Rebar Calculator
Count reinforcing bars for a slab or footing grid from size, spacing and cover, with lap splices, stock bar cutting, weight in lb or kg and steel area.
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About this toolWhat is the Rebar Calculator?
A rebar takeoff for a slab or footing mat comes down to three questions: how many bars fit across each direction at the chosen spacing, how long each bar is once the edge cover is taken off, and how many stock lengths you have to buy once long runs are lapped. This calculator answers all three for a rectangular grid, then multiplies the steel by the published unit weight of the bar so you can order by the piece or by the ton.
Enter the slab length and width, the edge cover, the spacing each way and the bar size. US sizes #3 to #11 use the nominal weights in ASTM A615 Table 1 (a #4 bar is 0.668 lb per foot); metric sizes 8 to 32 mm use the nominal masses from BS 4449 and IS 1786, which equal pi/4 x d² x 7,850 kg/m³. Choose one mat or top and bottom mats, set the stock length you can buy, and pick a lap either in bar diameters or as a fixed length.
The result lists bars in each direction, cut lengths, laps per run, total length including laps, weight in both unit systems, stock bars to buy with an offcut figure, and the steel area per foot or metre of width that a designer checks against the drawings.
- US bar sizes #3 to #11 with ASTM A615 weights and metric 8 to 32 mm bars
- Single or double mats, edge cover, hooks and lap splices
- Stock-length cutting estimate with offcut and lap count
- Steel area per foot or per metre of width for design checks
- Copy, share link and cost per stock bar; runs in your browser
How to use the Rebar Calculator
- Pick imperial or metric. Plan dimensions are in feet or metres; cover, spacing and laps are in inches or millimetres.
- Enter the slab or footing length and width and the edge cover. Add a hook or bend length per end if the bars turn up at the edges.
- Choose the bar size and the centre-to-centre spacing of the bars running along the length and along the width.
- Select a single mat or top and bottom mats, the stock length your supplier sells, and the lap length in bar diameters or as a length.
- Read the bar counts, total length, weight and stock bars to buy, then copy the takeoff or share a link with the inputs.
Worked examples
20 x 10 ft patio slab, #4 at 12 in each way
Input
20 ft x 10 ft, 3 in cover, #4 bars at 12 in both ways, single mat, 20 ft stock
Output
30 bars (10 along the length at 19.5 ft, 20 across at 9.5 ft), 385 ft, 257 lb, 20 stock bars
No run is longer than 20 ft, so there are no laps. The 9.5 ft bars come two to a stock bar, leaving 15 ft of offcut in total.
Metric footing pad with two mats
Input
6 m x 4 m, 50 mm cover, 12 mm bars at 200 mm both ways, top and bottom mats, 12 m stock
Output
100 bars, 470 m, 417.4 kg, 40 stock bars of 12 m
Each mat has 20 bars of 5.9 m and 30 bars of 3.9 m. Two 5.9 m pieces or three 3.9 m pieces come out of one 12 m bar, so only 10 m is wasted.
40 x 20 ft slab that needs laps
Input
40 ft x 20 ft, 3 in cover, #5 at 12 in, 20 ft stock, lap 40 bar diameters (25 in)
Output
60 bars, 1,653 ft including 40 laps, 1,724 lb, 84 stock bars
Each 39.5 ft run is made from two full 20 ft bars and a 3.7 ft closer with two laps. Nesting five closers per stock bar keeps the offcut to about 27 ft.
Background and insight
The bar count uses the fence-post rule: a clear width of W - 2c at spacing s holds floor((W - 2c) / s) + 1 bars, with a bar at each edge. Counting only (W - 2c) / s is the classic takeoff error that leaves you one bar short on every run. Bars along the length are spaced across the width, so the two directions swap dimensions.
When a bar run is longer than the stock length, it is made from n pieces that overlap by the lap length, so n pieces cover n x S - (n - 1) x lap and n = ceil((run - lap) / (S - lap)). The lap itself is a design value. ACI 318 sets tension lap splices at 1.0 or 1.3 times the development length, which depends on concrete strength, bar size, cover and position; 40 bar diameters is a common starting value on site drawings, not a code check. Cover also comes from the code: ACI 318 asks for 3 in where concrete is cast against and permanently exposed to earth, which is why footings use 3 in (75 mm).
Common use cases
Ordering steel for a slab on grade
Turn a spacing note such as "#4 @ 12 in EW" into a piece count and a weight that a supplier can quote.
Checking a fabricator quote
Compare the supplier's weight and bar list against an independent takeoff before the delivery arrives.
Footings and mat foundations
Add hooks at each end and a second mat to estimate a spread footing or raft.
Comparing bar size against spacing
Swap #4 at 12 in for #5 at 16 in and compare steel area per foot, weight and the number of bars to tie.
Tips and limitations
- Add 5 to 10% to the stock count for bars bent wrong, damaged or needed around openings and penetrations.
- Chairs, bolsters, tie wire and dowels are not included. Allow for them separately.
- The cutting estimate does not share offcuts between the two directions. A fabricator with optimisation software may save a few bars.
- Laps should be staggered on real slabs, which can change which bars are cut where but not the total steel.
- Epoxy-coated bars and top bars with more than 12 in of concrete below need longer development and lap lengths under ACI 318.
Frequently asked questions
How do you calculate the number of rebar in a slab?
Subtract the edge cover from both sides of the width, divide by the spacing, round down and add one. That gives the bars running along the length. Do the same with the length to get the bars running across. A 10 ft wide slab with 3 in cover at 12 in spacing holds floor(114 / 12) + 1 = 10 bars.
How much does #4 rebar weigh per foot?
ASTM A615 lists #4 bar at 0.668 lb per foot (0.994 kg/m), with a nominal diameter of 0.500 in and an area of 0.20 in². A 20 ft stick of #4 weighs about 13.4 lb. #3 is 0.376 lb/ft and #5 is 1.043 lb/ft.
What is the weight of 12 mm rebar per metre?
0.888 kg per metre. Metric bar mass is pi/4 x d² x 7,850 kg/m³, which site engineers often shorten to d² / 162 with d in millimetres: 144 / 162 = 0.889 kg/m. A 12 m bar of 12 mm weighs about 10.7 kg.
How long should a rebar lap splice be?
It depends on the code and the design. ACI 318 makes a tension lap 1.0 or 1.3 times the development length, which varies with concrete strength, bar size, cover and coating. Drawings often show 40 to 60 bar diameters. Use the length on your structural drawings; the calculator only adds the steel it implies.
What rebar spacing is typical for a 4 inch slab?
Residential slabs on grade often use #3 or #4 bars at 12 to 18 in each way, placed near mid-depth on chairs. ACI 318 caps spacing of flexural reinforcement in slabs at the lesser of 3 times the slab thickness and 18 in. Many slabs on grade use welded wire or fibres instead; follow the design.
Does the calculator include hooks and bends?
Yes, as a straight length added at each end of every bar. Standard hook dimensions come from ACI 318 or the CRSI Manual of Standard Practice and depend on bar size, so enter the extension your bending schedule shows.
Why do I need more stock bars than total length divided by stock length?
Because offcuts are rarely usable. A 9.5 ft bar from a 20 ft stock leaves 1 ft, and a 19.5 ft bar leaves 6 in. The cutting estimate counts whole bars per piece and nests short pieces only when they fit, so it is closer to what you will actually buy.
Is there an in-depth guide to the Rebar Calculator?
Yes. “Rebar Takeoff for Slabs: Bar Counts, Laps, Cutting and Weight” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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