Rebar Calculator
Enter your slab's length and width and the grid spacing you want, and the calculator lays out a standard two-way rebar mat: bars running both directions at the chosen on-center spacing. It returns how many bars you need, the total linear feet (with a lap allowance), how many 20 ft sticks to buy, and the total steel weight. Pick the bar size to update the weight and cost.
Your measurements
The longer side of the slab.
The shorter side of the slab.
Distance between bars, both ways. 12" is typical for driveways and footings, 16–18" for patios and light slabs.
#4 (1/2 in) is the standard for 4 in residential slabs. Heavier loads use #5.
20 ft sticks to buy
46
#4 (1/2")
Total rebar
901 lin ft
incl. laps & waste
Steel weight
602 lb
0.668 lb/ft
What to buy
- #4 rebar — 20 ft sticks
- 46 sticks
- Tie wire
- 3 rolls
A "stick" is one standard 20 ft length of rebar from the store. This is the only rebar you actually buy — cut it down on site to fit.
Thin wire to lash the bars together everywhere they cross.
Estimates only. Verify against your supplier's coverage figures before ordering.
Rebar layout
Rebar mat to scale
21 × 21 bars · 12" o.c.
How the grid is laid out
- Bars running the length
- 21 bars(every 12" across the 20 ft width)
- Bars running the width
- 21 bars(every 12" along the 20 ft length)
- Total bars in the mat
- 42 bars
- Length of each bar
- 19.5 / 19.5 ft
- Total bar length
- 819 lin ft(before overlaps)
- Laps & waste (+10%)
- 82 lin ft
- Total rebar to order
- 901 lin ft
On center ("o.c.") means measured from the middle of one bar to the middle of the next.
Bars are cut 3 in short at each end (the "cover") so the steel stays buried and won't reach the edge and rust.
Every bar added end to end — the running length of steel before any overlap.
Extra length for where two bars overlap end to end (a "lap splice") and for off-cuts.
Divided by the 20 ft stick length and rounded up, that's the 46 sticks above.
The formula
Bars each way = ⌊span ÷ spacing⌋ + 1 · Total length = Σ(bars × (slab side − 2× cover)) × (1 + lap %) · 20 ft sticks = ⌈total length ÷ 20⌉
Example: A 20 × 20 ft slab with #4 bar at 12" on center and 3" cover gives 21 bars each way (42 total), about 901 lin ft with a 10% lap allowance — 46 twenty-foot sticks weighing roughly 600 lb.
How it works
- 1A slab is reinforced with a two-way grid (mat) of rebar — bars running both directions at the same on-center spacing.
- 2The number of bars in each direction is the slab side divided by the spacing, plus one for the closing bar at the far edge.
- 3Each bar is cut a little short so the steel stays 3 in (the concrete cover) back from the slab edges and won't rust.
- 4Total linear feet is the sum of all the bars' lengths, plus ~10% for the overlaps where bars splice together and for cut-offs.
- 5Divide by the stock length (usually 20 ft) and round up for the number of sticks to buy; multiply the length by the bar's lb-per-foot for the weight.
Frequently asked questions
How much rebar do I need for a concrete slab?
Lay out a two-way grid at your chosen spacing — 12 in on center is standard for driveways and footings, 16–18 in for patios and light slabs. Count the bars each way (slab side ÷ spacing, then add one), add up their lengths, and add about 10% for lap splices. A 20 × 20 ft slab at 12 in on center needs roughly 900 linear feet, or about 46 twenty-foot sticks.
What size rebar is used in a residential slab?
#4 rebar (1/2 inch diameter) is the standard for typical 4-inch residential slabs, patios, driveways and garage floors. Step up to #5 (5/8 inch) for heavier loads such as commercial floors or driveways that carry trucks and RVs. #3 (3/8 inch) is used for light-duty sidewalks.
How far apart should rebar be spaced?
Common residential spacing is 12 to 18 inches on center in both directions. Tighter spacing means more steel and a stronger slab: driveways and footings usually use 12 inches, patios and light slabs 16–18 inches. The IRC/ACI maximum is the lesser of three times the slab thickness or 18 inches.
How much does rebar weigh?
Weight per linear foot is 0.376 lb for #3, 0.668 lb for #4, 1.043 lb for #5 and 1.502 lb for #6. A 20-foot stick of #4 weighs about 13.4 lb. Multiply your total linear footage by the per-foot weight to size the load — and check whether your vehicle can carry it.
How much overlap do rebar splices need?
Where two bars join end to end they must overlap (lap splice) by at least 40 times the bar diameter — about 20 inches for #4 and 25 inches for #5 — and be tied together. This calculator adds a 10% allowance by default to cover lap splices and cut-off waste; raise it if your slab is much longer than your bar stock.