Enter your slab or footing size, the bar spacing and the cover, and get how many bars run each way, the total length and weight of reinforcing bar, the tie intersections and the number of stock bars to buy - sized in mm with the H designation to BS 4449.
Bars one way (20)
5.85 m
Bars other way (30)
3.85 m
Total length
244 m
Total weight
217 kg
Tie wire assumes about 0.3 m per tie - tie every intersection, or every other one to save wire.
A rebar mat is a grid: one set of bars running the length, a second set running the width, crossing at right angles and tied where they meet. Each set starts one edge clearance (cover) in from the slab edge, then repeats at the grid spacing:
bars one way = (width − 2 × clearance) ÷ spacing, rounded DOWN, + 1 (run the length)
bars other way = (length − 2 × clearance) ÷ spacing, rounded DOWN, + 1 (run the width)
grid length = (bars one way × bar length) + (bars other way × bar length)
total length = (grid length + lap splices) × waste
stock bars = total length ÷ stock-bar length, rounded UP to whole bars
weight = total length × bar mass per foot (or per metre)
intersections = bars one way × bars other way
The + 1 is the bar sitting on each edge. Any run longer than a single stock bar is spliced, adding one lap length at each joint. The same grid covers a slab, a footing or a wall - a footing is just a long, narrow slab, and a wall is the mat stood on its edge.
This example follows the unit system you pick in the calculator above.
A 6 m × 4 m slab, a 200 mm grid both ways, 75 mm edge clearance, 12 mm bars in 6 m stock lengths, +5% waste:
The bar size sets the weight. This chart lists the common US #-bars and the metric bars side by side, with the weight per foot and per metre - the figures the calculator uses:
| Bar | Diameter | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|---|
| #3 | 3/8 in (9.5 mm) | 0.376 | 0.560 |
| #4 | 1/2 in (12.7 mm) | 0.668 | 0.994 |
| #5 | 5/8 in (15.9 mm) | 1.043 | 1.552 |
| #6 | 3/4 in (19.1 mm) | 1.502 | 2.235 |
| #7 | 7/8 in (22.2 mm) | 2.044 | 3.042 |
| #8 | 1 in (25.4 mm) | 2.670 | 3.973 |
| 10 mm (H10 / N10) | 10 mm | 0.41 | 0.617 |
| 12 mm (H12 / N12) | 12 mm | 0.60 | 0.888 |
| 16 mm (H16 / N16) | 16 mm | 1.06 | 1.578 |
| 20 mm (H20 / N20) | 20 mm | 1.66 | 2.466 |
| 10M (Canada) | 11.3 mm | 0.53 | 0.785 |
| 15M (Canada) | 16 mm | 1.05 | 1.570 |
| 20M (Canada) | 19.5 mm | 1.58 | 2.355 |
Metric bar weight follows a tidy rule: mass in kg/m ≈ 0.00617 × (diameter in mm)², from steel at 7,850 kg/m³. Canadian M-bars are sized by area, so their weight is the nominal area in mm² × 0.00785.
In the United Kingdom, reinforcing steel is known as reinforcing bar (rebar). Sizes are metric diameters in millimetres - 8, 10, 12, 16, 20, 25 and 32 mm - shown on drawings with an H prefix (H12 = a 12 mm high-yield bar). Grade B500B (500 MPa) ribbed bar is standard. Pick the bar designation you are actually buying in the calculator above so the weight and the bars-to-buy count match what your supplier quotes.
It is normally supplied in 12 m as the standard mill length, with 6 m widely sold by merchants - set the stock length above so the splice and order figures line up with the sticks you can get. For the bar grades, cover and lap detail, see BS 4449 for the steel and BS 8666 for bar scheduling, with cover to Eurocode 2 and BS 8500.
Cover (the clearance from the concrete face to the steel) and lap-splice lengths are set by your design and the code above, not by this estimator - it sizes the mat and the quantity, not the structural reinforcement schedule.
The calculator has two modes, because a footing is not a flat mat:
In footing mode, the longitudinal bars carry the lap splices (the footing is usually longer than a stock bar), and each stirrup is a single closed loop - its length is the section perimeter measured to the cover line, plus roughly 6 in (150 mm) for the hooks and bends. Enter the bars top and bottom as one combined longitudinal count.
Work out how many bars run each way: take the slab dimension, subtract twice the edge clearance, divide by the bar spacing and add one for the bar at each edge. A 6 m × 4 m slab on a 200 mm grid needs about 20 bars one way and 30 the other - 50 bars, roughly 233 m. Add 5% for waste and divide by your stock-bar length for the number to buy. The calculator does every step from your slab size, spacing and clearance.
The grid is two sets of bars crossing at right angles. Bars one way = floor((width − 2 × clearance) ÷ spacing) + 1, each as long as the slab length less two clearances; bars the other way swap length and width. Add the two sets for the total bars, multiply by their lengths for the total length in metres, add laps for any run longer than a stock bar, then add waste. The calculator returns the bars each way, total length, weight, tie intersections and stock bars to buy.
Switch the calculator to footing / cage mode. A footing is not a flat mat - it is a cage of a few continuous longitudinal bars running its length (top and bottom) wrapped by closed stirrups at a spacing. Enter the footing length, width, depth, the number of longitudinal bars and the stirrup spacing. The calculator splices the long bars (a footing is usually longer than a stock bar), sizes each bent stirrup as the cage perimeter plus a hook allowance, and totals the steel and weight.
For house slabs, paths and small pads, 10 mm or 12 mm bar on a 300 to 450 mm grid is typical. Driveways and footings often step up to 12 mm or 16 mm. The structural size, spacing and cover come from your design or local code, not from this estimator - set the bar size, spacing and clearance to match the plan and the calculator works out the quantity and weight.
Rebar weight is its length times the mass per metre, which depends on the bar size. A 10 mm bar is 0.617 kg/m, a 12 mm is 0.888 kg/m and a 16 mm is 1.578 kg/m - the mass follows about 0.00617 × the diameter in mm squared. The calculator multiplies your total length (with laps and waste) by the figure for the bar you picked, so you get the weight to order and can compare it to a price per tonne.
Residential slabs are commonly reinforced on a 200 to 300 mm grid, and lighter slabs sometimes use welded mesh instead of loose bars. Closer spacing means more steel and more crack control. Enter your design spacing in the calculator - it assumes a bar at each edge set in by the clearance, then one every spacing across, the same way you would chalk the grid out on the formwork.
When a run is longer than a single stock bar, two bars overlap by a lap-splice length so the force transfers through the concrete. A common rule of thumb is about 40 times the bar diameter, but the real figure comes from your code and the concrete strength. The calculator adds one lap length for every joint a run needs, so the total length and the bars to buy already include the overlaps.
Tie wire fixes the bars where they cross. Count the intersections - bars one way times bars the other way - and allow roughly 0.3 m of wire per tie, or half that if you tie every other crossing as is common in slabs. The calculator shows the number of intersections and an approximate tie-wire length so you can buy a roll or two of the right size.
Turn a slab or footing into bars each way, linear feet, weight and bars to buy.
Read guide→Bars each way and linear feet for a concrete slab, with tables by slab size.
Read guide→Longitudinal bars, ties and lap splices for a strip footing, with a worked example.
Read guide→US #-bar and metric rebar sizes with diameter and weight, and what they mean.
Read guide→Weight per foot and per metre for every rebar size, and the formula behind it.
Read guide→Typical slab rebar grids, what spacing does, and cover - with a spacing table.
Read guide→Want this page set up for your region?