Angle iron is measured as leg length × leg length × thickness, with each leg read from the outer corner to the toe and thickness taken on the flat face.
A tape laid across the rounded corner instead of the heel reads short by the radius, and that error follows the whole job: wrong bracket, wrong weld gap, wrong order. Angle iron is sized by three numbers — leg, leg, thickness. Get those right and everything downstream lines up. This guide covers the measurement order that works, the two spots where tape readings go wrong, and how listing notation maps to what’s on the shelf.
What Do the Numbers in Angle Iron Sizes Mean?
Size is written as two leg lengths followed by thickness, all in the same unit. Equal-leg stock uses matching figures: 2-inch legs and 1/4-inch thickness reads 2 × 2 × 1/4. Unequal-leg stock carries two different figures, such as 4 × 3 × 3/8.
Metric listings drop the multiplication signs. Engineering ToolBox documents sizes with all three values stated together, and current industry listings run formats like 25x25x1.8, 30x30x3, and 50x50x4 — millimeters for legs and thickness.
- First number: length of one leg, heel to toe.
- Second number: length of the other leg — identical on equal-leg, different on unequal-leg stock.
- Third number: wall thickness of the steel.
Measure both legs before assuming they match — plenty of stock sold as “2 inch” turns out to be 2 × 1-1/2 when checked.
How to Measure Angle Iron Step by Step
A clean measurement takes about two minutes per piece with a tape, caliper, and square, on a flat, stable surface with the piece secured.
- Measure leg A. Hook the tape at the outer corner (the heel, the sharp outside edge where the legs meet) and run it straight to the tip. Record it.
- Measure leg B. Repeat on the other leg. If the figures differ by more than a hair, it’s unequal-leg stock and both numbers go in your notes.
- Measure thickness. Use calipers on the flat part of the leg, away from the rounded inside corner. The bend adds material near the heel, so a reading there runs heavy.
- Check the angle. Lay a square against the inside corner. Standard angle iron opens to 90 degrees; anything off-square should be measured and marked as-is.
- Measure overall length. Run the tape end to end, aligned to one edge so it doesn’t drift.
On longer stock, check dimensions at both ends and the middle — a bent or twisted piece won’t read the same in three places, and finding that out before you cut saves the material.
Size and thickness both have to clear your project’s load and fit requirements before you cut or weld — buying on leg length alone is how brackets end up sloppy. If you’re replacing a specific piece, our roundup of tested 1 1/2 inch angle iron picks covers what’s in stock and how the sizes compare.
Common Measuring Mistakes to Avoid
Nearly every wrong order traces back to one of four habits.
- Measuring from the inside corner. The inside radius is not the leg. Start at the outer heel or your number comes up short.
- Reading thickness at the bend. The rounded corner is thicker than the flat. Slide off the radius before closing the calipers.
- Assuming equal legs. Never trust the label over the tape. Check both legs on every piece.
- One loose tape pass. A tape held off-angle adds or loses fractions. Keep it tight and aligned, and check multiple points.
The two rules that prevent all four: measure at the heel, and measure the flat.
Reading a Supplier Listing Correctly
Stock listings use their own shorthand, and thickness is the field people skim. McMaster-Carr’s angle iron category, for example, runs thickness options from 1/8″ up through 7/8″ alongside an outside-width field, so wall thickness is a separate pick from leg width, not bundled into it.
Overall length is quoted separately from the cross-section. One manufacturer stocks lengths of 6 m, 6–7.2 m, and 6–13 m depending on size, so the third number in a callout is thickness while cut length is a different question. Confirm both before ordering.
For reference values on standard sections, the Engineering ToolBox steel angles tables list equal-leg dimensions in one place.
FAQs
Do I measure angle iron from the inside or outside corner?
Always from the outside. Hook your tape at the heel — the sharp outer edge where the two legs meet — and run it to the tip of the leg. Measuring from the inside corner catches the rounded radius instead of the true leg, so the reading comes out short and your sizing is wrong from the start.
Why does thickness read differently near the corner?
The bend adds material at the inside radius, so calipers closed on the rounded corner measure heavier than the actual wall. Slide the tool onto the flat portion of the leg, a comfortable distance from the heel, and take the reading there. That flat-face number is the thickness the supplier quotes.
How can I tell equal-leg from unequal-leg stock?
Measure both legs separately. If the figures match, it’s equal-leg and sizes as 2 × 2 × 1/4 style. If they differ, you have unequal-leg stock and both numbers belong in your notes, written in the order your supplier lists them — leg, leg, then thickness.
References & Sources
- Engineering ToolBox. “Steel Angles – Equal Legs” Reference tables for standard equal-leg angle dimensions.
- McMaster-Carr. “Angle Iron” Current thickness options and outside-width listings.
- Metal Supermarkets. “Angle Iron: Sizes, Uses & Buying Advice” Size notation formats and stocked length ranges.