Bar count, laps, length, weight and what to order — for a rectangular slab
← Back to the calculatorThe net steel in the slab is 938 ft, which is 47 bars. The other 2 bars are laps — 20 joints × 25 in each. Most slab calculators stop at 47 and leave you short.
Open the calculator to run your own numbers, or read the four steps below.
If you would rather read the answer off a table than run the formula — a patio, shed, garage, driveway or walkway slab, in bars, feet and pounds — the how much rebar do I need page lists it by project type.
Everything on this page comes out of these four lines. Nothing else is needed for a rectangular slab.
The first four lines are the standard grid calculation, and any slab calculator will give you them. The last three are the ones that decide whether your order arrives long enough.
A 24 ft × 20 ft slab. #4 bar at 12 in on centre. 3 in edge clearance all round. Your yard sells 20 ft bars.
| Step | Working | Result |
|---|---|---|
| Grid | 24 − 2(0.25) × 20 − 2(0.25) | 23.5 × 19.5 ft |
| Bars running the length | floor(19.5 ÷ 1) + 1 | 20 bars @ 23.5 ft |
| Bars running the width | floor(23.5 ÷ 1) + 1 | 24 bars @ 19.5 ft |
| Net length | 20(23.5) + 24(19.5) | 938 ft |
| Net weight (#4 = 0.668 lb/ft) | 938 × 0.668 | 626.6 lb |
| Laps needed | 23.5 ft lines > 20 ft stock → 1 lap each; 19.5 ft lines → none | 20 laps |
| Lap length (#4) | 1.3 × 18.97 in | 25 in = 2.08 ft |
| Lap allowance | 20 × 25 in | 41.7 ft ≈ 27.9 lb |
| Order length | 938 + 41.7 | 979.7 ft |
| Bars to order | ceil(979.7 ÷ 20) | 49 bars = 654.6 lb |
Swipe the table sideways for more columns →
That is the whole job. 47 bars if you ignore laps, 49 if you count them. Two bars is a cheap correction at the yard and an expensive one when the crew is standing around waiting for a delivery.
The whole takeoff is ten cells. Put the inputs in the first four and let the rest follow — then you can change the slab size or the bar size and read the new answer straight off.
| Cell | What it is | Type this | On the 24 ft × 20 ft job |
|---|---|---|---|
| A2 | Slab length (ft) | 24 | 24 |
| B2 | Slab width (ft) | 20 | 20 |
| C2 | Spacing (ft) | 1 | 1 |
| D2 | Edge clearance (ft) | 0.25 | 0.25 |
| E2 | Grid length | =A2-2*D2 | 23.5 |
| F2 | Grid width | =B2-2*D2 | 19.5 |
| G2 | Bars running the length | =ROUNDDOWN(F2/C2,0)+1 | 20 |
| H2 | Bars running the width | =ROUNDDOWN(E2/C2,0)+1 | 24 |
| I2 | Net length (ft) | =G2*E2+H2*F2 | 938 |
| J2 | Net weight (lb) | =I2*0.668 | 626.6 |
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Two things to watch. ROUNDDOWN is not the same as ROUND — you are counting whole bars, so a 19.7 result is 19 bars plus the closing bar, not 20 plus one. And the 0.668 in J2 is the weight per foot of #4: swap it for 0.376, 1.043, 1.502 or 2.670 when you change bar size. Those figures are on the rebar weight calculator.
A lap is the overlap where two bars join to make one continuous line. The steel in that overlap is real steel you have to buy, and it is not part of the slab's net length.
It gets left out because it is invisible in the plan. The drawing shows one continuous line, the grid calculation produces one continuous length, and the two agree with each other — so the number looks finished. It is only when you cut the bars that the joint appears, and by then the order is placed.
Three things make it worse on a slab:
The usual field rule for a tension lap is 40 times the bar diameter, and never less than 12 in. It is short at every size — by 5 in on the #4 this page is working — so the lap carried into the order above is the ACI 318-19 value, Class B = 1.3 ld:
| Bar | Diameter (in) | 40 × db (in) | Class B, 4,000 psi (in) | Lap (ft) | 40 × db is short by |
|---|---|---|---|---|---|
| #3 | 0.375 | 15 | 19 | 1.58 | 4 in |
| #4 | 0.500 | 20 | 25 | 2.08 | 5 in |
| #5 | 0.625 | 25 | 31 | 2.58 | 6 in |
| #6 | 0.750 | 30 | 37 | 3.08 | 7 in |
| #8 | 1.000 | 40 | 62 | 5.17 | 22 in |
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If you are comparing tools, these are the four things that change the order quantity. A slab calculator that gives you fewer than four has not finished the job.
The calculator on the home page returns all four, plus a cost estimate from a price per bar or per pound. The rebar size and weight chart has the per-foot weights for every imperial and metric size if you want to check the arithmetic by hand.
The same arithmetic, already run for the four slabs people ask for most. All are #4 bar, 6 in edge clearance, grid steel only — laps, waste and offcut still go on top, as in the worked example on this page. Spacing is the design decision: 12 in on centre is the usual residential figure, 18 in for lighter duty.
| Slab type | Size | Grid | Bars each way | Total length | Weight | Steel per ft² |
|---|---|---|---|---|---|---|
| Driveway | 12 × 24 ft | #4 @ 18 in | 16 + 8 | 360 ft | 240.5 lb | 0.835 lb/ft² |
| Patio | 12 × 12 ft | #4 @ 12 in | 12 + 12 | 264 ft | 176.4 lb | 1.225 lb/ft² |
| Garage | 20 × 20 ft | #4 @ 12 in | 20 + 20 | 760 ft | 507.7 lb | 1.269 lb/ft² |
| Shed slab | 10 × 12 ft | #4 @ 18 in | 8 + 7 | 149 ft | 99.5 lb | 0.829 lb/ft² |
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Read one row so the columns are clear: the 12 × 24 ft driveway on an 18 in grid needs 16 bars running one way (11 ft each = 176 ft) and 8 the other (23 ft each = 184 ft) — 24 bars, 360 ft, 240.5 lb of #4 at 0.668 lb/ft. Move the spacing to 12 in and the same driveway becomes 36 bars, 540 ft, 360.7 lb: spacing is worth 120 lb of steel on one slab, which is why it is the engineer's number and not the estimator's. Want more project types — walkways, wide driveways, pads — the how much rebar do I need page lists those, on the same basis.
Subtract twice the edge clearance from the slab length and width to get the grid. Bars each way = floor(grid dimension ÷ spacing) + 1. Net length = (bars across × grid length) + (bars along × grid width). Then add the lap allowance: every line longer than the bar you can buy needs one lap per joint, and a lap is roughly 40 times the bar diameter. Order length = net length + (number of laps × lap length).
Class B at 4,000 psi, from the ACI 318-19 table: #3 = 19 in, #4 = 25 in, #5 = 31 in, #6 = 37 in, #7 = 54 in, #8 = 62 in. The field rule of thumb of 40 times the bar diameter gives 15, 20, 25, 30, 35 and 40 in — short at every size, and 22 in short on a #8. Class A is 1.0 ld instead of 1.3, and the drawings govern.
Because the net length is the length of steel in the finished slab. It does not include the overlap where two bars join. If a line is longer than one bar, the joint eats an extra lap length that was never counted, so the order is short by that amount. On a 24 ft × 20 ft slab with 20 ft bars, that is about 33 ft of extra steel, or two extra bars.
Slab rebar is commonly placed at 12 to 18 inches on centre in residential work and 18 to 24 inches on centre in lighter duty slabs, both ways. Spacing is set by the design and the local code, not by the calculator.
A 12 ft × 24 ft driveway on a #4 at 18 in grid with 6 in edge clearance takes 24 bars — 360 ft, 240.5 lb — and the same driveway at 12 in takes 36 bars, 540 ft, 360.7 lb. That is grid steel; laps, waste and offcut go on top, and the spacing itself is a design decision set by the load and the local code.