PEX Pipe Sizes: Nominal vs Metric, and How to Size a Line

PEX pipe sizing looks simple until you hit the two conventions — nominal versus outer-diameter — and realise a "½-inch" PEX and a "16 mm" PEX are not the same pipe. Get the size wrong and fittings don't fit, or you under-size a branch and starve a shower of flow. This guide lays out the common PEX sizes, how nominal and metric OD map to each other, how to size a line for the fixtures it feeds, and the sizing quirks of manifold layouts and PEX-AL-PEX — so you order the right diameter and the right fittings correctly the very first time.

For the wider picture on PEX types, connections, and fittings, see the complete PEX pipe guide; this article focuses on getting the size right.

The Common PEX Sizes

PEX is sold in a handful of standard sizes covering everything from a small fixture drop to a main. In North American systems, sizes are quoted nominal (the rough inside bore): ⅜", ½", ¾", and 1" are the everyday sizes, with larger diameters for mains. In metric markets, PEX is quoted by outer diameter: 16, 20, 25, and 32 mm are the common sizes. The two systems do not line up one-to-one at all — which is easily the single biggest sizing trap for buyers.

Nominal	Metric OD (approx)	Typical use
⅜"	~12 mm	Short fixture drops, icemaker lines
½"	~16 mm	Most fixtures (sinks, toilets, showers)
¾"	~20–25 mm	Branch lines, mains, high-demand runs
1"	~32 mm	Main lines, manifolds, larger buildings

The metric column is approximate — confirm the exact OD of the specific product, because a ½" nominal PEX and a 16 mm metric PEX have different actual dimensions and need their own matching fittings.

Nominal vs Outer Diameter — Don't Mix Them

This is where orders go wrong. Nominal sizing (the North American convention) names the approximate inside bore, so a "½-inch" PEX refers to a size family, not a literal measurement. Metric OD sizing names the actual outer diameter. The two are separate systems with separate fittings: a 16 mm metric fitting won't correctly fit a ½" nominal pipe even though both are loosely "half-inch," because the real diameters and the fitting tolerances differ. When you buy, state the convention explicitly — "½" nominal PEX" or "16 mm OD PEX" — and buy the pipe and fittings in the same system. Mixing conventions is the most common cause of fittings that won't seal.

Sizing a Line for Its Fixtures

Size a PEX line to the flow it must carry, not just habit. As a rough guide: ⅜" suits a single low-flow fixture on a short run (a lavatory, an icemaker); ½" is the workhorse for most individual fixtures — sinks, toilets, showers; ¾" carries branch lines feeding several fixtures and mains where demand is higher; and 1" serves main lines and manifolds. The key is diameter versus simultaneous demand: a ½" line feeding one fixture is fine, but a ½" branch feeding several fixtures that run at once will drop pressure. When multiple fixtures share a run, step up to ¾". Long runs also lose pressure to friction, so on a long line consider the next size up. Size for the worst-case simultaneous demand, not the average.

Sizing for a Manifold Home-Run

A manifold (home-run) layout changes the sizing logic. Instead of a big trunk with branches, each fixture gets its own dedicated line straight from a central manifold — so each line is sized for just its one fixture, usually ⅜" or ½", while the manifold itself and its feed are larger (¾" or 1") to supply all the lines at once. Because each fixture has its own line, pressure stays even when several taps run together, and you can isolate any fixture at the manifold. The trade-off is more total pipe than a trunk-and-branch layout, but the balanced pressure and per-fixture control are why manifolds are popular in new builds. Size the feed and manifold for the sum of the demand; size each home-run for its single fixture. That split is the whole trick to a balanced manifold system that holds pressure even when the house is busy.

PEX-AL-PEX and PE-RT Sizes

Multilayer PEX-AL-PEX and PE-RT — common for underfloor heating and radiator circuits (see cross-linked polyethylene) — are quoted by metric OD, in sizes like 16, 20, 25, and 32 mm. For underfloor heating, 16 mm and 20 mm are the typical loop sizes, with larger diameters for the manifold feeds and distribution. Because these pipes use press or sliding-sleeve fittings sized to the pipe's exact OD, matching the fitting to the precise size matters even more than with plain PEX. IFAN manufactures PEX-AL-PEX and PE-RT across the common metric range with the matching press and sliding-sleeve fittings, so the pipe and fittings come dimensionally matched from one source. If your project is heating rather than potable plumbing, work in the metric OD system and confirm the fitting size against the exact pipe OD.

Flow and Friction by Size

Diameter drives flow, and it's why size matters more than it looks. A larger bore carries more flow at a given pressure and loses less pressure to friction over distance — so a ¾" line delivers noticeably more than a ½" line, and the gap widens the longer the run. Two practical consequences: first, on a long run, friction eats pressure, so a line that's adequate at 3 metres may starve a fixture at 20 metres — step up a size for long runs. Second, on a shared branch, several fixtures drawing at once multiply the demand, so a ½" branch that's fine for one fixture drops pressure feeding three — use ¾" where a line serves multiple fixtures. Remember too that a crimp fitting's insert barb narrows the bore slightly at each joint, so a run with many fittings has a little less effective flow than the pipe size alone suggests. Size generously on long or shared runs; you rarely regret one size up.

Common Sizing Mistakes

Mixing nominal and metric. Ordering "½ inch" pipe and 16 mm fittings — the single most common error. State the convention on every line.

Under-sizing a shared branch. Running ½" to a branch that feeds several fixtures starves them under simultaneous demand; use ¾".

Ignoring run length. A size that works short can lose too much pressure on a long run — step up for distance.

Forgetting the manifold feed. Sizing home-runs correctly but feeding the manifold with too small a supply throttles the whole system — size the feed for total demand.

Coils, Lengths, and the Marking

PEX ships in coils — from short 100 ft / 50 m rolls up to large 1000 ft / 200 m spools — and in straight lengths. Coils suit long flexible runs with few joints; straight lengths help where a rigid, tidy run is wanted. Whatever the size, check the printed marking along the pipe: it states the type (PEX-a/b/c, PE-RT, PEX-AL-PEX), the size, the pressure/temperature rating, the standard, and the potable approval. That marking is your check that the right size and type arrived — read it against your order and the certification standards for your market, since colour (red/blue) tells you hot/cold but nothing about size or rating. A pipe whose marking doesn't match the order is unverified until the discrepancy is resolved — and on a bulk order, checking the printed size and type on the delivered coils against the packing list before installation catches a wrong-size shipment before it goes into a wall.

Frequently Asked Questions