Waste Pipe Sizes Explained: 32mm, 40mm or 50mm — Which Do You Need?

The wrong waste pipe size is a wasted trip back to the merchant

Every plumber knows the feeling: you're on a bathroom fit-out in Ealing, you've got the vanity unit in and the basin seated, and you reach into the van to find you've got 40mm fittings on a job that needs 50mm. Job stops. You've either got to strip it back or make a run to the merchant. Getting waste pipe sizing right before you start is basic — but it's easy to grab the wrong box in a hurry.

This guide covers when to use 32mm, 40mm, or 50mm waste pipe, the fittings you'll need for each, and how to make sure you're stocked before the job starts. We hold all three sizes across solvent weld and push-fit at our trade counter in Acton, West London — same-day collection, no minimum spend.

32mm Waste Pipe — Basins, Bidets, and Small Sinks

32mm is the standard for light-duty domestic drainage. It handles low flow rates from a single fixture and is typically used for:

• Wash hand basins
• Bidets
• Small kitchen prep sinks
• Overflow connections

Keep runs short on 32mm — BS EN 12056-2 recommends a maximum unventilated branch length of 1.7m for a basin. Longer runs need either a ventilated stack or an upgrade to 40mm. A 1-in-48 fall (approximately 20mm per metre) is your standard gradient. Go steeper and you drain the trap; go shallower and solids accumulate.

Browse our 32mm Waste Solvent Fittings collection for elbows, straight couplings, traps, and access fittings.

40mm Waste Pipe — Baths, Showers, Kitchen Sinks

40mm is the workhorse of domestic plumbing. It handles higher flow rates and longer runs than 32mm, making it the right choice for:

• Bath waste
• Shower trays and shower enclosures
• Kitchen sinks (single and double bowl)
• Washing machine and dishwasher discharge

With 40mm you can comfortably run up to 3m unventilated to the stack in most domestic situations. On kitchen installs where the sink is far from the soil stack — common in West London Victorian terraces with rear extensions — you'll often need to check you're not exceeding recommended run lengths. If you are, drop in an air admittance valve (AAV) to keep it building-regs compliant without breaking out a new stack.

Browse our 40mm Waste Solvent Fittings collection for the full range.

50mm Waste Pipe — High Flow, Commercial, and Longer Runs

50mm is where most domestic plumbers venture when the job is commercial or when multiple fixtures share a branch run. Typical applications:

• Commercial and restaurant kitchen sinks with waste disposal units
• Multiple appliances connected to a shared branch
• Long horizontal runs where 40mm would self-siphon the traps
• Condensate drains from large commercial boilers
• Urinal drainage

On domestic jobs, you'll most often see 50mm at the base of a soil stack before it transitions to 110mm underground drainage. Make sure your reducer fitting is rated for the temperature range if you're connecting near a boiler.

The Aquaflow 50mm Access Plug with Screw Cap is one of our top sellers — a clean, professional finish on any accessible cleanout point, and it pulls out for rodding without tools. Available in grey, white, and black:

• Aquaflow 50mm Access Plug with Screw Cap — Grey
• Aquaflow 50mm Access Plug with Screw Cap — White
• Aquaflow 50mm Access Plug with Screw Cap — Black

Also stock the Aquaflow 50mm 92° Waste Bend and 135° Bend for routing around joists and changing direction without cutting flow. See the full 50mm Waste Solvent Fittings range online.

Key Fittings You'll Need on Any Waste Run

Whatever size you're running, a properly installed waste system needs more than just straight pipe. Keep these on the van:

• Access plugs — rodding points at every change of direction or long straight run
• 92° and 135° bends — 135° is better practice for flow; 92° where space forces it
• Straight couplings — for extending runs cleanly
• Reducer fittings — when transitioning between pipe sizes
• Solvent cement — use the right type for your pipe material (grey for uPVC solvent weld waste)

For push-fit waste systems — common on retrofit jobs where you can't easily use solvent weld — check our Plumbing Fittings collection which covers both jointing methods across all pipe sizes.

Installing Waste Pipe: The Basics Done Right

A few things experienced plumbers still occasionally trip over on waste installs:

Fall: 1:48 (20mm per metre) for 32mm, 1:80 (12mm per metre) acceptable for 40mm and 50mm in longer runs. Measure from the trap outlet, not the wall.

Pipe support: Clip solvent weld waste pipe at 500mm centres maximum on horizontal runs. On longer verticals, use the manufacturer's recommended bracket spacing — unsupported pipe sags, creates standing water, and eventually cracks at fittings.

Trap depths: 75mm for above-ground waste connected to a ventilated stack system; 50mm where mechanical seal traps with resealing are used.

Back falls: Don't. Even a slight upward gradient on a horizontal waste run causes blockages within weeks on domestic and months on commercial. Use a spirit level on every horizontal run before you commit to clips.

Get It Same Day — Trade Counter in Acton W3

We stock Aquaflow solvent weld waste fittings in 32mm, 40mm, and 50mm in-store at 24 Western Avenue, Acton, London W3 7TZ. Open Monday to Friday, with Saturday morning hours for the trade. No need to order ahead — walk in and pick up what you need.

Prefer to order online and collect? Browse our full Plumbing Fittings range at apmi.uk and select click-and-collect at checkout. Call us on 020 8702 8080 to check stock on a specific fitting before you travel.

We're a trade merchant — we speak plumber. No queuing behind retail customers, no explaining what a 92° swept bend is. Just grab your order and get back on the job.