Plumbing Pipe Size Chart (Nominal vs Actual)

A plumbing pipe size chart is the first thing to check before buying pipe, fittings, or fixtures — and the first thing that confuses new DIYers. A pipe labeled 1/2 inch is not half an inch across, and the same nominal size means different actual diameters depending on whether the pipe is copper, PVC, PEX, or ABS. This guide explains nominal vs actual sizing, gives you the charts for the four most common residential pipe materials, and shows you how to measure a pipe you already have so you buy the right fitting the first time.

Nominal vs Actual: Why Pipe Sizes Lie

Nominal pipe size (NPS) is a naming system, not a measurement. It dates back to the early days of iron pipe, when the nominal size roughly matched the inside diameter of the pipe. As manufacturing improved and pipe walls got thinner, the outside diameter stayed the same so fittings would still fit, but the inside diameter grew — and the name stayed put. The result is that a 1/2-inch pipe has an outside diameter of about 5/8 inch for copper and 0.840 inch for PVC, and neither one is actually half an inch anywhere.

There are two sizing systems in common residential use. Copper, PEX, and CPVC use CTS (copper tube size), where the outside diameter is the fixed reference: a 1/2-inch CTS pipe always has an outside diameter of 5/8 inch, no matter the material. Iron pipe, PVC, and ABS use IPS (iron pipe size), where the outside diameter is larger for the same nominal size — a 1/2-inch IPS pipe has an outside diameter of 0.840 inch. This is why a 1/2-inch copper fitting will not fit a 1/2-inch PVC pipe, even though both say 1/2 inch on the label.

The practical rule is simple: never trust the label alone. Measure the pipe you have, identify the material, and use the chart for that material. Fitting manufacturers label their products by nominal size and by the system they fit, so a fitting marked 1/2-inch CTS fits copper and PEX, while one marked 1/2-inch IPS or Schedule 40 fits PVC and ABS. Mixing the two systems is the most common sizing mistake in DIY plumbing.

Copper Pipe Size Chart

Copper pipe is sized by CTS, and it comes in three wall thicknesses: Type M (thin, for residential water supply), Type L (medium, the most common in homes), and Type K (thick, for underground and commercial work). The outside diameter is the same for all three types at a given nominal size — only the inside diameter changes, because a thicker wall leaves less room inside. Here is the chart for the sizes you will see in a home.

For most home water supply work, Type L is the standard choice — it costs a little more than Type M but resists corrosion and physical damage better, and most plumbers prefer it. Type M is fine for straightforward interior runs on a budget. Type K is overkill inside a house and is usually reserved for underground service lines. Copper DWV (drain) pipe uses the same outside diameters but is sold in a thinner DWV wall, and it is increasingly rare since ABS and PVC took over drain work. If you are joining copper, see our guide on how to solder copper pipe.

PVC and ABS Pipe Size Chart

PVC and ABS drain and water pipe use IPS sizing, and the standard wall thickness for home plumbing is Schedule 40. The outside diameter is the same for Schedule 40 and Schedule 80 at a given nominal size — Schedule 80 just has a thicker wall and a smaller inside diameter. Here is the Schedule 40 chart for the sizes used in homes.

Notice how different these outside diameters are from copper: a 1/2-inch PVC pipe is 0.840 inch across, while a 1/2-inch copper pipe is 0.625 inch. That 0.215-inch difference is why the two need a transition fitting and can never share a fitting. For ABS pipe specifically, the outside diameters match PVC Schedule 40 at each nominal size, which is why ABS-to-PVC transition fittings exist — see our guide on how to connect ABS to PVC. For more on ABS sizing, see our ABS pipe size guide.

PEX Pipe Size Chart

PEX tubing uses the CTS system, so its outside diameters match copper exactly at each nominal size. That is convenient: a 1/2-inch PEX pipe and a 1/2-inch copper pipe have the same outside diameter (0.625 inch), and many transition fittings are designed around that shared measurement. The inside diameter of PEX is smaller than copper at the same size, because PEX walls are thicker relative to the pipe, and the barbed fittings used with PEX narrow the flow path a little more.

The flow figures in this table are typical residential ranges at normal water pressure, not code minimums — actual flow depends on pressure, pipe length, and how many fixtures run at once. One practical consequence of the smaller inside diameter is that PEX is often upsized one step compared with copper: a run that would be 1/2-inch copper is sometimes run in 3/4-inch PEX to keep the same flow, especially on long runs or where several fixtures share a line. If you are connecting PEX to an existing copper system, see our guide on how to connect PEX to copper.

How to Measure a Pipe You Already Have

To identify a pipe, measure its outside diameter with a tape measure or calipers. Wrap a piece of string around the pipe, mark where it overlaps, measure the string, and divide by 3.14 to get the diameter — or use calipers directly for a faster reading. Then match the outside diameter to the chart for the material you have. Copper is easy to identify by its color; PEX is usually red, blue, or white flexible tubing; PVC is white or gray rigid plastic; ABS is black rigid plastic. If you are unsure of the material, see our guide to identifying ABS pipe.

Measuring the outside diameter is more reliable than measuring the inside, because the inside diameter varies with wall thickness and is harder to measure accurately on an installed pipe. Once you have the outside diameter and the material, the nominal size follows from the chart. For example, a copper pipe measuring 7/8 inch outside is 3/4-inch nominal, and a white plastic pipe measuring 1.9 inches outside is 1-1/2-inch nominal PVC or ABS.

Also check whether the pipe is threaded or smooth. Threaded pipe, usually galvanized steel or brass in older homes, is measured differently — the nominal size refers to the inside diameter of the pipe, and fittings are sold by the thread size. A 1/2-inch threaded pipe has an outside diameter of about 0.840 inch, the same as 1/2-inch PVC, because both use the IPS system. If you are working with threaded pipe, see our guide on pipe thread sealant vs Teflon tape.

Choosing the Right Pipe Size

Pipe size is a trade-off between flow and pressure. A larger pipe carries more water with less pressure loss, but it costs more, takes longer to deliver hot water, and can actually cause problems if it is oversized for the fixture — water that moves too slowly through an oversized pipe can stagnate and lose heat. A smaller pipe is cheaper and delivers hot water faster, but it restricts flow and can leave a shower weak when another fixture runs at the same time.

For water supply lines, the standard residential sizes are 3/4 inch for main runs and 1/2 inch for branch lines to individual fixtures. A typical single-family home is served by a 3/4-inch or 1-inch main, depending on the number of bathrooms. For drain lines, the sizes are set by code and by fixture type: a bathroom sink drain is 1-1/4 or 1-1/2 inches, a tub or shower drain is 1-1/2 or 2 inches, a toilet drain is 3 inches, and the main building drain is 4 inches. Never reduce a drain below the size required for the fixture — undersized drains are a code violation and a clog waiting to happen.

When in doubt, match the existing pipe size. If you are replacing a section of pipe, use the same nominal size and material (or an approved equivalent) as the original. If you are adding a new fixture, size the branch to match the fixture's requirements — a shower needs a 1/2-inch supply and a 2-inch drain in most codes, while a toilet needs a 1/2-inch supply and a 3-inch drain. Local plumbing codes can vary, so check with your building department for anything beyond a simple repair.

Common Size Mistakes

The most common mistake is buying a fitting by nominal size without checking the sizing system. A 1/2-inch copper elbow does not fit 1/2-inch PVC pipe, and a 1-inch PEX fitting does not fit 1-inch PVC pipe, even though the labels match. Always buy fittings marked for the same material and system as your pipe — CTS fittings for copper and PEX, IPS or Schedule 40 fittings for PVC and ABS.

The second mistake is measuring the inside diameter and ordering by that number. Because the inside diameter changes with wall thickness, two pipes with the same outside diameter and nominal size can have different inside diameters — and a fitting ordered by inside diameter will be the wrong size. Always measure outside diameter and use the chart for your material.

The third mistake is mixing Schedule 40 and Schedule 80 fittings without realizing it. The outside diameters match, so the fittings will physically go together, but Schedule 80 fittings on a Schedule 40 system (or vice versa) create a step inside the pipe that catches debris and restricts flow. Keep the schedule consistent throughout a run. And finally, do not assume that a pipe's printed size is still readable after years in a wall — measure it yourself rather than guessing from a faded label.

Pipe Size FAQs

One question that comes up often is whether you can connect two different nominal sizes. Yes — reducer fittings exist for exactly that purpose, and stepping down from a 3/4-inch main to a 1/2-inch branch is normal plumbing practice. What you should not do is reduce a drain line below the size the fixture requires, or reduce a water main so much that fixtures starve for flow.

Another common question is whether PEX and copper of the same nominal size are interchangeable in fittings. Their outside diameters match under CTS, which is why many push-fit fittings accept both — but crimp and clamp fittings are made specifically for PEX, and solder fittings are made specifically for copper. Use the fitting designed for the material, even when the diameters match.

If you are planning a larger project and want a professional opinion on sizing, our plumbing pipe installation guide covers what to expect from a pro, and you can reach a local plumber at (877) 762-1902. Getting the size right before you buy is the cheapest part of any plumbing job — the expensive part is the second trip to the store.