What size water supply pipe do I need?

3/4-inch main supply line for most homes. 1/2-inch branch lines to individual fixtures. 1-inch main for homes with 3+ bathrooms or long runs from the water meter. Size is based on fixture units and maximum demand per IPC Table 604.3.

What size drain pipe for a bathroom?

Toilet: 3-inch minimum (4-inch for main building drain). Shower/tub: 2-inch. Bathroom sink: 1-1/4 to 1-1/2 inch. All per IPC Table 709.1. The main drain collecting all bathroom fixtures needs to be 3–4 inches.

How do fixture units affect pipe size?

Fixture units (FU) are a standardized way to compare water demand across different fixtures. A kitchen sink is 2 FU, a bathtub 4 FU, a toilet 4 FU, a washing machine 4 FU. Total fixture units determine the minimum pipe diameter per IPC tables.

What is the recommended water velocity in pipes?

Maximum 8 feet per second (fps) for cold water, 5 fps for hot water per industry standards. Higher velocities cause noise (water hammer), erosion, and pressure loss. Optimal is 4–6 fps for most residential applications.

Should I use PEX, copper, or CPVC?

PEX: cheapest, flexible, freeze-resistant, easy DIY installation. Copper: most durable (50+ years), required by some codes, recyclable. CPVC: middle ground, rigid, good for hot water. PEX has largely replaced copper for new residential construction due to cost and ease.

What size pipe do I need for my house?

Most homes use 3/4" or 1" main supply lines with 1/2" branch lines to individual fixtures. The exact size depends on your flow rate, number of fixtures, and distance from the water meter.

What is the difference between PEX and copper pipe?

PEX is flexible, cheaper, and easier to install. Copper is rigid, more durable, and has been the standard for decades. PEX has slightly smaller inner diameters than equivalent nominal copper sizes.

What flow rate do I need?

A typical shower uses 2-2.5 GPM, a kitchen faucet 1.5-2 GPM, and a toilet fill valve about 3 GPM. Add up your peak simultaneous fixtures to estimate total flow rate.

What are fixture units?

Fixture units are a standardized way to rate plumbing fixtures by their water demand. They are used in code calculations to size supply and drain pipes. A lavatory is 1 FU, a shower is 2 FU, and a bathtub is 4 FU.

Why is my water pressure low?

Low pressure can result from undersized pipes, long runs, excessive fittings, mineral buildup, or supply pressure issues. This calculator helps identify if pipe size is the bottleneck.

Can I reduce pipe size for short runs?

Short branch runs to individual fixtures can use smaller pipe (typically 1/2") because only one fixture draws at a time. But main lines serving multiple fixtures should be sized for combined peak flow.

Pipe Size Calculator

Calculate correct pipe diameter for water supply and drainage based on flow rate, fixture units, and plumbing code requirements.

Pipe Size Calculator

Calculate the recommended pipe diameter based on flow rate, pipe material, run length, and maximum velocity. Covers copper, PEX, PVC, and CPVC piping systems.

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Enter Your Numbers

Quick Presets

Flow Rate

Pipe Material

Pipe Length

Fixture Units (optional)

Leave 0 to use GPM directly

Max Velocity

Recommended: 5-8 ft/s to prevent water hammer

How This Calculator Works

Pipe Sizing Method

Flow velocity is calculated as:

V = (GPM × 0.4085) / A

Where A is the cross-sectional area of the pipe in square inches. The calculator selects the smallest pipe where velocity stays under your limit.

Pressure drop uses the Hazen-Williams equation, which accounts for pipe material roughness via the C-factor (140 for copper/CPVC, 150 for PEX/PVC).

Why Velocity Matters

Water velocity above 8 ft/s can cause water hammer, pipe erosion, and noise. Most codes recommend keeping velocity between 5-8 ft/s for supply lines.

Frequently Asked Questions

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Estimates only — consult a licensed professional and verify with local building codes.

How to Size Plumbing Pipes

Correctly sized pipes ensure adequate water pressure at fixtures and proper drainage flow. Undersized supply pipes cause low pressure; undersized drains cause slow flow and clogs. The International Plumbing Code (IPC) and Uniform Plumbing Code (UPC) specify minimum sizes.

Pipe Sizing Formula

Supply pipes: Based on total fixture units per IPC Table 604.3
Drain pipes: Based on drainage fixture units (DFU) per IPC Table 709.1
Velocity: Q = A × V (Flow = Area × Velocity, keep under 8 fps)

Common Residential Pipe Sizes

Main water supply: 3/4" to 1" (from meter to house)
Hot/cold branch lines: 1/2" to individual fixtures
Toilet drain: 3" minimum
Shower/tub drain: 2"
Sink drain: 1-1/4" to 1-1/2"
Main building drain: 3" to 4"
Vent stack: 1-1/2" to 3"

Pipe Material Comparison

PEX: $0.50–$1.50/ft. Flexible, freeze-resistant, easy DIY. 25+ year lifespan. Best for supply lines.
Copper: $2–$5/ft. Durable (50+ years), recyclable. Requires soldering. Still preferred for exposed locations.
CPVC: $0.75–$2/ft. Rigid plastic, glued joints. Good for hot water. Not freeze-resistant.
PVC (Schedule 40): $0.50–$2/ft. Drain and vent only (not for supply). Standard for residential DWV.
ABS: $0.50–$1.50/ft. Drain and vent. Black plastic. Common in western US.
Cast iron: $3–$8/ft. Quiet, durable. Used for vertical drains in multi-story and noise-sensitive areas.

Frequently Asked Questions

What size supply pipe do I need?

3/4" main supply for most homes, 1/2" branches to fixtures. 1" main for 3+ bathroom homes or long runs.

What size drain for a bathroom?

Toilet: 3". Shower: 2". Sink: 1-1/4" to 1-1/2". Main drain: 3–4".

How do fixture units work?

Standardized demand units. Total FU determines minimum pipe diameter per code tables. Kitchen sink = 2 FU, toilet = 4 FU, tub = 4 FU.

What water velocity is recommended?

4–6 fps optimal. Max 8 fps cold, 5 fps hot to prevent noise and erosion.

PEX, copper, or CPVC?

PEX for most new residential work (cheapest, easiest). Copper where durability or code requires it. CPVC as a rigid alternative.

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Pipe sizing must comply with IPC, UPC, or local plumbing codes. All plumbing work should be performed by or inspected by a licensed plumber. Consult local codes for specific requirements as they vary by jurisdiction.