mark every day or two. Use this calculator after any tank change, such as removing a lid, adding a fan, or moving the tank to another room. Planned top-ups keep the water stable, the equipment safe, and the tank looking full all week.
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