Understanding Pipe Insulation Sizes and Dimensions
This technical bulletin serves as a practical guide to pipe insulation sizing, helping engineers, designers, contractors, and maintenance personnel understand the difference between nominal pipe sizes, actual pipe dimensions, and insulation thickness requirements.
A common source of confusion in piping systems is that both pipe and insulation are often specified using nominal dimensions rather than their actual physical measurements. For example, a pipe designated as NPS 2 (Nominal Pipe Size 2) does not have an outside diameter of exactly 2 inches. Instead, it has an actual outside diameter of 2.375 inches. Likewise, a specified insulation thickness refers to the radial thickness of the insulation and not its overall outside dimension.
Our detailed pipe insulation size chart provides the dimensional information needed to properly match insulation thicknesses with the correct pipe outside diameters. This ensures proper fit-up, minimizes installation issues, and helps maintain the thermal performance of the insulation system.
Because the piping industry relies on terms such as Nominal Pipe Size (NPS) and Nominal Insulation Thickness, selecting insulation for a given pipe size often requires verifying the actual pipe outside diameter. For example, ordering insulation for a 2-inch pipe requires knowledge of the pipe’s true outside diameter to ensure a secure and properly fitted insulation assembly.
Similarly, when selecting insulation for smaller pipe sizes, such as 1¼-inch pipe insulation, it is important to verify dimensions using a steel pipe size chart or pipe outside diameter table. Confirming the relationship between the insulation inside diameter (I.D.) and the pipe outside diameter (O.D.) helps prevent excessive gaps or improper fit, both of which can reduce thermal efficiency, increase energy loss, and potentially contribute to moisture intrusion and corrosion under insulation (CUI).
Understanding these dimensional relationships is essential for designing and installing effective insulation systems that provide long-term thermal performance, energy conservation, and mechanical reliability.
| NOMINAL INSULATION THICKNESS (in.) | INSULATION O.D. (in.) | APPROX. INSULATION THICKNESS (in.) |
Pipe size: 1/2" (O.840 O.D.) |
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| 1 | 2 7/8 | 1 |
| 1 1/2 | 4 | 1 9/16 |
| 2 | 5 | 2 1/16 |
| 2 1/2 | 6 5/8 | 2 7/8 |
| 3 | 7 5/8 | 3 3/8 |
| 3 1/2 | 8 5/8 | 3 7/8 |
| 4 | 9 5/8 | 4 3/8 |
Pipe size: 3/4" (1.050 O.D.) |
||
| 1 | 2 7/8 | 7/8 |
| 1 1/2 | 4 | 1 7/16 |
| 2 | 5 | 1 15/16 |
| 2 1/2 | 6 5/8 | 2 3/4 |
| 3 | 7 5/8 | 3 1/4 |
| 3 1/2 | 8 5/8 | 3 3/4 |
| 4 | 9 5/8 | 4 1/4 |
Pipe size: 1" (1.315 O.D.) |
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| 1 | 3 1/2 | 1 1/16 |
| 1 1/2 | 4 1/2 | 1 9/16 |
| 2 | 5 9/16 | 2 1/8 |
| 2 1/2 | 6 5/8 | 2 5/8 |
| 3 | 7 5/8 | 3 1/8 |
| 3 1/2 | 8 5/8 | 3 5/8 |
| 4 | 9 5/8 | 4 1/8 |
Pipe size: 1 1/4" (1.66 O.D.) |
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| 1 | 3 1/2 | 15/16 |
| 1 1/2 | 5 | 1 11/16 |
| 2 | 5 9/16 | 1 15/16 |
| 2 1/2 | 6 5/8 | 2 1/2 |
| 3 | 7 5/8 | 3 |
| 3 1/2 | 8 5/8 | 3 1/2 |
| 4 | 9 5/8 | 4 |
Pipe size: 1 1/2" (1.90 O.D.) |
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| 1 | 4 | 1 1/16 |
| 1 1/2 | 5 | 1 9/16 |
| 2 | 6 5/8 | 2 3/8 |
| 2 1/2 | 7 5/8 | 2 7/8 |
| 3 | 8 5/8 | 3 3/8 |
| 3 1/2 | 9 5/8 | 3 7/8 |
| 4 | 10 3/4 | 4 7/16 |
Pipe size: 2" (2 3/8" O.D.) |
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| 1 | 4 1/2 | 1 1/16 |
| 1 1/2 | 5 9/16 | 1 9/16 |
| 2 | 6 5/8 | 2 1/8 |
| 2 1/2 | 7 5/8 | 2 5/8 |
| 3 | 8 5/8 | 3 1/8 |
| 3 1/2 | 9 5/8 | 3 5/8 |
| 4 | 10 3/4 | 4 3/16 |
Pipe size: 2 1/2" (2 7/8" O.D.) |
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| 1 | 5 | 1 1/16 |
| 1 1/2 | 6 5/8 | 1 7/8 |
| 2 | 7 5/8 | 2 3/8 |
| 2 1/2 | 8 5/8 | 2 7/8 |
| 3 | 9 5/8 | 3 3/8 |
| 3 1/2 | 10 3/4 | 3 15/16 |
| 4 | 11 3/4 | 4 7/16 |
Pipe size: 3" (3 1/2" O.D.) |
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| 1 | 5 9/16 | 1 |
| 1 1/2 | 6 5/8 | 1 9/16 |
| 2 | 7 5/8 | 2 1/16 |
| 2 1/2 | 8 5/8 | 2 9/16 |
| 3 | 9 5/8 | 3 1/16 |
| 3 1/2 | 10 3/4 | 3 5/8 |
| 4 | 11 3/4 | 4 1/8 |
Pipe size: 3 1/2" (4" O.D.) |
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| 1 | 6 5/8 | 1 5/16 |
| 1 1/2 | 7 5/8 | 1 13/16 |
| 2 | 8 5/8 | 2 5/16 |
| 2 1/2 | 9 5/8 | 2 13/16 |
| 3 | 10 3/4 | 3 3/8 |
| 3 1/2 | 11 3/4 | 3 7/8 |
| 4 | 12 3/4 | 4 3/8 |
Pipe size: 4" (4 1/2" O.D.) |
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| 1 | 6 5/8 | 1 1/16 |
| 1 1/2 | 7 5/8 | 1 9/16 |
| 2 | 8 5/8 | 2 1/16 |
| 2 1/2 | 9 5/8 | 2 9/16 |
| 3 | 10 3/4 | 3 1/8 |
| 3 1/2 | 11 3/4 | 3 5/8 |
| 4 | 12 3/4 | 4 1/8 |
Pipe size: 4 1/2" (5" O.D.) |
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| 1 | 7 5/8 | 1 5/16 |
| 1 1/2 | 8 5/8 | 1 13/16 |
| 2 | 9 5/8 | 2 5/16 |
| 2 1/2 | 10 3/4 | 2 7/8 |
| 3 | 11 3/4 | 3 3/8 |
| 3 1/2 | 12 3/4 | 3 7/8 |
| 4 | 14 | 4 1/2 |
Pipe size: 5" (5 9/16" O.D.) |
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| 1 | 7 5/8 | 1 |
| 1 1/2 | 8 5/8 | 1 1/2 |
| 2 | 9 5/8 | 2 |
| 2 1/2 | 10 3/4 | 2 9/16 |
| 3 | 11 3/4 | 3 1/16 |
| 3 1/2 | 12 3/4 | 3 9/16 |
| 4 | 14 | 4 3/16 |
Pipe size: 6" (6 5/8" O.D.) |
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| 1 | 8 5/8 | 15/16 |
| 1 1/2 | 9 5/8 | 1 7/16 |
| 2 | 10 3/4 | 2 |
| 2 1/2 | 11 3/4 | 2 1/2 |
| 3 | 12 3/4 | 3 |
| 3 1/2 | 14 | 3 5/8 |
| 4 | 15 | 4 1/8 |
Pipe size: 7" (7 5/8" O.D.) |
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| 1 | 9 5/8 | 1 |
| 1 1/2 | 10 3/4 | 1 1/2 |
| 2 | 11 3/4 | 2 |
| 2 1/2 | 12 3/4 | 2 1/2 |
| 3 | 14 | 3 1/8 |
| 3 1/2 | 15 | 3 5/8 |
| 4 | 16 | 4 1/8 |
Pipe size: 8" (8 5/8" O.D.) |
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| 1 | 10 3/4 | 1 1/16 |
| 1 1/2 | 11 3/4 | 1 1/2 |
| 2 | 12 3/4 | 2 |
| 2 1/2 | 14 | 2 5/8 |
| 3 | 15 | 3 1/8 |
| 3 1/2 | 16 | 3 5/8 |
| 4 | 17 | 4 1/8 |
Pipe size: 9" (9 5/8" O.D.) |
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| 1 | 11 3/4 | 1 1/16 |
| 1 1/2 | 12 3/4 | 1 1/2 |
| 2 | 14 | 2 |
| 2 1/2 | 15 | 2 5/8 |
| 3 | 16 | 3 1/8 |
| 3 1/2 | 17 | 3 5/8 |
| 4 | 18 | 4 1/8 |
Pipe size: 10" (10 3/4" O.D.) |
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| 1 | 12 3/4 | 1 |
| 1 1/2 | 14 | 1 9/16 |
| 2 | 15 | 2 1/16 |
| 2 1/2 | 16 | 2 9/16 |
| 3 | 17 | 3 1/16 |
| 3 1/2 | 18 | 3 9/16 |
| 4 | 19 | 4 1/16 |
Pipe size: 11" (11 3/4" O.D.) |
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| 1 | 14 | 1 1/8 |
| 1 1/2 | 15 | 1 9/16 |
| 2 | 16 | 2 1/16 |
| 2 1/2 | 17 | 2 9/16 |
| 3 | 18 | 3 1/16 |
| 3 1/2 | 19 | 3 9/16 |
| 4 | 20 | 4 1/16 |
Pipe size: 12" (12 3/4" O.D.) |
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| 1 | 15 | 1 1/8 |
| 1 1/2 | 16 | 1 9/16 |
| 2 | 17 | 2 1/16 |
| 2 1/2 | 18 | 2 9/16 |
| 3 | 19 | 3 1/16 |
| 3 1/2 | 20 | 3 9/16 |
| 4 | 21 | 4 1/16 |
Pipe size: 14" (14" O.D.) |
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| 1 | 16 | 2 |
| 1 1/2 | 17 | 1 7/16 |
| 2 | 18 | 1 15/16 |
| 2 1/2 | 19 | 2 7/16 |
| 3 | 20 | 2 15/16 |
| 3 1/2 | 21 | 3 7/16 |
| 4 | 22 | 3 15/16 |
| Large sizes, insulation O.D. are in 1" increments. Insulation thicknesses through 36" same as nominal. All dimensions rounded to the nearest 1/16". |
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Q: Why is a pipe insulation size chart necessary for ordering materials?
A: A pipe insulation size chart is necessary because nominal pipe sizes (names) do not always match the actual outside diameter of the pipe. Using a chart ensures that the insulation’s inner diameter matches the pipe’s outer diameter for a proper seal.
Q: Does the “2-inch” in insulation for a 2-inch pipe refer to the ID or OD?
A: In standard nomenclature, insulation for a 2-inch pipe generally refers to the Nominal Pipe Size (NPS) it is intended to cover. However, the actual internal diameter of the insulation is slightly larger to fit over the 2.375-inch OD of a standard 2-inch steel pipe.
Q: How do I use a steel pipe size table for insulation selection?
A: You use a steel pipe size table to cross-reference the Nominal Pipe Size (e.g., 4″) with the actual outside diameter (4.500″). This ensures that the insulation you purchase has the correct ID to accommodate the steel pipe.
Q: Is 1 ¼ pipe insulation the same size as the pipe itself?
A: No. 1 ¼ pipe insulation is manufactured with an internal diameter specifically designed to fit over a pipe with a nominal bore of 1 ¼ inches (which has an actual OD of 1.660 inches).
Q: Where can I find a reliable pipe insulation size guide?
A: This page functions as a pipe insulation size guide, providing a comparison of nominal sizes versus actual dimensions of both pipe size diameter and insulation thickness to assist in the accurate fabrication of pipe shoes, saddles, and insulation covers.