Rebar Calculator
The reinforcing bars for a slab grid: how many bars each way, the total length, the stock bars to buy with lap joins, and the weight.
Result…
How it's calculated
Bars each way = (span inside the cover ÷ spacing, rounded down) + 1 Bar length = slab side − 2 × cover, + one lap for each extra stock bar it needs Weight per length = π ÷ 4 × diameter² × 7850 kg/m³
The grid has two sets of bars. Bars running lengthways are spaced across the width, and bars running widthways are spaced along the length. Each set fits inside the edge cover: the count is the clear span divided by the spacing, rounded down, plus one for the bar at the start. When a bar is longer than the stock length it is made of two or more stock bars that overlap by the lap length. Cut pieces are fitted several to a stock bar where they fit. Weight assumes steel at 7850 kg/m³ and a round bar of the diameter you enter; for ribbed bars, enter the nominal diameter. Units are exact: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 lb = 0.45359237 kg.
Your numbers, step by step
About the Rebar calculator
This rebar calculator works out the reinforcing bars for a rectangular slab laid out as a grid. From the slab length and width, the spacing of the bars each way, the edge cover, the length the bars are sold in and the lap length, it gives the number of bars running each way, the total length of bar, the lap joins where one bar is too short to span the slab, and the number of stock bars to buy. Enter a bar diameter as well and it adds the weight, using steel at 7850 kg/m³. It works in metres and centimetres or in feet and inches. It is for estimating and ordering; the spacing, cover, lap and bar size themselves are structural choices that come from your drawings, engineer or local building code. The steps show each count and cut with your numbers. For the concrete itself, use the concrete calculator.
How to use it
- Choose “Metric” or “Imperial” under “Units”.
- Enter the “Slab length” and “Slab width”.
- Enter the “Spacing of bars running lengthways” and the “Spacing of bars running widthways”, centre to centre, and the “Edge cover”.
- Enter the “Stock bar length” you can buy and the “Lap length” for joins.
- Optionally enter the “Bar diameter (optional)” for the weight, then read the bars each way, total length and stock bars to buy.
Worked example
A 5 m × 4 m slab with bars at 20 cm each way and 5 cm edge cover has 4.9 m × 3.9 m inside the cover. Lengthways bars: 3.9 ÷ 0.2 = 19.5, rounded down to 19, plus 1 = 20 bars of 4.9 m. Widthways bars: 4.9 ÷ 0.2 = 24.5, rounded down to 24, plus 1 = 25 bars of 3.9 m. That is 20 × 4.9 + 25 × 3.9 = 195.5 m of bar, or 19 stock bars of 12 m with two or three pieces cut from each. With 10 mm bar at 0.6165 kg/m it weighs about 120.5 kg. In imperial, a 24 ft × 12 ft slab at 12 in spacing with 3 in cover, 20 ft stock and a 24 in lap needs 12 bars of 25.5 ft (each with one lap) and 24 bars of 11.5 ft: 582 ft, 40 stock bars and about 389 lb of 1/2 in bar.
Updated
Questions people ask
How do I calculate how much rebar I need for a slab?
Subtract twice the edge cover from each side, divide the clear width by the spacing, round down and add one to get the bars running lengthways, and do the same along the length for the bars running widthways. Multiply each count by its bar length and add them.
How do I work out the weight of rebar?
The weight per metre is π ÷ 4 × diameter² × 7850 kg/m³, with the diameter in metres. A 12 mm bar weighs about 0.888 kg per metre; multiply by the total length for the weight. The weight converter turns kilograms into pounds.
What is a lap length?
The distance two bars overlap where they join, so that the force passes from one to the other. It is usually set by your engineer or code as a multiple of the bar diameter, so use the figure on your drawings.
Does this include a top and bottom layer?
No. It counts one layer, or mat, of bars. A slab with top and bottom reinforcement needs double, and bends, hooks, chairs and tie wire are extra.
Why buy more than the total length divided by the stock length?
Offcuts are often too short to use. The calculator cuts each piece from a stock bar, fitting several where they fit, and does not reuse offcuts between the two directions, so its count is closer to what a site actually uses.