Use measured output to answer a batch-time question
How long RO/DI water takes depends on the amount you want and the output rate your installation actually produces under the observed conditions. A gallons-per-day rating is a specification under stated conditions, not a timed collection from your aquarium setup today. The RO/DI Water Production Calculator deliberately starts from measured product volume and elapsed time instead of treating a membrane rating as a guaranteed schedule.
Define product water as the collected output stream intended for the batch and keep it distinct from reject water. Also define where the measurement begins and ends: startup water, flushing and stored water can change the collection boundary. The estimate is conditional on a representative rate continuing. It is not an automatic filling controller or an assurance that a container can be left unattended.
Measure one coherent collection interval
Follow the exact system's setup and collection instructions, then record product-water volume over a known duration. For an example observation, collect 2 L over ten minutes. Output rate is 2/10 = 0.2 L/minute, equivalent to 12 L/hour. You do not need to collect for an entire hour to express the rate per hour; the time conversion does not create additional measured data.
If reject water is also measured, collect it over the identical interval and use a consistent system boundary. Combining ten minutes of product with an hour of reject would distort the ratio by a factor of six. Do not include an earlier flush in one container while excluding its time or product stream elsewhere. State unmeasured quantities instead of assuming they were negligible.
Calculate the chosen batch duration
Let Vp be measured product water, tobs the observation duration and Vgoal the desired product batch. Projected duration = Vgoal/Vp × tobs. With 2 L in ten minutes and a desired 20 L, duration is 20/2 × 10 = 100 minutes, or one hour forty minutes. The desired quantity is product water, not product plus reject.
Use consistent water units for both volumes and consistent time units in the calculation. The same test can be entered as 2 L over 600 seconds or ten minutes, and the physical duration should remain unchanged. A US-gallon and an imperial-gallon representation can also describe the same batch only after correct conversion. Simply keeping the number 20 while changing its unit changes the batch size.
Add reject and feed quantities only when measured
Suppose the same ten-minute observation also collected 6 L of reject alongside 2 L of product. Reject-to-product ratio is 6/2 = 3:1. If that ratio and rate persist, a 20 L product batch corresponds to 60 L reject and 80 L combined feed under the simple balance. The 20 L goal is not multiplied by four to obtain a different product amount; the additional volume belongs to the feed balance.
Recovery is product/(product + reject) × 100, so this observation gives 2/(2 + 6) × 100 = 25%. Recovery compares water quantities. It is not contaminant rejection or a TDS-removal percentage. The tool cannot determine water purity, membrane performance against a chemical criterion, DI resin life or suitability for a particular aquarium from those volume measurements alone.
Check rate changes rather than inventing correction factors
Manufacturer descriptions identify feed conditions and filter condition as reasons actual output can vary. Bulk Reef Supply specifically notes pressure, cold feed water and clogged prefilters in its production discussion. That establishes why a rating and an observation may differ; it does not provide one universal mathematical correction model for every membrane and system configuration.
If a second observation instead produces 1.5 L in ten minutes, the measured rate is 9 L/hour. A 20 L batch projects 20/1.5 × 10 = approximately 133.33 minutes, or two hours thirteen minutes twenty seconds. Use the observation corresponding to the conditions you are modeling. Do not average unrelated tests and call the result a validated forecast if conditions are expected to change.
Account for excluded time and uncertainty explicitly
The 100-minute example assumes constant observed production without interruptions. If the intended procedure includes separately timed setup, flushing or handling, those are additional planning components rather than quantities magically included in the rate. For an invented scheduling exercise, a separately established ten-minute task would make the combined arithmetic 110 minutes, but this is not a recommended system flushing duration.
Short tests and coarse volume markings can make rate estimates unstable. Repeat observations when appropriate under the manufacturer's procedure and keep records of conditions. This version does not average trials, predict temperature changes, calculate pressure corrections or supply a statistical interval. A precise duration display should therefore be read as a conditional projection, not a guarantee to the nearest second.
Use optional fields without manufacturing missing data
In NumUtility, enter measured product, collection duration and desired product quantity. Leave reject volume blank if it was not measured. The rate and duration can still be calculated, while reject, feed, ratio and recovery remain unavailable. Entering zero is a claim about an observed quantity, not a harmless substitute for a blank field.
If you enter a zero reject observation, a numeric recovery result reflects those entered numbers only. It is not proof of a zero-waste installation or permission to adjust a restrictor. Recheck unexpected observations and the equipment instructions. Invalid required positive values remove numeric answers; they should not be interpreted as a production time of zero or as authorization to proceed.
Keep production separate from water preparation and safety
Producing a batch, choosing its aquarium use and preparing its chemistry are separate steps. This calculator does not choose GH or KH targets, scale salt mix, validate a storage container or assess contamination. Use the RO-mixing and related guides for their separate arithmetic while following the applicable product and animal-care requirements. A measured flow rate cannot certify the resulting water.
This guide provides educational arithmetic, not water-quality, installation or animal-care approval. Record measured volumes, elapsed time, system boundary and excluded activities with the result. Follow manufacturer supervision and overflow-prevention requirements rather than treating an estimated duration as a failsafe timer. The defensible conclusion is conditional: if this measured output persists, the selected product quantity requires approximately this much collection time.