How to Measure Tube
Tube is named by its outside size and its wall. Measure both the way the specification does, away from the cut end and away from the weld, and most arguments about size go away.
What to Measure, and Where.
Measure at least 2 inches from a cut end: the saw can flare or pinch the last inch of tube.
Outside Size
Square and rectangle: across the flats, with calipers square to the side, not across the corners. Round: the outside diameter, turning the calipers to find the largest and smallest readings.
Wall
At the center of a flat side, with a tube micrometer or ball micrometer. Avoid the corners, which thicken as they form, and the weld. Both A500 and A513 allow 10% either way on a square or rectangle.
Corner Radius
Use a radius gauge on the outside corner. A500 allows up to three times the wall; mills usually run about twice the wall. A tight corner matters when the tube must fit a cap or a bracket.
Squareness
Hold a square against two sides. A500 allows 2 degrees either way; A513 allows .006 inch for each inch across the flats.
Twist
Lay a 3 foot section on a flat table and measure how far one corner rises at the far end. Both specs allow .062 to .087 inch in 3 feet, by size.
Straightness
Stretch a line along the tube, or roll it on a flat table, and measure the largest gap. A500 allows .025 inch for each foot of length; A513 allows 1/16 inch in 3 feet on shapes.
Outside Size Tolerances.
Plus or minus, in inches, from our A500 and A513 data sheets.
| A500 square and rectangle, largest side | ±in |
|---|---|
| 2-1/2 in and under | .020 |
| Over 2-1/2 to 3-1/2 in | .025 |
| Over 3-1/2 to 5-1/2 in | .030 |
| Over 5-1/2 in | 1% |
| A513 | ±in |
|---|---|
| Round, over 1-1/8 to 2 in OD | .005 |
| Round, over 2 to 2-1/2 in OD | .006 |
| Round, over 2-1/2 to 3 in OD | .008 |
| Round, over 3 to 4 in OD | .009 |
| Shapes, over 1-1/2 to 2 in widest side | .008 |
| Shapes, over 2 to 3 in | .010 |
| Shapes, over 3 to 4 in | .020 |
| 5 x 2 and 5 x 3 | .030 |
| 6 x 2 | .036 |
On A500 rectangles with sides in a ratio of 1.5 to 3, the small side's tolerance is 1.5 times the large side's. Round tube may be out of round by 50% more than its OD tolerance, as long as the average diameter is in tolerance.
Questions.
Measuring for a claim? Our claims policy asks where on the tube the defect is and for a 4 foot sample.
Is 2 x 2 tube really 2 inches?
Yes, across the flats, within the tolerance for its spec: about .020 inch either way for A500 and .008 inch for A513. Across the corners it measures less than the diagonal of a sharp square, because the corners are rounded.
Why does the wall measure thicker at the corner?
Steel thickens slightly on the inside of a bend as the corner is formed. Specs measure wall at the center of the flat for that reason.
Where is the weld on a square tube?
On a flat side, and on rectangles usually on a long side (the short side on request). The flash, the bead of steel the weld pushes out, is trimmed off the outside; inside it is usually left in place.
How do I measure a round tube that is slightly oval?
Take the largest and smallest diameters at the same spot and average them. A513 allows ovality up to 50% more than the OD tolerance if the average is in tolerance.
More Lessons
Put it to use: Size Finder
Tell Us the Size,
Wall and Footage.
One size or a whole program, in mill lengths or cut to your length. The quote form takes about two minutes.