What velocity should I use for pipe sizing?

Liquid suction is typically 0.6–1.5 m/s, discharge 1.5–3.0 m/s, and gas/steam around 15–30 m/s depending on service, noise limits, and erosion constraints.

Is Nominal size (DN) equal to the inside diameter?

No, DN is a reference size. The actual inside diameter depends on the schedule (wall thickness). You must use the actual ID to calculate velocity.

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Piping

Pipe Sizing Calculator

Determine the right pipe diameter for a target flow rate and allowable velocity, for liquid and gas service.

When to use this calculator

Use when selecting line size for a new pipe run or checking velocity on an existing line.

Required inputs

Flow rate
Fluid density
Allowable velocity
Pipe schedule

Expected outputs

Required inside diameter
Recommended nominal size
Actual velocity

Formula overview

Uses SI units (m³/s, m/s, m) in the equation — the actual app supports m³/h, L/min, and imperial units.

A = Q / v, then D = sqrt(4A / π). Velocity is checked against typical limits for the fluid service.

Worked example

Example: Q = 0.05 m³/s, selected v = 2 m/s (pump discharge)
A = Q / v = 0.05 / 2 = 0.025 m²
D = sqrt(4 × 0.025 / π) = 0.178 m ≈ 178 mm
→ Select DN200 (8") and verify actual velocity.

Common mistakes

Using the same velocity for suction and discharge — suction should be lower to prevent cavitation.
Forgetting to round up to the actual nominal size and not re-checking the actual velocity.
Confusing inside diameter with nominal size (DN is not the true ID).