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Rebar Calculator

Bar counts, linear feet and ties for a reinforced slab.

Your measurementsEvery assumption editable01

12 to 18 in on centre is typical for a residential slab.

Concrete cover from the slab edge to the first bar.

Rebar requiredCalculated live02

Result

24bars

24 × 20 ft bars — 464 linear ft

Bars running lengthwisespaced across the 12 ft width
12
Bars running widthwisespaced along the 20 ft length
20
Total linear feet
464 ft
Bars to buy
24 × 20 ft
Intersections to tieone tie wire each
240

No lap allowance is included. Where bars are spliced end to end, codes typically require an overlap of 40 bar diameters or more — on #4 bar that is around 20 inches per splice, and on a large slab those add up.

Rebar has to sit in the middle third of the slab depth to do anything, which means chairs or bolsters holding it up. Bar laid on the ground and pulled up as the concrete goes in rarely ends up where it should.

Reinforcement design for anything structural — footings, suspended slabs, retaining walls — is an engineering question, not an arithmetic one. This calculates a simple mat for a ground-bearing slab.

Enter the slab dimensions, your bar spacing and the edge cover, and it lays out a grid: how many bars run each way, the total linear footage, and how many standard lengths that is. Lap splices are not included and are explained in the notes.

Why use this tool?What it does differently03

Why use this tool?

Both directions counted

Bars running lengthwise and widthwise are counted separately, because the spacing divides differently across each dimension.

Bars to buy

Total linear feet converted to whole bars at the length your supplier sells, usually 20 feet.

Tie count included

Every grid intersection needs a tie wire. The count follows directly from the two bar counts.

Honest about laps

Splice overlaps are not included, and the notes explain why they matter on a large slab.

How this worksThe method04

How this rebar calculator works

The usable dimension in each direction is the slab size less twice the edge cover. Dividing that by the spacing and adding one gives the bar count for that direction, because a grid of n gaps has n+1 lines.

Total linear footage is each bar count multiplied by the length that direction's bars actually run, then added together. Bars to buy follows from dividing by the stock length and rounding up.

Lap splices are deliberately excluded. Where bars are joined end to end, codes typically require an overlap of around 40 bar diameters — roughly 20 inches on a #4 bar. On a slab longer than the stock length those laps add up quickly, so add them separately once you know your splice count.

How to use itStep by step05

How to use it

  1. Step 1: Measure the slab

    Length and width of the pour, at the finished dimensions.

  2. Step 2: Set the spacing

    Twelve to eighteen inches on centre is typical for a residential slab. Closer spacing for heavier loads.

  3. Step 3: Set the edge cover

    The distance from the slab edge to the first bar, typically about 3 inches so the steel stays fully encased in concrete.

  4. Step 4: Add laps separately

    If the slab exceeds your bar length, count the splices and add roughly 20 inches of overlap for each one.

Example usageWorked figures06

Example usage

A 20 by 12 slab at 12 inch spacing
With 3 inch cover, 12 bars run lengthwise and 20 run widthwise. That is roughly 462 linear feet, or 24 twenty-foot bars, with 240 intersections to tie.
Widening the spacing
The same slab at 18 inch centres drops to 8 and 14 bars — about 306 linear feet and 16 bars. Spacing is the biggest lever on rebar cost.
A slab longer than the bar
A 30 foot slab reinforced with 20 foot bars needs a splice in every lengthwise run. Twelve bars means twelve laps at roughly 20 inches each — an extra 20 feet of steel the grid calculation does not show.
Frequently asked questionsCommon questions07

Frequently asked questions

How much rebar do I need for a slab?

Count bars in each direction from the slab dimensions and your spacing, then add the runs together. A 20 by 12 slab at 12 inch centres needs roughly 462 linear feet, or 24 twenty-foot bars.

What spacing should rebar be?

Twelve to eighteen inches on centre is typical for a residential ground-bearing slab. Closer spacing carries more load. Anything structural should be designed rather than estimated.

Does this include lap splices?

No. Codes generally require overlaps of about 40 bar diameters where bars join — roughly 20 inches on a #4 bar. Count your splices and add that length separately.

Where in the slab should rebar sit?

In the middle third of the depth, held up on chairs or bolsters. Bar laid on the ground and pulled up as the concrete goes in rarely ends up where it should, and steel at the bottom of a slab does very little.

Can I use mesh instead of rebar?

For light residential slabs welded wire mesh is common and simpler to place. For anything carrying real load, or where a slab is designed by an engineer, the specification will say which and where.

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