Tubing OD vs ID: Chemical Feed Sizing Reference | Apex Flow
Tubing is sized differently from pipe: tubing is measured by its actual outside diameter, while traditional pipe is sized by a nominal bore that no longer matches any physical dimension. That difference matters because you select compression fittings, sleeves, and ferrules by the tubing OD — not by a nominal size. A 3/8" OD tube is 0.375" outside; a 3/8" nominal pipe is 0.675" outside. This guide gives you the OD, ID, and wall thickness for common tubing sizes and explains why OD matters for fitting selection.
Apex Flow Solutions stocks compression fittings for copper, stainless, and plastic tubing in the common OD sizes. The figures below are nominal; wall thickness varies by material, pressure rating, and tubing series — always confirm wall thickness from your tubing's spec sheet before selecting fittings.
Compression fittings grip the tubing OD via a ferrule; the fitting size must match the exact OD. If you're unsure of your tubing's OD, measure it or send us the tubing spec and we'll confirm the correct fitting size.
In This Guide
- OD, ID, and Wall Thickness Explained
- Tubing OD/ID/Wall Sizing Chart
- Copper Tubing Types (K, L, M)
- Stainless Tubing OD Series
- Plastic & Flexible Tubing
- Selecting Fittings by OD
- Standards & References
- Frequently Asked Questions
OD, ID, and Wall Thickness Explained
The outside diameter (OD) is the total outer width of the tube wall and does not change with wall thickness for a given tubing series — it is what compression fittings, sleeves, and ferrules grip. The inside diameter (ID) is the actual flow bore; it changes with wall thickness. The wall thickness determines pressure rating: thicker walls handle more pressure but reduce ID and flow capacity. The relationship is: ID = OD − (2 × wall thickness). For fitting selection, use the OD. For flow calculations, use the ID.
OD is fixed for a given tubing series and is what fittings are sized to. Wall thickness determines pressure rating; ID = OD minus two times wall thickness.
Tubing OD/ID/Wall Sizing Chart
Representative copper Type L (most common for water) and 316 stainless instrument tubing dimensions. Actual dimensions vary — always verify from the specific tubing's spec sheet.
| Nominal size | OD (in) | Copper Type L wall | Copper Type L ID | 316 SS instrument wall (common) | 316 SS ID |
|---|---|---|---|---|---|
| 1/4" | 0.375" | 0.030" | 0.315" | 0.035" | 0.305" |
| 3/8" | 0.500" | 0.035" | 0.430" | 0.049" | 0.402" |
| 1/2" | 0.625" | 0.040" | 0.545" | 0.049" | 0.527" |
| 5/8" | 0.750" | 0.042" | 0.666" | 0.065" | 0.620" |
| 3/4" | 0.875" | 0.045" | 0.785" | 0.065" | 0.745" |
| 1" | 1.125" | 0.050" | 1.025" | 0.083" | 0.959" |
Metering pump tubing selection: specify by OD for the compression fitting seat and by ID for the flow rate and pump head compatibility. Wall thickness determines pressure rating — confirm against the pump's discharge pressure.
Copper Tubing Types (K, L, M)
Copper tubing in the U.S. is sold in three standard wall grades for the same OD. Type K (thickest) is used underground and for high-pressure service; Type L is the general plumbing standard; Type M (thinnest) is used for low-pressure indoor water supply where local codes allow. All three types for a given nominal size have the same OD and therefore use the same fitting. The difference is wall thickness and hence pressure rating and ID:
| Copper type | Wall (1/2" nominal) | ID (1/2" nominal) | Use |
|---|---|---|---|
| Type K | 0.049" | 0.527" | Underground, high-pressure |
| Type L | 0.040" | 0.545" | General plumbing standard |
| Type M | 0.028" | 0.569" | Low-pressure indoor supply |
Stainless Tubing OD Series
316 stainless instrument tubing follows an OD-based sizing system independent of nominal pipe. Common instrument tubing ODs are 1/8", 3/16", 1/4", 3/8", 1/2", 5/8", 3/4", and 1". Wall thicknesses for each OD vary by pressure class; a 1/4" OD tube might be offered in 0.028", 0.035", 0.049", and 0.065" walls, each with a different pressure rating and ID. Fittings from compression-fitting manufacturers (Swagelok, Parker CPI, etc.) are sized directly by tubing OD and use the same ferrule across all walls for the same OD — the ferrule seals on the OD, not the ID. Specify tube OD and wall when ordering to ensure the pressure rating matches your system.
Plastic & Flexible Tubing
Polyethylene (PE), nylon (PA), polyurethane (PU), and PTFE tubing are also sold by OD. Push-to-connect fittings grip the tubing OD; compression fittings for plastic tubing require an insert sleeve inside the tube to prevent collapse under the ferrule load. Common ODs for pneumatic and chemical-feed tubing are 4 mm, 6 mm, 8 mm, 10 mm, 12 mm (metric), and 1/4", 3/8", 1/2", 5/8" (inch). Specify tubing by material, OD, and wall thickness or minimum bend radius. The ID determines flow capacity; for chemical-feed metering pumps, confirm the pump head's tubing ID and OD requirements before ordering.
Selecting Fittings by OD
For compression fittings, push-to-connect fittings, and flared fittings: always select the fitting by the tubing OD. A compression fitting for 1/2" OD tubing accepts any tubing that is exactly 0.500" OD regardless of wall thickness or material — the ferrule seats on the outer surface. Two tubes of the same nominal size but different OD series (e.g., 1/2" nominal copper OD = 0.625", versus 1/2" nominal pipe OD = 0.840") are not interchangeable in compression fittings. Measure the OD with calipers before ordering if there is any doubt about which series of tubing you have.
Standards & References
Copper tubing dimensions follow ASTM B88 (Types K, L, M). Stainless instrument tubing follows ASTM A269 (general), ASTM A213 (seamless), or manufacturer standards. Polyethylene tubing for gas and water follows ASTM D3350 or ASTM F1973. Nylon and polyurethane tubing for pneumatics follow manufacturer OD standards. Compression fitting engagement is per manufacturer spec (Swagelok MS-01, Parker CPI, or equivalent). Thread connections on fitting run-ends are NPT per ASME B1.20.1.
Frequently Asked Questions
Is tubing sized by OD or ID?
By OD. Fittings, ferrules, and push-to-connect grippers all seat on the outside of the tube. ID varies with wall thickness; it is used for flow calculations, not fitting selection.
Why doesn't the nominal size match the actual OD?
Copper and stainless tubing nominal sizes are a legacy naming convention, not a dimension. A 1/2" nominal copper tube is 0.625" OD. The only way to know the actual OD is to measure it or look up the spec for that tubing standard.
Can I use a Type L and Type M copper tube in the same compression fitting?
Yes — both have the same OD for a given nominal size. The compression fitting doesn't care about wall thickness; the ferrule grips the OD. Just confirm the wall meets your pressure requirement.
What size compression fitting do I need for 1/2" copper tubing?
A 5/8" OD compression fitting. "1/2" nominal copper" has a 0.625" OD, so the fitting is specified as 5/8" OD. This is one of the most common sources of sizing confusion.
Related Resources
- Compression Fitting Sizing Chart — size ferrule fittings by tubing OD
- Hose Barb Sizing Chart — when flexible hose replaces rigid tube
- NPT Thread Sealant & Torque Guide — sealing the threaded end of compression fittings
- Technical Resource Center
Shop Related Products
Compression Fittings · Tube Fitting Adapters · Tube Fitting Elbows · Tube Fitting Tees
Technical data disclaimer: Specifications, dimensions, pressure/temperature ratings, and compatibility information on this page are compiled from manufacturer publications and are provided for reference only. Always verify suitability for your application against the manufacturer’s current documentation before purchase or installation. Apex Flow Solutions assumes no liability for errors, omissions, or misapplication of this information.