Free Pipe Sizing Calculator - Water Supply & Drain (2026)

Fixture units are a standardized way to measure the probable demand each plumbing fixture places on the piping system. Instead of calculating exact flow rates for every fixture, the plumbing code assigns unit values that account for typical use patterns. A toilet uses 2.2 WSFU for supply and 3 DFU for drainage. A lavatory uses 1 WSFU and 1 DFU. These values let you size pipe by adding up the total fixture units and looking up the required pipe diameter in a code table.

WSFU (Water Supply Fixture Units) measure demand on the water supply system. They account for flow rate and the probability that multiple fixtures run at the same time. DFU (Drainage Fixture Units) measure the load on the drain, waste, and vent system. They account for the volume of water each fixture discharges. The two values are different for the same fixture because supply demand and drainage load are measured differently. A toilet is 2.2 WSFU but 3 DFU.

Most single-family homes with 2-3 bathrooms need a 3/4-inch water main from the meter to the house. A typical 2-bath home has around 10-14 WSFU, which fits within the 3/4-inch capacity of 14 WSFU at 60 PSI. Larger homes with 3+ bathrooms, multiple hose bibs, or irrigation often need a 1-inch main, which handles up to 30 WSFU. Individual fixture supply lines are typically 3/8-inch or 1/2-inch depending on the fixture.

A toilet requires a minimum 3-inch drain per both UPC and IPC code. This applies to the closet bend and the horizontal branch serving that toilet. The building drain (main sewer line) that collects all fixtures is typically 4-inch for residential buildings with up to 160 DFU on a horizontal branch or 256 DFU on a building drain. Never reduce pipe size downstream in a drain system.

Yes. Lower street pressure means you need larger pipes to deliver the same flow. This calculator assumes 60 PSI street pressure, which is typical for most municipal systems. If your static pressure is below 40 PSI, you may need to upsize the supply main by one diameter. Homes on well systems or at high elevations often have lower pressure and require larger supply pipes or a booster pump. Always check your actual static pressure at the meter before finalizing pipe sizes.

A horizontal branch is a drain pipe that receives discharge from one or more fixtures on a single floor and connects to a vertical stack or building drain. A building drain is the main horizontal pipe that collects all the vertical stacks and carries wastewater to the building sewer. Building drains are allowed higher fixture unit totals because they serve the entire structure. A 3-inch pipe can handle 20 DFU on a horizontal branch but 48 DFU as a building drain.

This calculator covers fixtures commonly found in residential and light commercial buildings. For larger commercial projects with flush valve toilets, commercial dishwashers, or specialized fixtures, use the flush valve toilet option and add additional fixture groups. Commercial projects over 100 fixture units should always be reviewed by a licensed engineer for proper pipe sizing, especially for fire suppression and multi-story pressure calculations.

Once you know the pipe sizes, you can estimate material costs. Use the construction cost estimator to build a complete plumbing estimate with labor, materials, and overhead. EstimationPro generates full plumbing estimates from photos and notes, including pipe materials, fittings, labor hours, and markup, so you can quote the job in minutes instead of hours.