To figure out exactly how much pipe you need, it depends on whether you are measuring for linear length (how long the pipe run is) or volume (how much liquid it will hold).
Example of How to Calculate Pipe Volume
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.
The standard formula for the interior volume (Vcap V𝑉) of a pipe is based on the volume of a cylinder:
Sizes pipe by velocity using Q = A × v. Outputs a required internal diameter and nearest nominal size from reference tables. Tender-stage only — verify with manufacturer data, pressure drop, and project spec. D = √(4Q / πv).
You can measure the outside circumference with a soft tape measure, then divide your measurement by pi. From there, you'll have to use a conversion chart to get the nominal pipe size (the size you'd buy in the store).
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.
40 mm waste pipes are used in areas where higher volumes of water will be entering the system such as baths, showers and kitchen sinks.
In plumbing, the "135-degree rule" is a code standard that dictates how much a horizontal drain pipe can bend or change direction. It limits the total cumulative angle of pipe fittings before a system requires an access point (cleanout) or must be vented, ensuring proper flow and preventing chronic clogs.
With modernisation these imperial fittings have now been given a metric name, but they are the same thread spec. So a 3/4 fitting is now also called 20mm. 1/2 inch fittings are now also called 15mm. The 15mm or 20mm is not the size of the threads, it's still the ID of the pipe.
Plumbing pipes are typically measured by their diameter. For instance, internal supply pipes in domestic properties are commonly 22mm or 15mm in diameter, while commercial plumbing often uses pipes with a diameter of 28mm.
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.
The 1.414 formula is a quick construction and plumbing trick used to find the exact length of a pipe needed to bridge a 45-degree gap or offset. It is the square root of 2, representing the mathematical constant used to find the hypotenuse of an equal-sided right triangle [𝑐2=𝑎2+𝑏2].
Pipes are generally measured by their Inside Diameter (ID), often referred to as the "nominal size" (e.g., a 2" nominal pipe does not have a 2" outer or inner diameter). However, pipes 12 inches and over are measured by their Outside Diameter (OD).
You'll find 15mm everywhere in a typical UK home because it handles the flow rate for individual outlets perfectly well. The 22mm pipe and valve combination is reserved for primary circuits where flow rate matters more.
Let's clear up a common question in plumbing and construction: 👉 Why is 1/2 inch PVC pipe equal to 20mm PVC pipe? ✅ Answer: The 1/2 inch size comes from the imperial system, while 20mm comes from the metric system — but both refer to similar pipe dimensions used for the same purpose.
No, 1/2 inch is not equal to 22mm. They represent completely different measurements, depending on how they are used:
Easily determine the correct pipe size using the continuity equation: 𝐷=4⋅𝑄𝜋⋅𝑉, where Dcap D𝐷 is internal diameter, Qcap Q𝑄 is volume flow rate, and Vcap V𝑉 is fluid velocity. This helps you select standard commercial pipe sizes by matching the calculated diameter and keeping flow within optimal velocity ranges.
Different pipe schedules mean different wall thicknesses for the steel pipe in the same diameter. The most frequent indications of schedule are SCH 5, 5S, 10, 10S, 20, 20S, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160. The larger the table number, the thicker the surface pipe wall, the higher the pressure resistance.
Accuracy
Proper Pipe Fitting: When pipes are joined together, their diameters must match precisely to create a tight seal and prevent leaks. Inaccurate measurements can lead to loose connections, which can compromise the integrity of the piping system and potentially cause significant issues.
To measure a pipe size, find the Outside Diameter (OD) by wrapping a flexible measuring tape around the pipe and dividing by πpi𝜋 (3.1415). Compare this OD to a standard reference chart to determine the nominal size, as nominal size often refers to the approximate inside diameter rather than the exact measurement.
A 4-inch pipe typically has an Outside Diameter (OD) of 114.3 mm.
How to Measure Pipe Fittings