Rebar Calculator
Rebar calculator works out Bars running the length and Bars running the width from Units, Length (feet, or metres in metric) and Width (feet, or metres in metric). The default figures on this page give total bars of 40; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Clear dimensions: 19.5 x 19.5 ft after 3 in of cover at each edge.
- Bars running the length: floor(234 / 12) + 1 = 20 per mat, x 1 mat(s) = 20.
- Bars running the width: floor(234 / 12) + 1 = 20 per mat, x 1 mat(s) = 20.
- Total length = 780 ft; at 0.994 kg/m that is 236.3 kg (521 lb).
- Steel area provided: 0.2 sq in per bar at 12 in o.c. = 0.2 sq in per foot of width.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
Keep this result
Save it, change one figure, then compare the two
The calculation stays free. Save this scenario, adjust one assumption, save that too, and put them side by side to see exactly what the change costs you. A free account keeps 5 scenarios and 2 comparisons; Personal makes both unlimited and adds exports and private links. Nothing is saved unless you press Save.
What the rebar calculator calculates
Counts the reinforcing bars a slab, footing or wall panel needs at a given spacing and cover, and totals the linear length, the weight in pounds and kilograms, the steel area per foot and the cost.
How to use this rebar calculator
Enter Units, Length (feet, or metres in metric), Width (feet, or metres in metric) and Bar size. Bars running the length updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Bars running the length, Bars running the width and Total bars.
The method follows ASTM A615/A615M, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, Table 1 (nominal diameter, area and mass), ACI 318-19, Building Code Requirements for Structural Concrete and NIST Special Publication 811 (exact inch, pound and foot conversions).
20 ft x 20 ft slab, #4 bars at 12 in each way, 3 in cover example
With Length (feet, or metres in metric): 20, Width (feet, or metres in metric): 20, Bar size: 4, Bar spacing on centre (inches, or millimetres in metric): 12, the calculator returns Bars running the length: 20, Bars running the width: 20, Total bars: 40. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Bars running the length, Bars running the width and Total bars. The inputs you can vary are Units, Length (feet, or metres in metric), Width (feet, or metres in metric), Bar size, Bar spacing on centre (inches, or millimetres in metric) and Spacing of the cross bars (0 = same as the main spacing).
Understanding Bars running the length, Bars running the width and Total bars
This is a quantity take-off, not a reinforcement design. Bar size, spacing, cover, lap length and where the steel sits in the section must come from a design by a licensed engineer working to ACI 318 (or the code adopted where you build). Nothing here checks whether the reinforcement you entered is adequate.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do I calculate how much rebar I need?
Counts the reinforcing bars a slab, footing or wall panel needs at a given spacing and cover, and totals the linear length, the weight in pounds and kilograms, the steel area per foot and the cost. Open “How this is worked out” beside the result to see each step for your own figures.
What size rebar do I need for a 4 inch slab?
Counts the reinforcing bars a slab, footing or wall panel needs at a given spacing and cover, and totals the linear length, the weight in pounds and kilograms, the steel area per foot and the cost. On this page it is computed from Units, Length (feet, or metres in metric) and Width (feet, or metres in metric) and shown as Bars running the length, Bars running the width and Total bars.
How much rebar for a 30x40 slab?
On the figures this page opens with — length (feet, or metres in metric) 20, width (feet, or metres in metric) 20, bar spacing on centre (inches, or millimetres in metric) 12 and clear cover at each edge (inches, or millimetres in metric) 3 — the total bars is 40. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
How to calculate area of rebar?
It needs Units, Length (feet, or metres in metric), Width (feet, or metres in metric), Bar size, Bar spacing on centre (inches, or millimetres in metric) and Spacing of the cross bars (0 = same as the main spacing). Nothing else is required, and the figures stay in your browser.
How much rebar for a 10x10 slab?
The result gives Bars running the length, Bars running the width, Total bars, Total bar length, Total weight and Nominal bar diameter, each labelled with its unit so the figure can be read on its own.
What information do I need for the rebar calculator?
You need Units, Length (feet, or metres in metric), Width (feet, or metres in metric), Bar size, Bar spacing on centre (inches, or millimetres in metric), Spacing of the cross bars (0 = same as the main spacing), Clear cover at each edge (inches, or millimetres in metric) and Mat layout. The page opens with default figures that give total bars of 40, so you can see the scale before entering your own.