Every reef and marine tank loses pure water to evaporation, concentrating the salt left behind and demanding a steady supply of fresh top-off water. This calculator estimates how much your tank evaporates per day from surface area, temperature, and humidity, then sizes an auto-top-off reservoir for one, three, or seven days.
How it works
Evaporation rate rises with the vapour-pressure deficit — the gap between the saturated vapour pressure at the water surface and the actual vapour pressure of the room air — and with surface area:
e_sat(T) = saturation vapour pressure of water at temperature T
deficit = e_sat(Twater) − RH × e_sat(Troom)
daily L = area(m²) × coefficient × deficit
reservoir = daily L × daysWarmer water raises e_sat(Twater), and drier air lowers the RH × e_sat(Troom)
term, both widening the deficit and speeding evaporation. The coefficient is tuned so
a typical open reef surface in average room conditions matches observed top-off rates.
What drives your evaporation rate
Water temperature is usually the biggest lever. The saturation vapour pressure of water rises sharply with temperature — a reef tank at 27°C evaporates noticeably more than one at 24°C running in the same room.
Room humidity is the second-biggest factor. A centrally heated home in winter may drop to 30–40% relative humidity, dramatically increasing the vapour-pressure deficit and accelerating evaporation. The same tank in a humid summer room evaporates much more slowly.
Surface area, not tank volume. Evaporation happens at the air-water interface. A 400 L wide reef display with a large open surface loses more water than a 400 L tall, narrow tank with a lid — even though both hold the same volume.
Air movement. A fan cooling the sump or lights positioned over open water increases evaporation by replacing the saturated boundary layer above the water surface with drier ambient air. Halide or T5 pendant fixtures over an open canopy are common sources of unexpectedly high evaporation.
Reservoir sizing guidance
| Use case | Recommended reservoir capacity |
|---|---|
| Daily check available | 1–2 days’ estimated evaporation + 20% margin |
| Weekend trips (2–3 days) | 3 days’ evaporation + 30% margin |
| Longer absences (5–7 days) | 7 days’ evaporation + 50% margin |
| Warm summer month, no AC | Add a further 20–30% — evaporation rises with room temperature |
Always include a margin, because a hot week, lower humidity after a weather change, or additional surface agitation can push daily evaporation above your estimate.
Key reminders
- Always top off with fresh RODI water. Only pure water evaporates — salt stays behind. Topping off with saltwater pushes salinity steadily upward, stressing corals and fish.
- Measure your actual top-off over a few days and use that figure, not the estimate, to size a long-term reservoir. Run the calculator to get in the right ballpark, then measure to confirm.
- ATO sensors fail. Size your reservoir conservatively — a float valve that sticks open in an oversized reservoir can flood your display. Use a secondary high-water-level sensor or reserve the reservoir to two days’ capacity maximum.