What the standard pins down
ISO 228-1 specifies parallel pipe threads, designated with a G followed by a size in inches. “Parallel” means the thread has the same diameter along its whole length, unlike the tapered family that wedges tight as it goes in.
The document fixes the profile — the angle of the flanks, the rounding of crest and root — the pitch for each designated size, the tolerances, and the gauging. It covers the sizes that matter on a garden shelf: the half inch, the three-quarter inch and the one inch that between them account for nearly every tap, pump port, timer body and quick coupling in this niche.
And it fixes the crucial negative in its own title: the joint is not made pressure-tight on the threads. A parallel thread holds the parts together and locates them; something else seals them, and on garden equipment that something is almost always a flat elastomer washer squeezed against a face.
The size designation is historical. It descends from a nominal bore of old pipe and has not corresponded to any measurable diameter for a very long time. This trips up every buyer with a vernier caliper exactly once.
What it does not promise
It does not promise that a fitting is any good. Thread form is one thing; the material, the wall thickness and the quality of the moulding are another, and on this shelf they vary enormously between a brass coupling and the one that came free with a hose.
It does not promise a seal. If the washer is absent, perished, or replaced by something thicker from a different fitting, the joint leaks and the thread is blameless.
It does not promise compatibility with the click-fit systems that sit on top of it. The quick couplings that dominate garden shelves are proprietary shapes; what is standardised is the threaded end that screws onto the tap, not the snap end that clicks into the hose. Two brands’ snap fittings may or may not mate, and no standard decides it.
It does not promise a hose bore. The inch figure on a thread and the inch figure on a hose are unrelated conventions, and a half-inch hose does not want a half-inch thread for any reason of arithmetic.
The documents it sits next to
The tapered pipe thread family — Rp and R designations — is a sibling document, and the distinction between them is where pump ports go wrong. The hose itself, its bore and its reinforcement, has its own page.
An electrical appliance with a threaded water connection carries its appliance standard as well; the thread standard says nothing about the pump behind the port.
Which shelves it reaches here
The connector, reel, timer, gun, pump and drip irrigation families all cite it: it is the single most widely shared specification in this niche, and the least often printed correctly.
Where a listing on this site quotes a fitting size, the page prints it in the form the maker used and names the thread family where the maker names it. Where a listing says only “universal”, the page says so — on this shelf that word means the seller has not stated a thread at all.
The questions that keep coming in
My tap says 3/4in but measures 26 mm. Which is wrong?
Neither. The inch figure in a pipe thread designation is a traditional size name, not the outside diameter of the thread. A G 3/4 male thread measures close to 26 mm across its crests, and that is correct.
Why does my connector drip even when it is tight?
Because a parallel thread does not seal on the thread. The joint is made by a flat washer compressed against the face; if the washer is missing, hardened or the wrong thickness, tightening harder only deforms the plastic.
Is a G thread interchangeable with an R thread?
Partly, and the direction matters: a parallel male can enter a tapered female in some combinations, and a tapered male into a parallel female is the classic bad idea. The tapered family has its own page here, and mixing them is the second commonest source of leaks after a missing washer.
Last reviewed 26 September 2026