For most residential yard drainage, downspouts, and French drains, a 4-inch (100 mm) pipe is the industry standard. It offers the ideal balance of water capacity, debris clearance, and ease of installation. For low-flow areas or tight spaces, a 3-inch (75 mm) pipe may also be used to maintain water velocity.
Drain pipe sizes depend on the fixture, ranging from 114 inches for bathroom sinks to 4 inches for main sewer lines. Correct sizing ensures proper flow and prevents clogs. Always measure the outer diameter (OD) or verify codes to match branch and stack values.
The "135-degree rule" in plumbing states that the total (aggregate) horizontal change in direction for a drainage pipe—specifically trap arms—cannot exceed 135 degrees between the P-trap and the vent. Exceeding 135 degrees (such as using three 45-degree bends or a 90 and two 45s) requires a cleanout because sharp turns hinder flow and cause clogs.
Standard waste pipe is either 40mm (1 1/2 inch) or 32mm (1 1/4 inch). 40mm pipe is used for kitchen sinks / washing machine waste / baths / showers. 32mm pipe is used for basins / sinks found in bathrooms and cloakrooms.
10mm might not seem like a very large amount, but when it comes to clearing drains it can make a huge difference in the result. For a very long time, 100mm PVC has been the standard diameter for both stormwater and sewer drains. But in the past 5 years or so, 90mm PVC has become much more popular.
The 100mm stormwater pipe is a more traditional and widely used size for stormwater drainage. It offers a larger diameter, meaning it can handle more water flow and is more durable than 90mm pipes.
Minimum size of storm drains is 12-inch diameter for tie-ins and 15-inch diameter for main lines.
32 mm waste pipes are used at points where a low water volume will be entering the system. In the home, this is usually just bathroom basins.
In conclusion, using a larger sewer pipe at the start of a system is a proper choice to ensure proper flow, especially when the slope is shallow. As the slope steepens, a smaller sewer can be used since the water flows faster.
50mm waste pipes are used to move waste water, often referred to as foul drainage to the drainage system. 50 mm waste pipes are solvent welded with with ABS fittings, meaning they can be used for both commercial and domestic properties.
To protect your plumbing and the environment, you should never put fats, starches, harsh chemicals, or solid waste down your drains.
Horizontal drain piping, including building drains and building sewers, shall be installed in uniform alignment at the following uniform slopes: not less than 1/4 inch per foot for 1-1/4 inch.
The flow capacity of a pipe refers to the volume of fluid (liquid or gas) it can transport over a given period, typically measured in gallons per minute (GPM) or cubic feet per second (cfs). It is primarily determined by the pipe's internal diameter, the fluid's velocity, and pressure.
40mm waste system is the common size for bath and kitchen sink installation.
Solid PVC pipe (specifically Schedule 40 or SDR35) is generally considered the best, most durable option for underground drainage, lasting over 50 years with excellent crush resistance and smooth flow. It is ideal for gutter downspouts, as its watertight, glued joints prevent root intrusion.
Waste pipes are between 21.5mm and 50mm in diameter, while soil pipes which carry solid waste are usually 110mm.
If the drain pipe is oversized for the sink it's attached to, solids can settle at the bottom of the pipe, which increases the chances of blockages. Oversized pipes can also keep water from flowing at the right speed, as bigger piping decreases suction down into the sewer pipes.
Myth 1: “Using push-fits devalues the trade.”
Some installers believe that using push-fit fittings removes the skill from plumbing, but that couldn't be further from the truth. While push-fits are quick and easy to install, the technology behind them is highly advanced.
1/2" pipe refers to the internal diameter, 15mm pipe is the external diameter... In my experience they are so similar that you can even use solder fittings without a worry, so using 15mm olives on 12" pipe isn't a problem. Occasionally they are very marginally tight, but not enough to be an issue.
Also known as 32mm Alkathene pipe, 32mm water pipe and 32mm Potable Water Pipe, MDPE pipe is generally used for the supply of cold water.
Basin wastes are all the same size 1 1/4″ or 32mm. Sometimes people get confused and ask why is there 2 sizes of basin wastes? the simple answer is that basin wastes are 1¼” BSP (32mm), whereas bath and kitchen sink wastes are 1½” BSP (40mm).
You should note that a standard 25mm service connection will provide 0.6 L/s and a standard 32mm service connection will provide 1.0L/s at the point of connection and at our normal mains operating pressure (please see section 3.2: operation of the mains distribution for more information).
Discover the versatility and reliability of 50 mm waste pipe fittings, essential for any plumbing project. Designed to efficiently manage wastewater, these fittings are ideal for both domestic and commercial settings, ensuring a seamless flow while preventing leaks and blockages.
A rule of thumb that incorporates pipe size is to choose liquid lines to handle a velocity of 1.5 +d/10 where “d” is the pipe diameter, inches. This gives 1.6 m/s for 1-inch and 2.5 m/s for 10-inch piping, and about 20 kPa/100 m pressure drop.