How does the pipe size calculator work?

It sizes a water supply pipe from two numbers: the design flow the pipe must carry and a target flow velocity. It converts the flow from litres per minute to cubic metres per second, divides that by your velocity to get the cross-section area, then works back to the internal bore in mm. Finally it rounds up to the nearest standard nominal size stocked in India.

What inputs do I need and what values should I use?

You need the peak design flow in litres per minute and a target velocity in metres per second. For flow, add up the simultaneous demand the pipe serves rather than a single tap. For velocity, 1 to 1.5 m/s is a common supply band; the default here is 1.5. Staying in that range keeps the bore sensible without excessive noise.

What is a safe water velocity in a supply pipe?

Roughly 1 to 1.5 m/s is a comfortable target for most residential supply runs. Below about 0.9 m/s the pipe may be oversized for the duty. Above about 2 to 2.4 m/s you risk flow noise and, over years, erosion of the pipe wall. Higher velocity means a smaller bore, so treat the calculator output as a starting point, not a fixed answer.

Interactive Calculator · 2026

Pipe Size Calculator

Size a water supply pipe from flow and velocity. Enter the design flow the pipe must carry and the target flow velocity — get the required internal bore in mm and the nearest larger nominal pipe size to specify.

Required internal bore (at 1.5 m/s)0.0 mm→ nearest nominal size: 0 mm (In the common 1–1.5 m/s supply band.)

Required bore → nearest larger nominal pipe size

Your design point

Design flow (L/min)

Peak flow the pipe must carry — sum the simultaneous demand it serves.

Target velocity1.5 m/s

A velocity of 1–1.5 m/s is a common supply target — stay below ~2–2.4 m/s to limit noise and erosion.

Required bore

0.0 mm

Nearest nominal size

0 mm

Target velocity

0.00 m/s

Required bore vs chosen nominal size

The nominal size is picked one standard step above the exact bore the flow needs.

The required bore is the exact internal diameter that carries 30 L/min at 1.5 m/s. Real pipe comes in discrete sizes, so you round up to the next standard bore — here a 25 mm nominal pipe.

Nominal size is not the exact internal bore — verify against the manufacturer's bore table for your material (CPVC, PPR, GI, PEX) before you commit.

How this is calculated

Flow in m³/s = flow ÷ 60000 = 30 ÷ 60000 = 0.000500 m³/s.
Cross-section area = flow ÷ velocity = 0.000500 ÷ 1.5 = 0.000333 m².
Required bore = √(4 × area ÷ π) × 1000 = 20.6 mm.
Nearest nominal size = smallest standard size ≥ required bore = 25 mm.

Indicative sizing for concept planning — nominal size does not equal internal bore exactly, and friction loss over long runs may push you a size up. Verify against NBC 2016 Part 9 & manufacturer bore tables, and confirm with a qualified consultant before procurement.

Understand the number you just got

Plumbing Pipes Guide (India)Materials, sizes, pressure ratings and where each pipe belongs.Choosing Plumbing Pipes in IndiaCPVC vs PPR vs GI vs PEX — how to pick the right pipe for the job.

Frequently asked questions

How does the pipe size calculator work?: It sizes a water supply pipe from two numbers: the design flow the pipe must carry and a target flow velocity. It converts the flow from litres per minute to cubic metres per second, divides that by your velocity to get the cross-section area, then works back to the internal bore in mm. Finally it rounds up to the nearest standard nominal size stocked in India.
What inputs do I need and what values should I use?: You need the peak design flow in litres per minute and a target velocity in metres per second. For flow, add up the simultaneous demand the pipe serves rather than a single tap. For velocity, 1 to 1.5 m/s is a common supply band; the default here is 1.5. Staying in that range keeps the bore sensible without excessive noise.
What is a safe water velocity in a supply pipe?: Roughly 1 to 1.5 m/s is a comfortable target for most residential supply runs. Below about 0.9 m/s the pipe may be oversized for the duty. Above about 2 to 2.4 m/s you risk flow noise and, over years, erosion of the pipe wall. Higher velocity means a smaller bore, so treat the calculator output as a starting point, not a fixed answer.

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