How to use this ro/di water production calculator
Follow the system's setup and collection instructions, then record product-water volume and elapsed time. If measuring reject water, use the identical observation interval and a consistent system boundary. Enter the desired product-water batch. Choose liters, US liquid gallons or imperial gallons for all water fields and seconds, minutes or hours for the collection duration. Leave reject volume blank if it was not measured; do not use zero as a substitute for missing information.
How the calculation works
Measured production rate is collected product volume divided by elapsed time. Dividing the desired product quantity by that rate gives a projected duration. For example, a short observation can describe liters per hour without requiring the pump or membrane to run for a full hour. The projection assumes the observation remains representative. It does not include an automatic allowance for startup, flushing, interruptions, storage-tank backpressure, or changing operating conditions during a longer collection.
A system's gallons-per-day rating is not a measurement of what your installation produces today. Manufacturer descriptions identify operating conditions that can affect output, including feed pressure, temperature and filter condition. This tool deliberately asks for a timed collection instead of claiming to correct a rated output using unverified coefficients. Repeat observations when conditions change, and check equipment instructions separately. An unexpectedly slow result is a measurement to investigate, not a diagnosis of a particular failed component.
When reject water is also measured, divide reject volume by product volume to obtain the reject-to-product ratio. A ratio of 3:1 means three units of reject for each one unit of product, not that three quarters of the product is rejected again. Feed water for the selected boundary is their sum under this simple balance. This assumes no other uncounted stream or inventory change during the observation; mixed collection intervals would break that comparison.
Recovery uses a different denominator: product divided by the sum of product and reject, multiplied by 100. At a 3:1 reject-to-product ratio, recovery is 1 / (1 + 3) × 100 = 25%. Recovery is not membrane contaminant rejection, a TDS reading, or a water-quality certification. The calculator does not estimate DI resin life, verify chlorine removal, or decide whether product water meets the requirements of an aquarium, drinking-water use, or laboratory analysis.
All water amounts share one unit selector, so the measured product, optional reject and desired batch convert together. The resulting elapsed time and physical ratios should remain unchanged after a unit switch. US liquid and imperial gallons are explicitly separate choices rather than a generic gallon setting. Duration conversion changes only the representation of the observation: ten minutes, six hundred seconds and one sixth of an hour describe the same time, not three different production tests.
Formulas
Observed output rate
Projected batch duration
Optional water balance
Recovery, not contaminant rejection
Worked examples
A measured 2 L in ten minutes
Collecting 2 L of product over ten minutes gives 12 L/hour. A desired 20 L product batch would require 20 / 2 × 10 = 100 minutes if that rate stays unchanged. With no reject measurement, only rate and duration are numeric answers; the reject and total-feed cards ask for measured data rather than assuming a favorable ratio.
Add a simultaneous reject collection
If 6 L of reject was collected alongside those 2 L of product during the same ten minutes, reject-to-product ratio is 6 / 2 = 3:1 and recovery is 2 / 8 × 100 = 25%. The 20 L product scenario projects 60 L reject and 80 L total feed water. These conditional quantities exclude any additional flushing or separately handled startup water.
Common uses
Plan a product-water collection
Estimate the time associated with a batch you have already selected, using a recorded output test rather than a nameplate rating. The number is useful for comparing observations under stated conditions; it is not a timer controller and must not encourage leaving a filling container unattended.
Separate product, reject and feed quantities
Keep the three water streams conceptually distinct when considering a measured observation. If you later estimate costs, use the relevant tariff and actual billing basis separately. This worksheet does not include sewer allowances, filter replacement, electricity or a universal local water price.
Accuracy and limitations
The projection describes constant measured conditions only. Startup and flushing quantities must be accounted for separately rather than assumed included in a steady collection. Storage changes or unmeasured streams can prevent product plus reject from representing total inlet water for a particular observation.
An entered zero reject volume means a reported zero observation, not validated 100% recovery or permission to alter a flow restrictor. Recheck unexpected measurements and equipment instructions. Missing reject water leaves dependent outputs unavailable; invalid values remove every numeric result and conversion.
Sources and methodology
References support unit conversions, relevant material or energy calculations, and husbandry context where listed. NumUtility's geometric and arithmetic models are explained above; they do not establish safe stocking, maintenance requirements, or structural capacity.
Frequently asked questions
Can I enter only the membrane's GPD rating?
No. This version uses a measured volume and time, not rated gallons per day or a manufacturer-specific correction model. Rated and observed production are different kinds of information.
Why is total feed water missing when reject is blank?
The feed balance needs both product and reject quantities for the same interval. Blank means unmeasured, so the tool does not invent a waste ratio or report product alone as total feed.
Is recovery the same as TDS rejection?
No. Recovery compares water volumes. Contaminant rejection requires appropriate feed and product measurements on a consistent reporting basis. This calculator does not perform that analysis or certify water purity.
Are my product details and measurements saved?
No. Values are processed in your browser without an account and are not sent to NumUtility for calculation. Keep your own product instructions and dated measurement record if you need to repeat or audit a result.