Use the free Drain Pipe Size Selector below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
What this tool does
A drain that is too small gurgles, drains slowly, and can back up when two fixtures run at once, while a drain that is far too large can fail to carry solids properly. This free drain pipe size selector adds up the fixture units of everything connected to a drain run and suggests a minimum pipe size from that total. It is a planning aid for a bathroom addition, a laundry move, or a simple check on why an existing drain struggles, and it runs entirely in your browser.
Count each fixture that drains into the run and enter the numbers: toilets, showers, sinks, bathtubs, washing machines, and kitchen sinks. Press Calculate and the tool shows the total fixture units and the estimated minimum size. Fixture units are not litres or gallons: they are a weighting system used by plumbing codes, where a toilet counts far more than a hand basin because it discharges a large volume suddenly. That is why a run serving one toilet and one sink needs a larger pipe than a run serving two sinks alone.
The values used here are toilet four, shower two, sink one, bathtub two, washing machine four, and kitchen sink two. The size steps are equally plain: up to four units suggests one and a half inches, up to ten units suggests two inches, up to twenty-one units suggests three inches, and up to thirty-six units suggests four inches. Anything above thirty-six units is flagged for professional design rather than given a single size, because long runs, bends, and venting change the answer. These steps are a simplified guide, not a quotation from any particular code book.
Take a small bathroom plus laundry: one toilet, one shower, one sink, and one washing machine. That is four plus two plus one plus four, which is eleven fixture units, so the tool suggests three inches. Drop the washing machine and the total falls to seven units, which suggests two inches. Add a second toilet and a bathtub and the total jumps to seventeen units, still three inches, but now much closer to the next step. That sensitivity is the point: one extra fixture can move you across a boundary, so count honestly and include everything that will ever drain into the run.
Three things beside size decide whether a drain works. Slope comes first: too flat and solids settle, too steep and water outruns solids, so follow your local slope guidance for the pipe size. Venting comes second: a poorly vented drain gurgles and can siphon traps dry, letting smells in. Length and bends come third: a long run with many bends behaves worse than a short straight one of the same size. If an existing drain is slow, check for a blockage, a vent problem, or a sagging section before assuming the size is wrong.
Does a bigger drain always drain better? No: an oversized drain can run too shallow to carry solids, so bigger is not automatically better. Can I connect a toilet to a two-inch drain? Under this guide, a toilet alone is four units, which sits at the one-and-a-half-inch boundary, but toilets in practice need a larger drain, so always follow your local code for toilets specifically. Should I use this tool for a whole house? It is best for one run at a time; a whole-house design, with stacks, vents, and branches, needs a professional layout.
Important limits: an estimate, not a code approval
Every result on this page is an educational estimate only. It is not professional advice and it does not guarantee compliance with any building, plumbing, gas, or safety code. Local rules, soil, climate, pipe material, and the condition of an existing system can change the correct answer, and this tool cannot see any of those things. Gas work must be done by a licensed professional and checked against local gas codes; never size, install, or alter gas pipework from an online estimate. For any real installation, repair, or purchase decision, consult a licensed plumber or carpenter first, and get a second opinion when the job is large, expensive, or safety-critical. Use this tool to plan, compare, and ask better questions — not as a substitute for a qualified person on site.
Also available in Spanish: Spanish version.