Pipe insulation · Elastomeric foam
AEROFLEX Tube Insulation
- Tube bore
- 6–165 mm
- Wall thickness
- 6–50 mm
- Service temp.
- -57 – 125 °C
Pipe size names (inch / NPS) vs actual outside diameter (OD): what is the difference?
In short
A pipe name such as "1/2 inch" is a nominal size, not a dimension you can measure. A 1/2 inch steel pipe has an outside diameter (OD) of about 21.3 mm, while a 1/2 inch air-conditioning copper tube has an OD of 12.7 mm. Tube insulation is sold by inside diameter (bore) + wall thickness, so it must be chosen from the pipe's actual OD, not from the name alone.
How do nominal and actual sizes differ?
Steel pipe uses NPS (Nominal Pipe Size), with dimensions set out in ASME B36.10M. In Thailand it is usually called by "inches" or IPS. Three points matter:
- For NPS 12 and smaller, the OD is larger than the number in the name. NPS 1/2 has an OD of about 21.3 mm, not 12.7 mm. From NPS 14 upward, the OD equals the number in the name (in inches).
- The OD is fixed for each NPS, and the wall thickness changes with the schedule. A thicker-walled pipe of the same NPS has a smaller bore but the same outside surface, which helps insulation work because insulation fits on the outside.
- DN is the metric designation of the same size, for example DN 15 pairs with NPS 1/2. It is also a name, not a millimetre measurement.
Thai tradespeople often give sizes in "hun" (หุน), where 1 hun is 1/8 inch: 2 hun = 1/4 inch, 3 hun = 3/8 inch, 4 hun = 1/2 inch. It is only a way of saying fractions of an inch. You still need to know the pipe type to know the OD.
Why are steel and copper pipes with the same name different sizes?
Each pipe type has its own standard and its own naming rule.
| Pipe type | How the size is named | What "1/2 inch" actually measures |
|---|---|---|
| --- | --- | --- |
| Steel pipe (NPS / IPS) | Nominal; OD larger than the name | OD about 21.3 mm |
| Copper tube for air conditioning and refrigeration (ACR, ASTM B280) | Named by actual OD | OD 12.7 mm |
| Copper water tube (ASTM B88 Types K / L / M) | Nominal; OD is 1/8 inch larger than the name | OD 5/8 inch (about 15.9 mm) |
| Plastic pipe such as PVC, PPR | By each pipe's own standard; PPR is usually named by OD in millimetres | Check the pipe specification or measure |
So the single name "1/2 inch" can mean at least three different ODs. Ordering insulation by name without stating the pipe type is likely to give the wrong size.
How is tube insulation sized?
Pre-formed tube insulation is described by three numbers: bore × wall thickness × length per piece. The bore is made equal to or slightly larger than the pipe OD. Each manufacturer prints its matching table differently.
| Product | What the manufacturer's table prints | Example from the table |
|---|---|---|
| --- | --- | --- |
| AEROFLEX Tube | Bore in inches and millimetres, plus a steel pipe size for 14 bores | 7/8 inch (22 mm) bore pairs with 1/2 inch steel pipe |
| MAXFLEX FR Tube | Bore against copper, iron/PVC (IPS) and PPR pipe sizes | 22 mm bore pairs with 7/8 inch copper, 1/2 inch IPS and 20 mm PPR |
| ArmaFlex Class 1 | Bore against copper pipe OD (inch), steel pipe OD and DN | 22 mm bore pairs with 7/8 inch copper and DN 15 |
| SCG PCI | Steel pipe size (inch) with OD in millimetres | 1/2 inch steel pipe = OD 21.40 mm, 1 inch = 34.10 mm, 2 inch = 60.30 mm |
Note that insulation named "7/8 inch" bore fits a steel pipe named "1/2 inch". The number in the insulation name and the number in the pipe name do not have to match. What must match is the insulation bore and the pipe OD.
The AEROFLEX document prints matches for steel pipe only. For copper or other pipes, measure the OD and choose directly by bore. Our product page therefore offers both a "steel pipe size" selector and an "insulation bore" selector. SCG PCI is sized for steel pipe; copper tube is a separate model from the manufacturer.
Finding the right size in 3 steps
- Identify the pipe type: steel, AC copper, copper water tube, PVC or PPR.
- Measure the actual OD. Use a vernier caliper on the outside of the pipe, or wrap a tape around it and divide the circumference by 3.1416. If the pipe has old insulation or thick paint, measure on the bare pipe.
- Compare with the insulation manufacturer's table. Choose the bore the manufacturer lists for that OD. If the table does not cover your pipe type, send us the measured OD and we will help match it.
Bore is a separate question from insulation wall thickness, which must come from design for the pipe temperature and ambient conditions. Read on in How thick should insulation be? and How to choose pipe insulation.
A bore that is too loose or too tight affects the result
A loose bore leaves an air gap between pipe and insulation; on a cold pipe, water vapour can condense in that gap. A tight bore forces the insulation to stretch, thinning the wall and putting the seam in tension, so it may open later. See the related symptoms in Why do AC pipes drip?
Common mistakes
- Saying only "1/2 inch pipe" without stating steel or copper
- Using the number in the pipe name directly as the insulation bore
- Applying one insulation brand's matching table to another brand, although the manufactured bores can differ slightly
- Forgetting fittings, valves and flanges, which are larger than the pipe and need sheet insulation or a larger tube cut to fit
Sources
- ASME B36.10M Welded and Seamless Wrought Steel Pipe — read through Wermac's summary tables and the Nominal Pipe Size article (Wikipedia); the standard itself was not read
- ASTM B88 (copper water tube) and ASTM B280 (ACR copper tube) — read through The Engineering ToolBox summary tables
- Aeroflex Co., Ltd. — AEROFLEX brochure (AF/OCT 2024), standard packing table, p.7
- Vandapac Co., Ltd. — MAXFLEX FR STANDARD catalogue (FLEX 08/24), p.4
- Armacell — ArmaFlex Class 1 TDS (11/2019, APAC), tube size table
- SCG — SCG Insulation for Industrial : Pipe (PCI size table and installation guide)
Related products
Categories and example products for this topic
- AEROFLEX Tube Insulation — Its page lets you pick by steel pipe size or by insulation bore, 6–165 mm.
- ArmaFlex Class 1 Tube — 2 m tubes; the maker prints bore against copper OD, steel OD and DN.
- MAXFLEX FR STANDARD Tube — Bore 6–165 mm; the maker's table matches copper, iron/PVC and PPR pipes.
- SCG PCI / PCIF — Glass-wool sections sized for 1/2–18 inch steel pipe; 25, 38, 50 mm thick.
Pipe insulation · Elastomeric foam
Armacell ArmaFlex Class 1 Tube
- Tube bore
- 6–168 mm
- Wall thickness
- 6–50 mm
- Service temp.
- -50 – 105 °C
Pipe insulation · Elastomeric foam
MAXFLEX FR STANDARD Tube
- Tube bore
- 6–165 mm
- Wall thickness
- 6–50 mm
- Service temp.
- -57 – 125 °C
Pipe insulation · Glass wool
SCG PCI / PCIF
- Pipe size
- ½–18 in
- Thickness
- 25–50 mm
- Service temp.
- 10 – 540 °C