Plumbing
Plumbing GPM Calculator
Check flow rate against fixture demand and the capacity of the pipe serving it.
Result
The timed fill is the honest measurement and it is worth doing. Fixture demand figures here are typical simultaneous flows rather than code fixture units, which are a different system used for sizing rather than for measuring.
Get a Free Business AuditWhat moves the number
- Fixture type, since a shower, a hose bib and a toilet fill have very different profiles.
- Supply pressure, which caps flow through any given restriction.
- Pipe size on the branch rather than on the main.
Where this one goes wrong
Diagnosing from the main when the branch is the problem. Good flow at an outside tap and poor flow upstairs points at the branch or the elevation, not at the incoming supply.
Questions
About this calculator
Where should I measure first?
At the closest outlet to the meter, then at the worst-performing fixture. The difference between them tells you whether the problem is incoming or internal.
Does a water softener affect flow?
It can, particularly if it is undersized for the demand or overdue for service. It sits in the whole-house path, so a restriction there shows up everywhere.
What if flow is fine but pressure is low?
Check for a pressure reducing valve that has failed or been set low. They are frequently the cause and are cheap to test before anything is opened up.
The formula
GPM = gallons / seconds x 60
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How to use this calculator
- Work it out from. Enter your own figure; the page starts at Timing a container.
- Container size (gallons). Enter your own figure; the page starts at 5.
- Seconds to fill it. Enter your own figure; the page starts at 38.
- Showers. Enter your own figure; the page starts at 1.
- Basin taps. Enter your own figure; the page starts at 2.
- Toilets filling. Enter your own figure; the page starts at 1.
The result updates as you type, so you can try a figure, see what it does to flow, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Work it out from | Timing a container |
| Container size (gallons) | 5 |
| Seconds to fill it | 38 |
| Showers | 1 |
| Basin taps | 2 |
| Toilets filling | 1 |
| Washing machines or dishwashers | 1 |
What comes out
| Result | Value |
|---|---|
| Flow main result | 7.9 |
| From fixture demand | 8.7 |
| From the timed fill | 7.9 |
| Litres per minute | 29.9 |
Filling 5 gallons in 38 seconds is 7.9 GPM.
How sensitive the answer is
Holding everything else steady and changing only container size (gallons), this is what happens to flow. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Container size (gallons) | Flow |
|---|---|
| 2.5 | 3.9 |
| 3.75 | 5.9 |
| 5 example above | 7.9 |
| 7.5 | 11.8 |
| 10 | 15.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Work it out from
- Choose from Timing a container, Fixtures running at once.
- Container size (gallons)
- A number, between 0 and 100, starting at 5.
- Seconds to fill it
- A number, between 0 and 3600, starting at 38.
- Showers
- A number, between 0 and 40, starting at 1.
- Basin taps
- A number, between 0 and 60, starting at 2.
- Toilets filling
- A number, between 0 and 40, starting at 1.
- Washing machines or dishwashers
- A number, between 0 and 40, starting at 1.
What the results tell you
| Result | What it is |
|---|---|
| Flow | The headline result. A decimal figure. |
| From fixture demand | A decimal figure. |
| From the timed fill | A decimal figure. |
| Litres per minute | A decimal figure. |
Before you rely on this number
Pipe sizing, flow and pressure figures depend on the actual fittings, the run, the material and local code. Code requirements vary by jurisdiction and override any general calculation, so check what applies where the work is happening.
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.