Rebar Spacing Calculator
Bar count, linear feet, ties and chairs for a slab — with the wire mesh alternative costed beside it.
803 linear ft — 20 lengthwise, 14 crosswise
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How to work it out yourself
- 1
Take the cover off the dimensions
Steel does not run to the edge of the slab. Around 3 inches of clear cover keeps it inside the concrete where it cannot rust, so subtract twice the cover from both the length and the width before working out bar lengths.
- 2
Count the bars in each direction
Bars running the length of the slab are spaced across its width, and vice versa. Divide the usable dimension by the spacing and add one, because the first bar sits at the start of the run.
- 3
Turn linear feet into stock lengths
Rebar comes in 20 ft lengths as standard. Add up the total linear feet needed and divide by 20 — then allow for laps, because bars that meet must overlap by roughly 40 times the bar diameter, which is 20 inches for #4.
- 4
Do not forget the ties and chairs
Every intersection needs a tie, and the mat has to be held up in the middle third of the slab. Steel lying on the subgrade does nothing at all — it must be lifted on chairs or dobies, roughly 0.6 per intersection.
Worked example: A 20 × 30 slab with #4 bar at 18 in centres
| Usable length | (30 × 12) − 6 = 354 in |
| Usable width | (20 × 12) − 6 = 234 in |
| Bars running lengthwise | ⌊234 ÷ 18⌋ + 1 = 14 |
| Bars running crosswise | ⌊354 ÷ 18⌋ + 1 = 20 |
| Total linear feet | (14 × 29.5) + (20 × 19.5) = 803 ft |
| 20 ft bars | 803 ÷ 20 = 41 bars |
| Tie intersections | 14 × 20 = 280 |
Around 41 bars, 280 ties and about 170 chairs. Add a couple of bars for laps and offcuts.
Mesh sheets do not cover what the tag says
A sheet of welded wire mesh is 5 ft × 10 ft, which people quote as 50 sq ft. It is not. Sheets have to lap by one full square — usually 6 inches — so real coverage is closer to 45 sq ft. On a 600 sq ft slab that is the difference between 12 sheets and 14, and you will discover it on pour day. Mesh also only works if it is actually lifted; mesh walked flat into the subgrade is a line item, not reinforcement.
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Common questions
What size rebar for a concrete slab?
#4 bar, which is half an inch, is the standard for residential slabs and driveways. #3 is used in light-duty flatwork, and #5 for heavier loads or engineered slabs. Where a slab is structural, the size and spacing come from the engineer's drawing, not from a rule of thumb.
What spacing should rebar be in a slab?
Eighteen inches on centre each way is typical for a residential slab, tightening to 12 inches for driveways carrying heavy vehicles. Halving the spacing roughly doubles the steel, so it is a real cost decision rather than a free upgrade.
How much rebar do I need for a 20 × 30 slab?
At 18 inch spacing with #4 bar and 3 inches of cover, about 803 linear feet — roughly 41 lengths of 20 ft bar, plus around 280 ties and 170 chairs.
Is wire mesh or rebar better for a slab?
Rebar for anything carrying vehicles or spanning poor subgrade; mesh is acceptable for light residential flatwork on a good base. Mesh is cheaper and quicker to place, but only if it is lifted onto chairs — its main failure is being trodden to the bottom of the pour where it does nothing.
How much should rebar overlap?
About 40 bar diameters where bars are spliced, which is 20 inches for #4 and 25 inches for #5. Tie the lap in at least two places. Check local code, because some jurisdictions require more.
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