How this instrument works
Flow rate through a pipe is simply how much fluid crosses a cross-section of that pipe per unit of time, and it follows directly from two things: how fast the fluid is moving (velocity) and how much open cross-sectional area the fluid is moving through. Multiply velocity by area and you get a volume per unit time — cubic feet per second, in this instrument's internal units — which then just needs converting into whatever units your trade actually specifies.
In the US, that unit is almost always gallons per minute. Sprinkler and irrigation system design, residential and commercial pump sizing, fire-suppression flow testing and well-pump ratings are all specified in GPM, so an engineer or contractor working from a measured or target velocity needs that figure converted, not the raw cubic-feet-per-second number physics naturally produces. This instrument does that conversion in one step: it computes the pipe's circular cross-sectional area from the inside diameter you enter, multiplies by velocity, and applies the exact 448.831 gallons-per-cubic-foot-per-second conversion factor, a figure derived from the US gallon's exact legal definition of 231 cubic inches.
Pipe inside diameter, not outside or nominal diameter, is what actually matters here — a pipe's nominal size (like '2-inch PVC') is a trade label, and its true inside diameter, the number that determines how much water can physically pass through, is usually a little smaller once wall thickness is accounted for. Using the nominal size instead of the measured inside diameter is a common source of GPM estimates that run a bit optimistic.
- Enter the fluid's speed through the pipe into Flow velocity (ft/s) — measured or targeted, depending on whether you're checking an existing system or sizing a new one.
- Enter the pipe's true inside diameter into Pipe inside diameter (in) — use the actual measured bore, not a nominal trade size, for an accurate result.
- Read Pipe cross-sectional area (ft²) as an intermediate check — it's the circular area the velocity is being multiplied across.
- Read Flow rate (US gallons per minute) for the final GPM figure to compare against a pump, sprinkler zone or fire-flow specification.
- If your figures are in metric units (m/s and mm) rather than feet and inches, use this site's Liters per minute instrument instead — the same physics, converted for metric-unit trades.
Worked example — 5 ft/s through a 2-inch pipe
Enter 5 into Flow velocity (ft/s) and 2 into Pipe inside diameter (in). Pipe cross-sectional area (ft²) reads 0.021817 ft² — the area of a circle with a 2-inch diameter, converted to square feet by dividing the diameter by 24 (12 inches per foot, then halved to a radius) before squaring and multiplying by π.
Flow rate (US gallons per minute) reads 48.960 GPM. That comes from 5 ft/s × 0.021817 ft² × 448.831, the exact conversion between cubic feet per second and US gallons per minute — a flow rate in the range a residential well pump or a single sprinkler zone commonly runs at.
Questions
Why does the formula divide the diameter by 24, not 12?
Because two conversions happen at once: dividing by 12 converts an inch measurement to feet, and dividing by 2 converts a diameter to a radius, since the area formula needs the radius, not the diameter. Combining both steps into one division by 24 is a shortcut — (D/12)/2 is exactly the same as D/24 — that saves a step without changing the answer.
Where does the 448.831 conversion factor come from?
It comes from the exact legal definitions of the units involved: 1 US gallon equals 231 cubic inches exactly, and 1 cubic foot equals 1,728 cubic inches exactly, so 1 cubic foot equals 1,728/231 gallons. Multiplying that ratio by 60 seconds per minute gives 448.8311688..., rounded to 448.831 — a standard reference constant, not something specific to this instrument.
Should I use a pipe's nominal size or its actual inside diameter?
Always use the actual measured inside diameter if you can get it, since a pipe's nominal size (like '2-inch') is a trade label that doesn't always match the true bore once wall thickness is factored in — the real inside diameter is typically a bit smaller. Using the nominal figure in place of the measured one tends to overstate the flow rate, sometimes by a meaningful margin depending on the pipe schedule and material.
How is this different from the Liters per minute instrument on this site?
They compute the identical underlying physics — flow rate equals velocity times cross-sectional area — but this instrument takes feet-per-second velocity and inch diameters and reports US gallons per minute, the units American plumbing and irrigation trades specify in. The Liters per minute instrument takes metres-per-second velocity and millimetre diameters and reports liters per minute for metric-unit work. Use whichever matches the units on your spec sheet.
Can I use this for a non-round duct or channel instead of a pipe?
No — this instrument assumes a circular cross-section, computing area from a single diameter figure. A rectangular duct, open channel or irregular cross-section needs its area calculated separately (length times width for a rectangle, for instance) before multiplying by velocity to get flow rate; the 448.831 conversion factor still applies once you have that area in square feet.
What flow velocities are typical in residential plumbing?
Water supply lines are commonly designed for velocities in roughly the 4 to 8 ft/s range — fast enough to be efficient but slow enough to avoid excessive noise, erosion and water-hammer risk in copper or PVC piping. Drainage and irrigation lines often run at different target velocities depending on the specific system, so check the relevant plumbing code or irrigation design guide for the figure that applies to your situation.