How much rebar do I need for a 20x20 slab?

At 18 in on center, 15 bars each way and 600 linear feet, which is 30 twenty-foot bars. Both counts agree here, because every run is exactly one stock length: nothing is cut and there is no offcut to argue about. Tighten the spacing to 12 in and it takes 21 bars each way, 840 linear feet and 42 sticks — still whole bars. Change the slab to 12 × 12 ft and the two part company: 18 sticks to carry home against the 11 the footage suggests. Go the other way, to 40 × 40 ft, and each run needs three bars rather than two, because the 29 in lap at every splice means two 20 ft bars reach 37.6 ft and not 40.

How much overlap do you need on rebar?

22 in for #3, 29 in for #4 and 36 in for #5, from the Class B tension lap splice table for Grade 60 uncoated bars in normal-weight concrete, 'Other Bars' Case 1. Class A splices, epoxy coating, top bars and lower concrete strength all change the figure, so confirm against the table your code cites.

What spacing should rebar be in a driveway slab?

This calculator defaults to 18 in on center each way, from a municipal driveway specification that gives it as a maximum. That default is weaker than the rest of the constants here — one city's rule, and another city nearby says 24 in — and it is weak for a reason we can now state plainly: there is no national figure to cite. No association or extension service publishes a reinforcement spacing for residential driveways, and two of them argue that a properly built residential slab does not need distributed steel at all. Spacing is a design decision belonging to your local code or an engineer. Treat the default as a placeholder you replace.

Does this calculator tell me what size rebar to use?

No. It quantifies the grid you tell it to build. Bar size, spacing, cover, slab thickness and whether the slab needs reinforcement at all are structural decisions governed by local code and, above a certain scale, by an engineer's stamp.

Rebar Calculator

Rebar for a slab is a grid, and the quantity falls out of the slab dimensions, the spacing, and the bar lost to laps where one stick meets the next. This calculator counts the bars each way, adds the published lap length for your bar size, and returns the whole sticks you carry home rather than the total footage divided up. It estimates material; it does not design the slab.

How it works

A grid spans edge to edge, so the bar count is the number of spaces plus the closing bar. At 18 in on center a 20 ft slab has 13.3 spaces, rounded up to 14, which means 15 bars — and 15 the other way as well, for 600 linear feet on a 20 × 20 ft slab.

Laps are added only where a run is longer than the bar you can buy. With 20 ft stock a 20 ft slab needs none, but a 24 ft slab needs one splice in every bar: 34 splices on a 24 × 24 ft slab, adding 82.2 ft of overlap to the 816 ft of grid.

Lap lengths come from the Class B tension splice table for Grade 60 uncoated bars in normal-weight concrete at f'c of 3,000 psi or more, 'Other Bars' Case 1: 22 in for #3, 29 in for #4, 36 in for #5 — each roughly 58 times the bar's own diameter.

Formula: Bars each way = slab dimension ÷ spacing, rounded up, + 1. Linear ft = bars × their run, plus lap length × number of splices

Measuring on site

Measure the finished slab edge to edge. The grid is counted across the full dimension, so the cover you hold back at each edge leaves every bar slightly shorter than counted — the result runs a little long, which is the right direction to be wrong in.
The 18 in default spacing rests on a weaker source than anything else on this site, and it is worth knowing exactly how weak. It comes from a municipal driveway specification, which says No. 3 bars at not more than 18 inches on center, both directions — a maximum rather than a recommendation, for the approach in the right-of-way rather than the slab up to your garage, and with welded wire fabric accepted in place of bars. Other cities land elsewhere: Dallas details the same job at 24 in. We looked for a national figure and there is none — no association or extension service publishes a reinforcement spacing for residential driveways. So treat 18 in as a starting point and set the field to whatever your permit, local code or engineer specifies.
Stock length defaults to 20 ft. Change it to the length your supplier stocks before reading the bar count, because it drives both the number of splices and how the total divides into sticks.

Waste and what it covers

There is no waste percentage here: the lap allowance is the real addition, and it is calculated rather than guessed. What matters more is which of two counts you order from. The headline counts the whole bars each run takes. Where the run is shorter than a stock bar that is one bar and a wasted offcut — a 20 ft bar cut for a 12 ft run leaves 8 ft no other 12 ft run can use, so a 12 × 12 ft slab takes 18 bars. Where the run is longer, it is however many bars the run needs once you account for the overlap at each splice: two 20 ft bars lapped 29 in reach 37.6 ft, not 40, so a 40 ft run takes three. Dividing total linear feet by the stock length gives a lower figure, and it only holds if every offcut gets tied into a shorter run: normal on a plain rectangle, wishful where footings and blockouts cut the pour up. Both figures are on the result, and the site counts whole pieces the same way for block and plywood.

Slab grid at 18 in on center, #4 bars, 20 ft stock. The two bar columns are the two counts: what you carry home, and what the footage divides into if every offcut gets re-used. They part company as soon as a cut leaves an offcut that no other run can use, and close again where the cutting happens to come out even.
Slab	Bars each way	Splices	Linear ft with laps	20 ft bars to buy	If offcuts re-used
10 × 10 ft	8 × 8	0	160.0	16	8
12 × 12 ft	9 × 9	0	216.0	18	11
20 × 20 ft	15 × 15	0	600.0	30	30
24 × 24 ft	17 × 17	34	898.2	68	45

Questions

How much rebar do I need for a 20x20 slab?: At 18 in on center, 15 bars each way and 600 linear feet, which is 30 twenty-foot bars. Both counts agree here, because every run is exactly one stock length: nothing is cut and there is no offcut to argue about. Tighten the spacing to 12 in and it takes 21 bars each way, 840 linear feet and 42 sticks — still whole bars. Change the slab to 12 × 12 ft and the two part company: 18 sticks to carry home against the 11 the footage suggests. Go the other way, to 40 × 40 ft, and each run needs three bars rather than two, because the 29 in lap at every splice means two 20 ft bars reach 37.6 ft and not 40.
How much overlap do you need on rebar?: 22 in for #3, 29 in for #4 and 36 in for #5, from the Class B tension lap splice table for Grade 60 uncoated bars in normal-weight concrete, 'Other Bars' Case 1. Class A splices, epoxy coating, top bars and lower concrete strength all change the figure, so confirm against the table your code cites.
What spacing should rebar be in a driveway slab?: This calculator defaults to 18 in on center each way, from a municipal driveway specification that gives it as a maximum. That default is weaker than the rest of the constants here — one city's rule, and another city nearby says 24 in — and it is weak for a reason we can now state plainly: there is no national figure to cite. No association or extension service publishes a reinforcement spacing for residential driveways, and two of them argue that a properly built residential slab does not need distributed steel at all. Spacing is a design decision belonging to your local code or an engineer. Treat the default as a placeholder you replace.
Does this calculator tell me what size rebar to use?: No. It quantifies the grid you tell it to build. Bar size, spacing, cover, slab thickness and whether the slab needs reinforcement at all are structural decisions governed by local code and, above a certain scale, by an engineer's stamp.

Sources

Dayton Superior, tabulating ACI 318 — Rebar Splicing Handbook, Table 1.11“Table 1.11 Tension Lap Splice Lengths – Grade 60 Uncoated Bars ... f'c = 3,000 psi or greater, Normal Weight Concrete ... Tabulated values are based on Grade 60 reinforcing bars and normal-weight concrete. [Class B, Other Bars, Case 1 — #3: 22, #4: 29, #5: 36]”
City of San Marcos, Texas, adopting City of Austin Standard Specification Item No. 433S — Item No. 433S — P. C. Concrete Driveways, §433S.4(D) Reinforcement“Reinforcement for Type I driveways shall consist of 1 layer of 6 x 6 by W 1.4 x W 1.4 (150 x 150 by MW9 x MW9) wire fabric or No. 3 (10 M) bars placed not more than 18 inches (450 mm) on center, both directions. ... No. 433S-1 Type I Driveway (1 & 2 Family Residential Use Only)”

Figures and sources last checked by The BuilderTally Team in August 2026.