The short answer: For a round tube, record the outside diameter, wall thickness and required cut length. If a component must fit inside the tube, check the inside diameter and the tolerances that matter to your fit.
Tube measurements can be confusing because “20 mm tube” may refer to different dimensions in different contexts. Before ordering a replacement piece or material for a new build, identify which measurement your job actually needs.
Outside diameter
The outside diameter (OD) is the distance across the outside of a round tube. Measure across the widest part of the circular section, passing through its centre.
Outside diameter matters when the tube must fit into a clamp, bracket, hole or another component.
Wall thickness
The wall thickness is the thickness of the metal between the outside and inside surfaces. It affects the opening inside the tube and is an essential part of the product specification.
Do not assume that two tubes with the same OD have the same wall thickness. Check both dimensions on the product listing or drawing.
Inside diameter
The inside diameter (ID) is the width of the opening. For an ideal round tube, its nominal value can be calculated as:
Inside diameter = outside diameter − twice the wall thickness
For example, a tube with a nominal 20 mm OD and 2 mm wall has a nominal 16 mm ID. Real stock has manufacturing tolerances. If a close-fitting part must slide inside, confirm the actual dimensional requirements before ordering.
Cut length
Measure the length you want supplied, in millimetres. If you will face, machine or otherwise finish the ends, allow the extra length your process needs. Tell the supplier if a particular cut tolerance or end finish is required.
A clear tube order
Include material, grade where relevant, OD, wall thickness, length and quantity:
304 stainless steel round tube, 25 mm OD × 1.5 mm wall, six pieces at 400 mm long.
If you’re matching an existing tube, check whether you need the same grade and surface finish as well as the same dimensions.
Cut Length Metals supplies selected stainless steel, brass and aluminium tube cut to length for customers across Australia. Contact us with your measurements if you need help identifying an available size.
