Measuring guide

How to measure a hose: ID, OD, wall thickness and bend radius

Four numbers decide whether a replacement hose fits. Here is how to take each one, so the first sample is the right one.

Why one wrong number costs a week

Almost every failed first sample comes down to one of four numbers: the bore, the wall, the length, or the bend radius. Each is easy to take with the right tool and easy to get wrong with the wrong one. A five-minute measurement done properly saves a full round of courier, tooling and approval.

Inner diameter — the number that matters most

The inner diameter, or bore, sets the flow and determines what the hose will push onto. Measure it with a caliper or a bore gauge, not with a tape across the outside.

• Use calipers inside the bore, at least 20 mm in from the end, because the end flares when it is cut.
• Measure twice, at 90° to each other. A hose that has been on a spigot for years is often slightly oval.
• If the bore is oval, give both readings and tell us which way the oval runs relative to the bend.
• Do not measure a soft hose with a tape — the tape compresses it and reads small.

Outer diameter and wall thickness

The outer diameter alone does not define a hose. A 25 mm bore with a 4 mm wall and a 25 mm bore with a 6 mm wall are different parts with different pressure ratings and different stiffness.

• Measure the outside diameter with calipers over the reinforcement, not over a moulded bead.
• Calculate the wall as (OD − ID) ÷ 2, or measure the cut end directly.
• If the hose is reinforced, say so — and whether the reinforcement is yarn, fabric or wire.

Length — measured along the centreline

This is where most drawings go wrong. Length is measured along the centreline of the hose, following every bend, not as a straight line between the two ends.

The practical way: lay the hose flat on the bench exactly as it sits in the vehicle, mark both ends, then measure along the curve. If you are working from a part that is still installed, use a soft tape or a length of string.

Also tell us whether the length includes the ends or is the cut length between them. On a bent hose the difference is easily 30 mm.

Bend radius

Bend radius is the radius of the tightest curve, measured to the centreline of the hose. Two hoses with identical bore and length behave completely differently at 40 mm and 80 mm radius.

• Measure to the centreline, not to the inside of the bend.
• For a 90° bend, the radius is roughly the distance from the corner to the centreline curve.
• If the part has several bends, give the radius of each one, and say which is the tightest.
• If you do not know the radius, send a photo of the part standing on a sheet of graph paper.

End form — the part most often left out

How the hose attaches decides the tooling, and it is the single most under-specified detail on enquiries.

• Beaded — a raised ring for a spring or worm clamp.
• Barbed — a tapered, ribbed spigot to push into.
• Flanged — a moulded flange with bolt holes.
• Fitted — a metal or plastic connector crimped or clamped on.
• Plain cut — no end treatment at all.

Photograph each end straight on. Two end photos answer more questions than a paragraph of description.

What to send us

1. Bore, outer diameter and wall.
2. Length, and whether it is along the centreline.
3. Bend radius of every bend.
4. End form at both ends, with photos.
5. The medium, working temperature and pressure.
6. Quantity for the first order and per year.

Send those and we can usually confirm the part in one reply. If something is missing we will tell you exactly which number we need, rather than quoting a guess.

A note on measuring an old hose

An old hose has already taken a set. It may be swollen from the fluid, flattened where it was clamped, or stretched at the ends. Measure it, but also tell us how old it is and what it was doing — a 15-year-old hose is a shape reference, not a dimensional reference.

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