What it does
Reinforced concrete slabs, walls and footings usually carry a mesh of steel bars: one set running along the length and one across, at a set spacing, in one or two layers. This calculator turns that description into quantities:
- the number of bars in each direction, with a bar at each edge and no gap wider than your spacing,
- each bar’s length after the cover at both ends, plus any hooks or bends,
- laps: when a bar is longer than a stock length, it is made from several stock bars overlapping by your lap length,
- the total length and weight (mass per metre = 7850 kg/m³ × π d² / 4), with an optional waste allowance,
- the number of stock bars to order, found by fitting the cut pieces into stock lengths (first fit, longest first),
- a cut list (marks, sizes, lengths and numbers) as a table, CSV, image, PDF or Word file, and a plan drawing of the mesh.
For study and planning, not a substitute for professional judgement. Check your local code and a qualified engineer, and always order from the approved bar schedule.
How to use it
- Pick slab, wall or footing. This only changes the wording and the starting number of layers (two mats for a slab, two faces for a wall, one mat for a footing); change it as you need.
- Type the length and width (or height) in metres, the cover in mm (measured to the bar ends and edges), and how many identical elements there are.
- For each direction, choose the bar size, the spacing (centre to centre) and any extra length per end for a hook or bend.
- Type the stock bar length you can buy (often 6 m or 12 m) and the lap length from your drawings or code.
- Add a waste allowance if you want a margin for offcuts and mistakes.
Worked example
A made-up slab 6 m × 4.5 m with 40 mm cover, two mats of 12 mm bars at 150 mm along the length and 10 mm bars at 200 mm along the width, 12 m stock bars (the page opens with these values):
- Along the length: (4.5 − 0.08) ÷ 0.15 = 29.5, so 31 bars per mat, 62 bars of 5.92 m. 12 mm bar weighs 0.888 kg/m, so 367.0 m weighs 325.9 kg.
- Along the width: (6 − 0.08) ÷ 0.2 = 29.6, so 31 per mat, 62 bars of 4.42 m, 274.0 m of 10 mm bar at 0.617 kg/m = 169.0 kg.
- Total 494.8 kg of steel, 641.1 m of bar, about 18.3 kg per m² of slab. Two 5.92 m bars fit in each 12 m stock bar, and two 4.42 m bars in another, so 62 stock bars are needed.
Tips
- If your spacing does not divide the width exactly, the calculator adds a bar so that no gap is wider than the spacing.
- Laps are placed one per stock length. In real work they are staggered, and codes limit how many laps sit at one section; follow your drawings.
- The steel area per metre (π d²/4 × 1000 ÷ spacing) is shown so you can compare it with the area your design needs.
FAQ
Why is the weight different from my supplier’s? Bars are rolled to a tolerance on mass, and some standards give rounded masses per metre. This tool uses the exact circle area and 7850 kg/m³.
Does it include chairs, links or starter bars? No. Add them as another element, or by hand.
Can I enter the lap as so many bar diameters? Multiply it out: a lap of 50 diameters on a 12 mm bar is 600 mm. Use the figure your code and designer give you.
Where the numbers come from
- Bar mass from the density of steel, 7850 kg/m³, times the area of a circle π d² / 4 (general engineering fact). Bar counting, laps and first-fit-decreasing cutting are fibub's own arithmetic.
- No values are copied from any design code or bar schedule standard (ACI, Eurocode, IS, BS or others): cover, spacing, lap length and hooks are all entered by the user, and the example values are made up.
- Checked against an independent Python calculation written for the tests (mass per metre, counting loop, greedy lap split and a separate first-fit-decreasing packer).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.