To calculate an observed well-water flow rate, divide collected volume by elapsed time. To use that number for filter planning, keep the measurement location, flowing pressure, observation duration, pump behavior and other demand with it. A short collection tells you an average outlet rate during that interval; it does not establish sustained supply at a treatment inlet.
Flow observation calculator and worksheet
How to calculate GPM from volume and seconds
Observed litres/minute = litres collected ÷ seconds × 60
US GPM = litres/minute ÷ 3.785411784
A hypothetical collection of 3 US gallons in 30 seconds gives 3 ÷ 30 × 60 = 6 US GPM, or approximately 22.71 litres/minute. This is calculated arithmetic, not a tested property of your well. A 30-second record cannot establish whether the supply maintains that rate for ten minutes.
Use the volume actually recorded, not an assumed container capacity. The conversion uses the US liquid gallon. A British Imperial gallon is a different unit; it cannot be entered as a US gallon. The NIST conversion table gives 3.785411784 litres per US gallon.
Outlet flow, pump delivery, well yield and available treatment flow
| Number | What it describes | What it does not establish |
|---|---|---|
| Outlet observation | Water delivered at a particular outlet during a recorded interval. | Delivery at another location or over an entire backwash cycle. |
| Pump delivery | Discharge under the pump’s actual operating conditions. | A universal rate independent of pressure, lift and restrictions. |
| Well yield / recovery | Water the well can supply under the conditions and duration of an appropriate well assessment. | That the installed pump and plumbing deliver the same rate to a filter. |
| Available treatment flow | Delivery at the relevant inlet under the required operating conditions and duration. | Contaminant removal, service-flow suitability or installation approval. |
These numbers can differ. A useful supply record names what was observed and how. For an explanation of the well, pump and pressure tank as separate components, see the National Ground Water Association’s Well Owner’s Guide.
The observation record to keep before choosing a filter
| Record | Details to preserve | Decision it supports |
|---|---|---|
| Location and restrictions | Exact outlet or inlet; hose, fixture, treatment stages or other restrictions in the measured path. | Whether the observation is relevant to the proposed treatment inlet. |
| Flow and units | Collected volume, collection seconds, original units and calculated rate. | A reproducible calculation without silently mixing US gallons and litres. |
| Pressure during flow | Lowest and highest observed pressure, gauge location and when readings were taken. | Comparison with documented inlet conditions. A static reading alone is insufficient. |
| Duration and behavior | Collection interval, broader observation duration, changing rate, interruptions and pump behavior. | Whether a brief rate has any evidence of being maintained. |
| Concurrent demand | Household use, irrigation or other loads present during the observation. | Whether the same operating scenario is being compared. |
| System arrangement | Known pressure-switch, variable-speed or storage/booster arrangement; unknown if not established. | Selection of an appropriate measurement and interpretation method. |
| Date and documentation | Date, relevant professional record, and source of equipment details. | A traceable record that can be checked against later conditions. |
How to interpret a rate that changes
Suppose three hypothetical equal-volume collections imply 8, 6 and 4 US GPM. Reporting only the first reading hides the decline. Reporting their average still does not show that a 6 GPM requirement was maintained. Keep the sequence and pressure observations; the cause and a suitable assessment belong with a qualified professional.
A conventional pressure-tank system can deliver water from stored drawdown during part of an observation. Nominal tank volume is not the water available between the operating pressure limits. For example, Amtrol’s MC10188 (04/24), printed page 6, lists the WX-202 as a 20-gallon tank with 5.4 gallons of drawdown at 40/60 psig. That model-specific table is not a measurement of your tank or a backwash-storage design.
Variable-speed and storage/booster arrangements need their own interpretation. Record the actual arrangement rather than applying a conventional tank-refill estimate to every installation. No single collection time on this page is a universal well-yield test.
A pressure-tank drawdown/refill estimate is another kind of record. The Genesis manual, printed page 8, describes drawdown divided by cycle time. That estimate must not be entered here as if its refill seconds were the seconds spent collecting water. Preserve the original method and ask a professional how it relates to treatment-inlet delivery.
When this record is ready for the backwash checker
Keep location, rate, flowing pressure and duration together.
Use the assembled-system manual and applicable configuration.
A missing duration or pressure condition keeps the decision open.
Proceed to the backwash flow checker when you have a supported supply observation and the exact system’s requirements. The checker asks for the lowest flow and minimum pressure maintained during the applicable observation. It will not automatically treat this calculator’s interval average as sustained supply.
Questions about measuring well-water flow
Can I use a five-gallon bucket result as my well’s GPM?
You can calculate an observed outlet rate from an appropriately obtained volume-and-time record. Calling it sustained well yield or available filter-inlet flow requires additional evidence. Location, restrictions, pressure and observation duration matter.
Is a pump rated at 10 GPM proof of 10 GPM at my filter?
No. A pump label or nominal rating does not establish delivery at your actual inlet conditions. Keep pump documentation and relevant observations separate.
Does 40/60 mean I have 40 psi throughout backwash?
No. A stated pressure-switch setting is not a record of flowing inlet pressure throughout a cycle. Record actual observations and applicable documentation.
How long should the observation last?
There is no universal duration specified by this calculator. The relevant comparison includes the documented cycle duration; a suitable professional assessment may need more than one cycle’s worth of evidence.
Does adequate flow mean the water is safe to drink?
No. This worksheet does not measure contaminants or establish drinking-water safety. Water testing and treatment-performance questions require separate assessment.
Sources, methods and technical review status
The formulas and hypothetical examples are our stated calculations. Manufacturer and industry sources support specific equipment distinctions; they do not independently validate this worksheet or your installation. Source check: 6 October 2026. Independent professional review of a field procedure has not been completed.
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