How Much Rebar Do I Need?
Planning a concrete slab and wondering how much rebar you need? This free rebar calculator takes your slab's length and width plus your grid spacing (12″, 18″, or 24″ on center) and tells you exactly how many bars the grid requires, how many to buy with a 10% waste and lap allowance, and what they will cost.
Enter your slab details
Your results
| Bars per direction | — |
|---|---|
| Total bars in grid | — |
| Total linear feet | — |
| Estimated cost | — |
Estimate for a rectangular slab; assumes bars run straight across the full dimension each way. Confirm spacing and bar size with your local building code or a structural engineer.
How to use this calculator
Measure your slab in feet and enter the length and width, pick your grid spacing (12″, 18″, or 24″ on center), and tell the calculator what bar length you plan to buy — rebar is commonly sold in 20-ft or 40-ft lengths. If you know your supplier's price per bar, enter it for a material cost estimate. Results update instantly as you change any value.
How it works
This tool uses one counting rule for a rectangular slab grid. Convert a slab dimension to inches, divide by the spacing, round down, and add one for the edge bar:
Bars in one direction = ⌊slab inches ÷ spacing inches⌋ + 1. Each direction is counted across the opposite dimension (the bars that run along the length are spaced across the width, and vice versa), so total bars = the two direction counts added together. Total bar-length needed is each direction’s count times the slab dimension its bars span. Pieces to buy uses your purchased bar length — a slab side longer than the bars you buy takes ⌈side ÷ bar length⌉ purchased pieces per bar — and then adds a 10% waste and lap-splice allowance, rounded up.
Worked example — a 20×20 ft slab at 18″ spacing: 20 ft equals 240 inches. 240 ÷ 18 = 13.33, round down to 13, plus the edge bar = 14 bars per direction, or 28 bars total (14 × 20 ft + 14 × 20 ft = 560 linear feet). Buying 20-ft bars: 28 × 1.10 = 30.8, rounded up = 31 bars to buy. (For comparison, a tighter 12″ grid on the same slab gives ⌊240 ÷ 12⌋ + 1 = 21 bars per direction, 42 total.)
About the 10% allowance: rebar ends must overlap (lap splices) wherever bars join end to end, and cutting a grid from stock lengths always leaves offcuts. The 10% covers both. If your slab is long enough to need many spliced joints, choose a shorter purchased bar length above — the calculator will count the extra pieces the splices require.
Frequently asked questions
How much rebar do I need for a 20×20 slab?
At standard 18-inch grid spacing, a 20×20 ft slab needs 14 bars per direction — 28 bars total. The math: 20 ft equals 240 inches, 240 ÷ 18 = 13.33, round down to 13 and add the edge bar to get 14 per direction. With the 10% waste and lap allowance, buy 31 twenty-foot bars. At a tighter 12-inch spacing the same slab needs 21 bars per direction, 42 total.
What rebar spacing for a concrete slab?
Most residential concrete slabs use 18-inch on-center spacing. Use 12-inch spacing for heavier loads such as driveways, garage floors, and thick slabs, and 24-inch spacing only for light-duty flatwork like walkways. Spacing that is too wide will not control cracking, while spacing tighter than you need just adds cost. Your local building code or your engineer’s specification always overrides any rule of thumb.
How do I calculate rebar overlap/lap length?
Rebar ends are overlapped — lap spliced — so each joint is as strong as the bar itself. A common rule of thumb is a lap length of 40 to 60 times the bar diameter: for a #4 bar (1/2 inch) that is 20 to 30 inches, and for a #5 bar (5/8 inch) it is 25 to 37.5 inches. Many residential slabs use a 24 to 30-inch lap for #4 bars. The 10% waste allowance in this calculator covers typical laps and cutting offcuts.
What size rebar (#3 vs #4 vs #5) should I use?
#3 rebar (3/8 inch) suits thin flatwork and light residential slabs up to about 4 inches thick. #4 rebar (1/2 inch) is the most common choice for residential slabs, driveways, and patios. #5 rebar (5/8 inch) is used for thicker slabs, footings, and heavier loads. Bar size does not change the count from the grid rule, but bigger bars cost more per piece — enter your price per bar above for an accurate estimate. When in doubt, ask a structural engineer; undersized rebar is not the place to save money.