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 metres per second, in this instrument's internal units — which then just needs converting into whatever units your trade actually specifies.
Across most of the world outside the United States, that unit is liters per minute. Metric plumbing design, chemical and fertilizer dosing-pump ratings, drip-irrigation emitter specs and small-bore process piping are all commonly rated in LPM, so an engineer or installer working from a measured or target velocity needs that figure converted, not the raw cubic-metres-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 60,000-to-one conversion between cubic metres per second and liters per minute, a figure that follows directly from the SI definition of the liter as exactly 0.001 cubic metres.
Pipe inside diameter, not outside or nominal diameter, is what actually matters here — a pipe's nominal size (like 'DN50') is a trade label, and its true inside diameter, the number that determines how much fluid can physically pass through, is usually a little different once wall thickness and the specific pipe schedule are accounted for. Using the nominal size instead of the measured inside diameter is a common source of LPM estimates that run off from the real figure.
- Enter the fluid's speed through the pipe into Flow velocity (m/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 (mm) — use the actual measured bore, not a nominal trade size like DN50, for an accurate result.
- Read Pipe cross-sectional area (m²) as an intermediate check — it's the circular area the velocity is being multiplied across.
- Read Flow rate (liters per minute) for the final LPM figure to compare against a dosing pump, irrigation emitter or process-piping specification.
- If your figures are in imperial units (ft/s and inches) rather than metric, use this site's Gallons per minute instrument instead — the same physics, converted for US-unit trades.
Worked example — 1.5 m/s through a 50 mm pipe
Enter 1.5 into Flow velocity (m/s) and 50 into Pipe inside diameter (mm). Pipe cross-sectional area (m²) reads 0.00196350 m² — the area of a circle with a 50 mm diameter, converted to square metres by dividing the diameter by 2000 (1000 mm per metre, then halved to a radius) before squaring and multiplying by π.
Flow rate (liters per minute) reads 176.715 L/min. That comes from 1.5 m/s × 0.0019635 m² × 60,000, the exact conversion between cubic metres per second and liters per minute — a flow rate in the range a small commercial water heater or a 2-inch-class metric supply line commonly runs at.
Questions
Why does the formula divide the diameter by 2000, not 1000?
Because two conversions happen at once: dividing by 1000 converts a millimetre measurement to metres, 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 2000 is a shortcut — (D/1000)/2 is exactly the same as D/2000 — that saves a step without changing the answer.
Where does the 60,000 conversion factor come from?
It comes from the exact SI definitions of the units involved: 1 liter equals exactly 0.001 cubic metres, so 1 cubic metre equals 1,000 liters, and multiplying by 60 seconds per minute gives 60,000 — the exact conversion from cubic metres per second to liters per minute. This is a defined relationship, not an approximation, so no rounding is introduced by this step.
Should I use a pipe's nominal size (like DN50) or its actual inside diameter?
Always use the actual measured inside diameter if you can get it, since a nominal designation like DN50 is a trade label and doesn't always match the true bore once wall thickness and the pipe's specific schedule are factored in. Using the nominal figure in place of the measured one can meaningfully shift the calculated flow rate, particularly in thicker-walled pipe schedules.
How is this different from the Gallons per minute instrument on this site?
They compute the identical underlying physics — flow rate equals velocity times cross-sectional area — but this instrument takes metres-per-second velocity and millimetre diameters and reports liters per minute, the units most metric-world plumbing and dosing specs use. The Gallons per minute instrument takes feet-per-second velocity and inch diameters and reports US gallons per minute for American plumbing and irrigation trades. 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 60,000 conversion factor still applies once you have that area in square metres.
What flow velocities are typical for small dosing or drip-irrigation lines?
Small-bore dosing and drip lines often run at low velocities, commonly well under 1 m/s, since these systems prioritize precise, steady low-volume delivery over throughput. Larger metric supply and process piping is typically designed for higher target velocities, so check the relevant design standard or manufacturer's rating for the figure that applies to your specific system.