How to use the rebar calculator calculator
- 1
Enter the slab dimensions
Enter the length and width of the area being reinforced. For an L-shaped slab, split it into rectangles and run the calculation for each.
- 2
Enter the spacing from your drawings
Enter the on-center spacing the structural drawings call for. This calculator does not specify a spacing or a bar size.
- 3
Read the bars each way
Bars running the length are spaced across the width, and bars running the width are spaced across the length. The calculator handles that so the two counts are not mixed up.
- 4
Add a stock bar length
Enter the length rebar is sold in to convert the total linear feet into a bar count. Remember this excludes laps.
- 5
Add laps and accessories separately
Lap length, chairs or supports to hold the bars at the right height, and tie wire are all separate quantities taken from your drawings.
Formula
Total linear feet = (floor(width / spacing) + 1) x length + (floor(length / spacing) + 1) x width
Each direction uses the span it crosses, not the span it runs along. Laps are not included in this figure.
Worked example
A slab 30 by 20 feet with bars at 18 inches on center each way
- 1Spacing in feet: 18 / 12 = 1.5 ft.
- 2Bars running the 30 ft direction: floor(20 / 1.5) + 1 = 13 + 1 = 14.
- 3Bars running the 20 ft direction: floor(30 / 1.5) + 1 = 20 + 1 = 21.
- 4Linear feet: (14 x 30) + (21 x 20) = 420 + 420 = 840.
- 5Tie points: 14 x 21 = 294.
The grid needs 840 linear feet of bar and has 294 intersections to tie, before any lap allowance.
Worked examples
Converting to stock bars
The same 840 linear feet using 20 foot stock bars
- 1Bars needed: 840 / 20 = 42 exactly.
- 2This assumes every bar runs its full length with no lap.
- 3A real layout laps bars where they meet, which adds length.
42 stock bars cover the bare grid length. The lap allowance has to be added from your drawings, because lap length depends on the bar size and the code in force.
Tighter spacing on the same slab
The same 30 by 20 foot slab at 12 inches on center
- 1Bars running the 30 ft direction: floor(20 / 1) + 1 = 21.
- 2Bars running the 20 ft direction: floor(30 / 1) + 1 = 31.
- 3Linear feet: (21 x 30) + (31 x 20) = 630 + 620 = 1,250.
Going from 18 to 12 inch spacing raises the bar length by about 49 percent. Spacing is the dominant variable in a rebar quantity.
Which span sets which bar count
The detail people most often get backwards is which dimension governs each direction. Bars that run along the 30 foot length are spaced out across the 20 foot width, so it is the width divided by the spacing that determines how many of them there are.
Getting this the wrong way round on a square slab makes no difference, which is exactly why the error survives. On a long thin slab it matters a great deal: the counts and the total linear footage both come out wrong.
The calculator labels each direction by the length its bars actually run, so the output reads as bars running the 30 foot direction rather than as an ambiguous first and second count.
Why laps are excluded
Rebar comes in stock lengths, and any run longer than a stock bar has to be made up from two bars overlapped. That overlap is a lap splice, and it is structural: the bars have to overlap enough to transfer force between them.
How much overlap is required depends on the bar size, the concrete strength, the bar coating, the spacing, and the code in force. It is not a fixed multiple that a general calculator can apply, and applying a plausible-looking one would be worse than leaving it out.
So the linear footage here is the bare grid geometry. Take the lap requirement from your drawings, work out how many splices the layout needs, and add that length to the figure before ordering.
What the tie count is for
The intersections figure is the number of points where a bar running one way crosses a bar running the other. Those crossings are normally tied to hold the grid together while concrete is placed.
It is useful for two reasons. It gives a sense of the labor involved, since tying several hundred intersections is real work. And it sizes the tie wire order, once you know how much wire each tie uses.
Not every intersection is always tied, and how many are is a specification detail. The calculator gives the full count so you can apply whatever proportion your drawings call for.
Common mistakes
Where rebar quantities go wrong:
- Using the span a bar runs along instead of the span it is spaced across.
- Forgetting the closing bar, so each direction is one short.
- Ordering the bare grid length with no allowance for lap splices.
- Applying a remembered lap multiple instead of the one on the drawings.
- Forgetting chairs or supports, which hold the bars at the specified height.
- Treating an L-shaped slab as one rectangle.
- Reading a bar count as confirmation that the reinforcement design is adequate.
Sources and methodology
Every figure on this page comes from geometry rather than from a product database. Areas use the standard area formulas, counts divide an area by the size of one unit, and unit conversions use exact factors: 12 inches to a foot, 9 square feet to a square yard, and 100 square feet to a roofing square.
YardCalc publishes no product dimensions, coverage rates, or prices. Unit sizes differ between manufacturers and regions, and a figure that is right for one product is wrong for another, so the calculator asks you for the size of the product you are actually buying. Prices are yours alone.
Results are planning estimates, not order quantities. Read how every value on the site is derived and checked in the YardCalc calculation methodology.
Frequently asked questions
- How much rebar do I need for a slab?
- Divide each span by the spacing, round down, and add one to get the bars in each direction, then multiply each count by the length those bars run. A 30 by 20 foot slab at 18 inches on center needs 840 linear feet before laps.
- How do I calculate rebar spacing?
- This calculator does not determine spacing. It is a structural decision that depends on the loads, the slab, and local code, so take it from stamped drawings or an engineer and enter it here.
- Does this include lap splices?
- No, deliberately. Lap length depends on bar size, concrete strength, coating, and code, so there is no general multiple that would be safe to apply. Take the lap requirement from your drawings and add it to the linear footage.
- Which dimension sets the number of bars?
- The span the bars are spaced across, not the one they run along. Bars running the 30 foot direction of a 30 by 20 slab are spaced across the 20 foot width, so the width divided by the spacing gives their count.
- What is the intersections figure for?
- It is the number of points where bars cross, which is where ties go. It sizes the tie wire order and gives a sense of the labor involved.
- Does my slab need rebar at all?
- That is not something this page can answer. Whether reinforcement is required, and in what form, depends on the loads, the subgrade, and local code. Ask a qualified engineer or your building department.
Grid counts are each span divided by the spacing you enter, rounded down, plus the closing bar. Lap splices are excluded because lap length depends on bar size and code. No bar size, spacing, cover, or price is assumed, and nothing here constitutes reinforcement design. Counts come from exact geometry using the product dimensions you enter. Confirm the product size, the layout, cutting and breakage, and the final order quantity with your supplier before purchasing.
Results are planning estimates. Last reviewed 2026-08-30.