Pipe Diameter Calculator

Sizing a pipe is choosing a velocity

There is no such thing as the correct diameter for a flow rate — only the diameter that follows from a chosen velocity. The arithmetic is trivial once that choice is made, which is why the interesting engineering all sits in the velocity field. Go fast and pipes are cheap, compact and easy to route, but friction losses climb with roughly the square of velocity, the pump grows, and the running cost accumulates every hour for decades. Go slow and the reverse applies.

The second reality is that pipe is not sold on a continuum. A calculation returning 84 mm has no product behind it; the choice is the next standard bore, and that jump matters. Land in a 102 mm pipe and the actual velocity falls to about 1 m/s — comfortably lower than the target, with lower losses to match. Checking the resulting real velocity, rather than admiring the theoretical diameter, is what turns this into a design decision instead of an equation.

How to use the pipe diameter calculator

1. Enter the flow rate. In m³/h, litres per second or US gallons per minute — whichever your pump curve or specification uses; the tool converts internally.
2. Choose a design velocity. This is the real design decision. A discharge line at 2 m/s and the same line at 1 m/s differ by 40% in required bore and enormously in cost and friction.
3. Read the theoretical bore. The exact internal diameter that produces your chosen velocity — a target, not a purchasable item, since pipe comes only in standard sizes.
4. Take the suggested standard size. The tool picks the next standard bore at or above the requirement, because rounding down raises velocity and friction rather than lowering them.
5. Check the resulting actual velocity. Standard sizes jump in steps, so the real velocity in the selected pipe often lands well below target — confirm it still sits in an acceptable range.
6. Verify friction separately. Diameter sizing sets velocity; it does not tell you the pressure loss over a run. Follow up with the pressure drop calculator for the full picture.

Key formulas

• Continuity: Q = A × v, with A = π d² ÷ 4
• Required bore: d = √(4Q ÷ π v), Q in m³/s → d in metres
• Velocity in a chosen pipe: v = Q ÷ (π d² ÷ 4)
• Unit note: divide m³/h by 3,600 before applying the formula

Worked example

Sizing for 30 m³/h at a design velocity of 1.5 m/s: Q = 30 ÷ 3,600 = 0.008333 m³/s, so d = √(4 × 0.008333 ÷ (π × 1.5)) = 0.08410 m = 84.1 mm. No pipe exists at that bore, so the selection moves up to DN100 with a 102.3 mm bore — in which the actual velocity becomes 1.01 m/s, safely inside the acceptable band and with lower friction than the original target implied.

Standard bores, schedule 40 steel

Selected sizes from ASME B36.10M schedule 40. The calculator checks the full series from DN15 to DN400.

Things to keep in mind

• Size suction lines generously. One nominal size above the discharge is standard practice and protects the pump from cavitation.
• Diameter is a lifetime decision. Pipe is buried, boxed in and lived with; the pump behind it gets replaced several times over.
• Allow for future demand. Adding capacity later means re-piping — a modest allowance now is far cheaper than a second installation.
• Fittings add hidden length. Every bend, valve and tee behaves like extra metres of pipe; sizing on bore alone under-reads total loss.
• Verify with a pressure drop check. Velocity within range does not guarantee acceptable head loss over a long run.