A DIN 3015 pipe clamp is selected by one number: the outer diameter (OD) of the line it grips, in millimetres. Get that number wrong — or quote a DN or NPS value instead — and the clamp will not close, or will not grip. This is the single most common sizing error on a clamp RFQ.
DIN 3015 pipe clamps are sized by the pipe outer diameter (OD) in millimetres, not by DN (nominal bore) or NPS (nominal pipe size). To measure OD: wrap a flexible tape around the pipe and divide circumference by 3.1416, or use a caliper across the bare metal. Measure bare pipe, not over insulation or coating. For tube, the stated size is already the OD. For steel pipe given as DN or NPS, convert to actual OD — DN 25 pipe is 33.7 mm OD, not 25 mm.
- Best for
- Selecting the correct DIN 3015 clamp size for hydraulic tube, instrument tube and steel pipe by measured OD
- Not suitable for
- Using the DN or NPS number directly as the clamp size — the most common sizing error on clamp orders
- Selection steps
- 1 — Tube or pipe? → 2 — Measure or convert to actual OD in mm → 3 — Measure bare metal, not over coating → 4 — Round to nearest DIN 3015 OD step → 5 — Confirm wall thickness for high pressure
- RFQ information
- Measured OD (mm), tube or pipe, DN/NPS if known, wall thickness, quantity per size
§ 01 — Why OD, Not DN or NPS
A clamp closes around the outside of the line, so it is dimensioned by the outside diameter. DN (Diamètre Nominal / nominal bore) and NPS (Nominal Pipe Size) refer to an approximate inside bore and are not the physical outside measurement. They do not equal the OD and cannot be used directly to pick a clamp.
§ 02 — How to Measure OD
Two reliable methods, in order of preference:
- Circumference method (most accurate): Wrap a flexible tape or a dedicated pi-tape around the pipe, read the circumference, and divide by π (3.1416). A circumference of 78.5 mm ÷ 3.1416 = 25.0 mm OD.
- Caliper method: Place a vernier or digital caliper across the pipe and measure at two points 90° apart; take the larger reading (pipe is rarely perfectly round). Use the jaws on bare metal.
§ 03 — Tube vs Pipe
| Line type | How it is specified | Is the stated size the OD? |
|---|---|---|
| Hydraulic / instrument tube | By OD directly (e.g. "12 mm tube", "1/2" tube") | Yes — stated size = OD |
| Steel pipe (metric) | By DN (nominal bore) | No — convert DN → OD |
| Steel pipe (imperial) | By NPS (inch nominal) | No — convert NPS → OD |
Most wind-turbine hydraulic and cooling lines are tube, so the stated size is usually already the OD. Confirm before ordering — mixing a tube size with a pipe DN on the same RFQ is a frequent cause of wrong clamps.
§ 04 — DN / NPS to OD Conversion
For steel pipe, use the actual OD — these values are fixed by the pipe standard and do not change with wall thickness:
| DN | NPS | Actual OD (mm) | Nearest DIN 3015 step |
|---|---|---|---|
| DN 6 | 1/8" | 10.2 | 10 |
| DN 8 | 1/4" | 13.5 | 13.5 / 14 |
| DN 10 | 3/8" | 17.2 | 18 |
| DN 15 | 1/2" | 21.3 | 22 |
| DN 20 | 3/4" | 26.9 | 27 / 28 |
| DN 25 | 1" | 33.7 | 34 / 35 |
| DN 32 | 1¼" | 42.4 | 42 |
| DN 40 | 1½" | 48.3 | 48 |
| DN 50 | 2" | 60.3 | 60.3 |
| DN 65 | 2½" | 76.1 | 76.1 |
§ 05 — Common Mistakes
- Using DN as OD: the number-one error. DN 25 ≠ 25 mm. Always convert.
- Measuring over coating or insulation: oversizes the clamp; the insert loses grip and the line vibrates loose.
- Assuming tube = pipe: a "25 mm" tube is 25 mm OD, but "DN 25" pipe is 33.7 mm. Confirm which one.
- Ignoring wall thickness for high pressure: OD sets the clamp size, but heavy-wall pipe at the same OD may need a Part 2 heavy clamp — see the selection matrix.
Not sure whether your line is tube or pipe, or what the OD converts to? Send us the DN/NPS values or a photo with a caliper — we return the correct DIN 3015 size and a quote within one business day.
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