Rebar Calculator – From Slab Size to a Shopping List
Concrete is very strong in compression but weak in tension. Once it bends or shrinks, it cracks. Rebar (reinforcing bar) is cast into the concrete to carry that tension and keep cracks tight. This rebar calculator turns the size of a slab, wall, footing or beam into the numbers you need at the supply yard: how many bars, how long they are, how many stock bars to buy, the total weight, and the number of ties.
How to Read Rebar Sizes
US bar sizes follow ASTM A615. The bar number is the diameter in eighths of an inch, so #4 rebar is 4/8 = 1/2 in and #5 is 5/8 in. Metric bars are named by their diameter in millimetres (10, 12, 16 mm and so on), and Canadian bars (10M, 15M, 20M) are named by their approximate area. Metric weight comes from the density of steel, 7,850 kg/m³:
kg/m = (π / 4) × d² × 7850 / 1,000,000 ≈ d² / 162 (d in mm)| Bar | Diameter | Weight | Nearest metric bar |
|---|---|---|---|
| #3 | 3/8 in (9.5 mm) | 0.376 lb/ft | ≈ 10 mm (0.617 kg/m) |
| #4 | 1/2 in (12.7 mm) | 0.668 lb/ft | ≈ 12 mm (0.888 kg/m) |
| #5 | 5/8 in (15.9 mm) | 1.043 lb/ft | ≈ 16 mm (1.578 kg/m) |
| #6 | 3/4 in (19.1 mm) | 1.502 lb/ft | ≈ 20 mm (2.466 kg/m) |
Why Cover Matters
Concrete cover is the distance from the concrete surface to the steel. It protects the bar from moisture and chlorides that cause rust. Rust takes up more room than steel, so a rusting bar splits the concrete around it. Cover also shields the steel from fire. Concrete cast directly against soil usually gets about 3 in (75 mm), and formed surfaces get less. The calculator removes the cover from both ends of every bar, so a 20 ft slab with 3 in cover has bars 19 ft 6 in long.
How the Bars Are Counted
Bars in a rebar grid are spread across the width inside the cover. The count is rounded up so the real spacing is never wider than the spacing you asked for:
bars = ceil((width − 2 × cover) ÷ spacing) + 1A 10 ft wide slab with 3 in cover and 12 in spacing has 114 in to fill. That needs ceil(114 ÷ 12) + 1 = 11 bars at an actual 11.4 in spacing. The tool shows both the actual and the requested spacing, and counts a tie at each intersection.
Lap Splices and Why They Are Staggered
When a run is longer than a stock bar, two bars overlap so that one can pass its force to the next. This overlap is a lap splice. A common rule of thumb is 40 × d_b, 20 in for a #4 bar, but the real length depends on concrete strength, steel grade, cover and bar position. Each lap adds steel, so the calculator counts it in the length and the weight. Splices on neighbouring bars should be staggered so they don't all fall in the same weak section, and the drawing shows them offset on alternate bars.
Why You Should Buy Extra
Bars get cut wrong, bent on site, or turn out slightly short. Offcuts from one length rarely match another. The calculator only packs identical cut lengths into a stock bar, so its count is a little conservative. It then adds your waste percentage (5% by default) to the length, the weight and the number of stock bars. A spare bar or two costs far less than a trip back to the yard while the concrete truck waits.