Does doubling the pipe diameter double the flow?

No — it roughly quadruples it at the same velocity, because flow area grows with the square of the diameter. In real systems the gain is even larger since friction losses drop too.

Where do I find the internal diameter of my pipe?

Manufacturer dimension tables list the ID for each nominal size and schedule. Copper type L, PEX, PVC schedule 40 and schedule 80 all differ at the same nominal size.

Can I use this for compressed air?

The area and velocity math is the same, and the CFM output is useful for a first pass, but compressed air is compressible so you should verify against free air delivery at your working pressure.

What velocity should I design to?

For domestic cold and hot water, 5–8 ft/s is the usual target. Hot recirculation lines are often held under 3 ft/s to limit erosion-corrosion in copper.

Does this include friction loss?

No. It gives theoretical volumetric flow. For pressure drop over a run, use a Hazen-Williams or Darcy-Weisbach calculation that accounts for pipe material and fittings.

Trade Calculators

Pipe Size / Flow Rate Converter

Cross-sectional area and GPM, L/min and CFM conversions.

Inputs

Given

Inside diameter

Diameter unit

Fluid velocity

fps

Velocity unit

Results

Computed

Flow rate

58.75 GPM

At 6.00 fps through a 2.000 in bore

Cross-sectional area3.1416 sq in

Cross-sectional area (metric)20.268 sq cm

US gallons per minute58.75 GPM

Litres per minute222.40 L/min

Cubic feet per minute7.854 CFM

Cubic metres per hour13.344 m³/h

This is a theoretical volumetric flow from area and velocity. Real systems lose capacity to friction, fittings and elevation, so size pipework from a pressure-drop calculation for anything critical.

58.8 GPM · 222.4 L/min · 3.142 sq in

FIG. 4 — FLOW = AREA × VELOCITY

How this is calculated

Volumetric flow is the pipe's cross-sectional area multiplied by the average fluid velocity. Area comes from the internal diameter:

area = π × (diameter ÷ 2)²

flow = area × velocity

Everything is computed in SI internally — metres and metres per second give cubic metres per second — then converted out: multiply by 15,850.32 for US gallons per minute, by 60,000 for litres per minute, and by 2,118.88 for cubic feet per minute.

Use the actual bore, not the nominal size. A nominal 2 inch schedule 40 steel pipe has an internal diameter of about 2.067 inches, while schedule 80 is only 1.939 inches — a 12% difference in flow area. Because area scales with the square of the diameter, going up one pipe size roughly doubles capacity at the same velocity.

Keep design velocities sensible: 5–8 ft/s (1.5–2.4 m/s) for domestic water supply, under 4 ft/s on suction lines, and under 2 ft/s for gravity drainage. Excessive velocity causes water hammer, noise and long-term erosion of fittings and elbows.

Worked example

Sizing a drain line for a workshop

A 2 in inside-diameter line must move water at 4 ft/s.
Cross-section = π × (1 in)² ≈ 3.14 sq in ≈ 0.0218 sq ft.
Flow = area × velocity = 0.0218 × 4 ≈ 0.087 cu ft/s.
That is about 39 US GPM, or roughly 148 L/min.
If the fixture load exceeds that, step up to a 2½ in or 3 in line rather than accepting higher velocity.

Common mistakes

Using nominal pipe size as the diameter
A nominal 2 in steel pipe has an inside diameter near 2.07 in; a 2 in copper tube differs again. Flow scales with diameter squared, so small ID errors swing capacity by 10% or more.
Designing to maximum velocity
Cold water lines are typically designed near 4–6 ft/s and hot water near 3–5 ft/s to limit noise, erosion and water hammer. Running at the theoretical maximum leaves no headroom.
Ignoring friction on long runs
Area and velocity give ideal flow. Real runs lose pressure to friction and fittings; long runs or many elbows need a pressure-drop check, not just a diameter check.