When you order tubing for a maritime, offshore, or industrial application, one of the first questions you need to answer is deceptively simple: is the size referring to the inside or the outside of the tube? Getting this wrong can mean ordering material that does not fit your system, delaying your project, and costing you time you cannot afford to lose. This guide breaks down how tubing is measured, why the distinction matters, and how to make the right call when placing your order.
Is tubing measured by inside or outside diameter?
For most structural and mechanical tubing, the size refers to the outside diameter (OD). This is the most common convention in the steel industry and applies to round, square, and rectangular tube sections used in construction, fabrication, and offshore applications.
However, there is an important exception. Hydraulic and instrumentation tubing is sometimes specified by inside diameter (ID), because what matters most in those systems is the flow area inside the tube, not the outer profile. In practice, the majority of steel tube sizes you will encounter in maritime and industrial procurement are OD-referenced, with wall thickness specified separately to define the ID.
So the short answer is: most tubing is sized by OD, but always confirm which convention applies to your specific product and application before ordering.
Why does it matter whether you measure ID or OD?
The difference between ID and OD is not just a technical detail — it has real consequences for how components fit together in a system. If you are connecting tubing to fittings, flanges, or other tubes, the mating dimensions must align precisely. Ordering by the wrong reference dimension means the parts will not fit, and in critical applications such as pressure systems or hydraulic lines, a poor fit is a safety risk.
Wall thickness is the variable that links OD and ID. Once you know the OD and the wall thickness, the ID is calculated by subtracting twice the wall thickness from the OD. This is why most tubing specifications list OD and wall thickness together rather than OD and ID — it gives you complete dimensional information in the most practical format.
For buyers sourcing tubing for vessels or offshore platforms, where systems are often custom-built or maintained under tight schedules, understanding this relationship helps you communicate clearly with your supplier and avoid costly specification errors.
What’s the difference between pipe sizing and tubing sizing?
This is a question that trips up a lot of buyers, and it is worth explaining clearly. Pipes and tubes are not sized the same way, even though they look similar and are sometimes used interchangeably in conversation.
Pipe sizing uses a system called Nominal Pipe Size (NPS), which is a standardised designation that does not directly correspond to any actual measured dimension. For example, a 2-inch NPS pipe does not have a 2-inch OD. The actual OD is fixed per NPS designation, and the wall thickness is defined by the schedule — such as Schedule 40 or Schedule 80. This is the system used for ASTM pipes, which we supply extensively for maritime and industrial applications.
Tubing sizing, on the other hand, is more straightforward. The stated size typically refers directly to the actual OD, and wall thickness is specified separately in millimetres or fractions of an inch. This makes tubing dimensions more transparent and easier to verify with a caliper.
Understanding this difference matters when you are sourcing from a supplier or reading a technical drawing. If the specification says “2-inch pipe,” that is an NPS reference. If it says “2-inch tube,” it likely means the OD is genuinely 2 inches.
What are the most common tubing sizes and dimensions?
Steel tube sizes vary widely depending on the application, but there are standard ranges that cover the majority of industrial and maritime needs. Here are the most commonly encountered dimensions:
- Small diameter tubing: 6 mm to 50 mm OD, used for instrumentation, hydraulic lines, and precision applications
- Medium diameter tubing: 50 mm to 200 mm OD, common in structural fabrication and offshore equipment
- Large diameter tubing: 200 mm up to 20 inches OD, used in structural columns, heavy fabrication, and piping systems
- Wall thickness: Ranges from 1 mm for light-gauge tube up to 20 mm or more for heavy-duty applications
- Common standards: EN 10210, EN 10219 for structural hollow sections; ASTM A519 and A513 for mechanical tubing
Material grade also plays a role in dimensional tolerances. Stainless steel tubing, for example, often has tighter dimensional tolerances than carbon steel, which matters when you are fitting components in precision assemblies. If you are unsure which dimensions apply to your project, get in touch with our team and we will help you work through the specification.
How do you choose between ID or OD when ordering tubing?
When you are ready to place an order, follow this practical approach:
- Check your technical drawing or system specification first. It should state whether dimensions are OD or ID references. If it lists OD and wall thickness, you are working with the most common convention.
- Identify the application. Structural and mechanical tubing: order by OD and wall thickness. Hydraulic or flow-critical systems: confirm whether ID is the governing dimension.
- Measure existing components if replacing parts. Use a caliper to measure the outside of the tube and the wall thickness. Do not assume the nominal size matches the actual measurement — especially with older or non-standard material.
- Specify both OD and wall thickness in your order. This eliminates ambiguity and gives your supplier everything they need to pull the right material from stock.
- Ask your supplier if you are unsure. A knowledgeable supplier should be able to confirm the correct specification from a description of your application, the existing fitting dimensions, or a drawing.
You can browse the full range of available products on our Marine Steel products page to get a sense of what is in stock and what dimensions are available.
How Marine Steel helps you order the right tube, first time
Ordering the wrong tubing size is a mistake that costs time, money, and in some cases, project progress. We understand the pressure our clients are under — whether you are maintaining a vessel in port or sourcing for an offshore installation — which is why we do more than just fulfil orders.
- We stock steel tube sizes from small-diameter precision tubing up to 20-inch large-diameter sections
- Our team advises on OD vs ID specifications, wall thickness, and material grade based on your application
- We supply tube with full material certificates and documentation where required
- Custom cutting, bending, and processing are available from our Rotterdam and Houston warehouses
- One conversation is enough — we think along with you and come back with the right solution
If you are not sure which tubing specification you need, or you want a fast quote on a specific size, contact us directly and we will get back to you promptly.
