Pipe Sizing Calculator — Water Supply & Drain Pipe Diameter
Determine the correct pipe diameter for water supply lines and drain pipes based on flow rate, fixture count, and pipe material. Get code-compliant recommendations.
How to Use This Calculator
- Select whether you are sizing a water supply line or a drain/waste line
- Choose your pipe material
- Enter the number of each fixture type connected to this pipe run
- For supply lines, enter the pipe run length
- Click "Calculate Pipe Size" for recommended diameter and sizing details
How the Calculation Works
// Water Supply (fixture unit method)
fixture_units = sum of all fixture unit values
velocity_fps = flow_gpm / (pipe_area_in² × 0.3208)
recommended_size = smallest diameter where velocity ≤ 8 fps (supply) or ≤ 2 fps (low noise)
// Drain (DFU method — Drainage Fixture Units)
dfu_total = sum of all DFU values
min_drain_size = table lookup by DFU count and pipe slope Water supply sizing is based on fixture units (FU) — each fixture type is assigned a unit value representing its relative flow demand. Total FUs are converted to approximate GPM flow, then velocity is checked against 8 fps maximum (ASHRAE recommends 5–8 fps for supply, 2–4 fps for quiet operation). Drain sizing uses Drainage Fixture Units (DFU) from the IPC/UPC tables — the minimum pipe diameter is the size that can handle the DFU count at the required slope.
Variables:
- fixture_units: Sum of fixture unit values for all connected fixtures
- flow_gpm: Estimated peak flow in gallons per minute from FU conversion
- velocity_fps: Water velocity in feet per second through the pipe
- pipe_area_in²: Cross-sectional area of the pipe interior in square inches
- dfu_total: Total Drainage Fixture Units — drain pipe sizing metric
Pro Tips
- Size pipes for peak demand — multiple fixtures running simultaneously, not just one at a time
- Copper Type L is the most common residential supply pipe; Type M is thinner and acceptable in many areas
- PEX tubing is flexible, freeze-resistant, and easier to install than copper — now code-approved in most states
- Always increase pipe size at branches — going from 3/4" main to 1/2" branches is standard residential practice
- Drain lines must slope 1/4" per foot for 3" and smaller pipes; 1/8" per foot is acceptable for 4"+ pipes
Recommended Products
Frequently Asked Questions
What size pipe do I need for a bathroom?
A full bathroom (toilet, sink, shower) typically requires a 3/4" supply line to the bathroom with 1/2" branches to each fixture. The drain requires a 3" main drain for the toilet and can use 1.5"–2" for sink and shower drains, all connecting to a 3" or 4" main stack.
What is the difference between 1/2" and 3/4" water supply pipe?
3/4" pipe carries roughly 2.5× more flow than 1/2" pipe. Use 3/4" for main supply lines and branches serving multiple fixtures. Use 1/2" for final runs to individual fixtures like sinks, toilets, and single showers. Undersized supply pipe causes low pressure when multiple fixtures run simultaneously.
What is the minimum drain pipe size for a toilet?
Toilets require a minimum 3-inch drain pipe per most plumbing codes (IPC and UPC). The drain pipe from the toilet flange to the main stack must be at least 3 inches in diameter. A 4-inch drain is used when multiple toilets connect to the same line.
How do I size a main water service line?
Main service lines (from the street meter to the house) are typically 3/4" for homes with up to 2 bathrooms and 1" for larger homes or those with irrigation systems. For new construction, 1" is standard to allow for future additions and irrigation without undersizing.
What pipe material is best for residential water supply?
PEX (cross-linked polyethylene) is now the most popular choice — it is flexible, freeze-resistant, easy to install, and cost-effective. Copper (Type L) is the traditional standard and still widely used for durability. CPVC is a plastic alternative to copper that works well in hot water applications.