Start with measured loss, not a universal tank-size rule
To estimate ATO reservoir water for a planning period, divide measured evaporative replacement by observation days, then multiply by future planning days. Apply any extra allowance explicitly: planning volume = daily average × planning days × (1 + allowance / 100). This is a record-based quantity estimate. It is not a physical evaporation forecast or a guarantee that a particular reservoir lets an aquarium run safely without attention for that period.
Tank capacity alone cannot establish a universal daily top-off quantity. Two installations with similar water volumes can have different exposure and operating conditions. NumUtility's Aquarium Evaporation Calculator therefore asks what you observed rather than assigning a fixed percentage of tank volume. Its output is conditional on that record being representative. A chosen reserve makes an assumption visible, but does not turn a historical average into a reliable worst-case limit for future operation.
Build an observation record with a consistent waterline
Choose a repeatable reference waterline and record water added specifically to restore evaporative losses over a known period. For an automatic system, use a reliable record of actual delivery or reservoir usage that excludes unrelated refills and removals. Start and finish at comparable water levels. Otherwise the apparent loss can include an initial level difference rather than only evaporation. The instrument and container marks used to measure additions also introduce uncertainty.
Keep water changes, cleaning removals, sampling, leaks, and spills out of the evaporation total. A log of 1.5 L, 1.8 L, and 1.5 L added solely for evaporation over six days totals 4.8 L. If a separate 20 L water change occurred, do not add that to the 4.8 L or count its replacement again as top-off. When the different losses cannot be separated, you do not have a clean evaporation record for this calculation.
Worked example: daily average and future preparation volume
For 4.8 L of measured evaporative replacement over six days, average daily loss is 4.8 / 6 = 0.8 L/day. For a hypothetical twelve-day planning period, the base requirement is 0.8 × 12 = 9.6 L. An explicitly chosen 25% allowance adds 9.6 × 0.25 = 2.4 L, producing 12 L as the planning quantity. These inputs are illustrative, not a recommended unattended interval or a standard safety percentage.
The distinction between observed and planning rates is useful. The observed rate stays 0.8 L/day, while the allowance-adjusted rate is 0.8 × 1.25 = 1 L/day. You should not describe the latter as a measured evaporation rate. Recording both makes the estimate auditable: someone reviewing the plan can tell how much came from observation and how much came from the chosen assumption. A larger allowance changes the scenario but does not certify its adequacy.
Use deliverable water rather than the reservoir's brim capacity
The container's advertised capacity is not always usable ATO water. Some water may remain below the installed pump's required minimum level, and the intended fill level may be below the brim. Use the actual quantity the installed system can deliver while meeting its operating requirements. Do not infer that quantity from outside dimensions alone. Follow the manufacturer's instructions rather than running a pump dry merely to find a more exact theoretical capacity.
Suppose an illustrative 12 L container has 9 L of confirmed usable water. Divide 9 L by the previous example's planning rate of 1 L/day to obtain a nine-day theoretical interval. Using its 12 L brim capacity would incorrectly report twelve days for this installation. At the observed 0.8 L/day alone, 9 L would correspond to 11.25 days; the shorter displayed interval reflects the chosen allowance, not a second measurement or a built-in equipment safety assessment.
A US-gallon example and an independent unit check
Consider a record of 2.4 US liquid gallons replaced over six days. Daily average is 0.4 US gallon/day. Ten planning days require 4 US gallons before allowance; a chosen 25% produces 5 US gallons. A reservoir with 3 US gallons of usable water divided by the adjusted 0.5 US gallon/day gives a six-day theoretical refill interval. The result is a volume-to-rate relationship, not a promise about sensor or pump reliability.
The same record in liters should preserve that interval. Multiply US gallons by 3.785411784; the observation becomes about 9.085 L, usable capacity about 11.356 L, and planned quantity about 18.927 L. Convert unrounded values for the comparison rather than chaining rounded displayed results. Imperial gallons use 4.54609 L each and describe a different unit. An unlabeled gallon container can introduce a larger error than the last few decimals of the calculation.
Representative conditions matter more than extra decimal places
Evaporation can change when the installation or room conditions change. Covers, exposed surface, airflow, heating, and humidity can make an old average less representative. The calculator does not ask for those inputs and does not simulate heat or moisture transfer. It simply projects a measured record at a constant rate. Refresh the observation after meaningful changes instead of assuming that a fixed liters-per-day figure is a permanent property of the aquarium.
A scenario check can illustrate that uncertainty without pretending to forecast it. For 9 L usable water, a rate of 0.6 L/day corresponds to fifteen days before any added allowance, while 1.2 L/day corresponds to 7.5 days. Those chosen rates are not universal expected bounds; they show how the interval responds if the actual rate differs. More decimals in a constant-rate projection do not protect against an observation that misses the conditions encountered later.
Reservoir sizing is not unattended-operation approval
ATO equipment restores a water level, but a quantity calculation does not evaluate the entire installation. Sensor behavior, pump operation, minimum depth, tubing arrangement, controller limits, power, and the consequences of faults need independent consideration. Follow the instructions for the specific system and arrange appropriate supervision. An arithmetic interval cannot establish that any aquarium, animal population, or equipment arrangement can safely remain without attention until the modeled reservoir-empty date.
Do not automatically choose the largest possible reservoir on the basis of this calculation. More available water can change the consequences of uncontrolled delivery, and the tool does not assess overflow or other failure scenarios. Top-off also differs from a water change: it restores evaporated water without first draining the existing water. The quantity is not permission to add salt, medicine, buffers, or other products; water composition and dosing require a separate appropriate method. This is educational planning, not animal-care advice.
Enter a record and interpret missing or zero observations honestly
In the Aquarium Evaporation Calculator, select liters, US liquid gallons, or imperial gallons. Enter the measured evaporative replacement, observation days, planning days, and your explicit allowance. For the first example enter 4.8 L, 6 days, 12 days, and 25%. Optionally enter 9 L usable reservoir water. Read observed average 0.8 L/day, planning volume 12 L, and reservoir interval 9 days; the base quantity of 9.6 L appears separately below the result cards.
Leave usable capacity blank when it is unknown. The interface then shows a prompt rather than inventing a numeric interval, while retaining the other quantity estimates. Zero observed replacement produces zero projected quantities but no finite interval: a zero record is not proof of unlimited future autonomy. Observation and planning periods must be positive, and invalid entries hide results. Keep your own dated log, because the browser calculation does not save observations or monitor actual equipment and water levels afterward.