How Much Rebar Do I Need? Spacing Chart, Bar Counts & Weights

Short on time? The rebar calculator computes bars, linear feet, weight, and the 10% waste allowance for any slab in one step. This guide explains the convention behind it — spacing, worked tables, laps — so you can read a structural detail or a supplier's quote and know exactly what you are looking at.

Spacing chart by application

These are typical residential guidance figures, the spacing values you will see on standard details and contractor quotes. They are not engineering: your local building code, soil conditions, and any stamped structural drawings govern. When in doubt, check local code — the cost of asking is zero, and the cost of under-reinforcing a slab is not.

Bar size notation: #4 means 4/8 = ½ inch diameter, #5 is ⅝ in, #6 is ¾ in, and #3 is ⅜ in — the number is eighths of an inch. Residential work lives at #3 and #4; #5 shows up for driveways and heavier slabs; #6 and #8 are engineer-specified territory.

The bar-count formula

Every table below — and the calculator — uses one convention. For each direction of a rectangular slab grid:

• Bars per direction = floor((dimension in inches − 2 × edge clearance) ÷ spacing) + 1
• Bar length = dimension in feet − 2 × (clearance ÷ 12)

The default edge clearance is 3 inches — the ACI minimum for concrete in ground contact. So a 20 ft side at 18-in spacing: floor((240 − 6) ÷ 18) + 1 = floor(234 ÷ 18) + 1 = 13 + 1 = 14 bars, each 20 − 2 × 0.25 = 19.5 ft. That gives 14 × 19.5 = 273 ft in one direction. Do it again for the other direction and you have the grid: 14 bars/dir × 19.5 ft = 546 linear feet.

Why “+ 1”? Because the first bar sits at the clearance line, not one spacing beyond it: you count the bar at the start edge, the bar at the finish edge, and the spaces between them. This is the fence-post convention — N spacings need N + 1 posts.

Worked tables: 10×10 and 20×20 slabs

Both tables use 3-in edge clearance. “Per direction” means one axis of the grid; a slab needs both, so the total is doubled.

Check one by hand: 20×20 at 18 in. Each side is 240 in; subtract 2 × 3 in clearance → 234 in; floor(234 ÷ 18) = 13; + 1 = 14 bars. Bar length: 20 − 2 × 0.25 = 19.5 ft. Per direction: 14 × 19.5 = 273 ft. Both directions: 546 ft. In #4 bar at 0.668 lb/ft that is 546 × 0.668 = 364.7 lbs — about 401 lbs with the 10% waste allowance (see the weights section next).

Weight table by bar size

Nominal weights per foot (ASTM A615) are the second column; the third column applies them to the 20×20 @ 18 in example (546 ft) so you can feel the jump between sizes:

Suppliers price by weight (per pound or per ton), so the weight column is your order ticket. Standard rebar comes in 20-ft lengths from most yards — that matters for the bar-length column: a 19.5-ft bar fits a 20-ft stick with a half-foot cutoff, while a 29.5-ft bar needs two sticks plus a lap splice. Factor standard lengths into your waste allowance on big slabs.

Footings and walls

Footings do not use a grid — they use continuous longitudinal bars, typically 2 (sometimes 3 for wide footings), running the full footing length with standard end clearance. The formula is the same idea: bars × (length − 2 × clearance/12).

• 20 ft footing, 2 × #4: 2 bars × 19.5 ft = 39 ft — about 26.1 lb, order 29 lb with waste.
• 40 ft footing (two sides), 2 × #4: 4 bars × 19.5 ft = 78 ft — 52.1 lb.
• Vertical dowels: typically #4 at 48 in on center tying the wall to the footing — count them separately; they are short pieces the grid formula misses.
• Corners and T-intersections: continuous bars must turn corners with bent bars or extra lap splices; each splice costs one lap length (see below).

For footing volume and concrete sizing, pair this with the concrete footing calculator — it handles strip, square, and round footings and applies the same 10% overage.

Lap splices and waste

Rebar sticks max out at 20 ft from most suppliers, so long runs need lap splices — overlapping two bars so the concrete bonds them into one. The typical rule of thumb is a Class B splice of about 40× the bar diameter:

Label this as typical guidance: the exact splice length comes from ACI 318 and depends on concrete strength, bar coating, and position — check the structural detail on permitted work. The calculator does not count splice laps in its bar total; instead it rolls them into the 10% waste allowance alongside off-cuts. On jobs with many splices, corners, or openings, raise the allowance to 12–15% rather than cutting it fine.

Chairs, ties, and the other 10%

The bar count is the headline; the accessories are what keep the steel where the design put it. Chairs (or dobies and bricks on small jobs) hold the grid up at the right height — rebar belongs in the middle of the slab, not resting on the dirt, because cover below the steel matters as much as cover above it. Plan roughly one chair per 4–6 sq ft of grid. Tie wire (16-gauge, sold in coils or pre-cut loops) fastens every bar intersection or every other one — a 20×20 grid has 196 intersections, and a coil is cheap insurance against the grid walking during the pour. Neither is glamorous; both are the difference between reinforcement and expensive decoration.

Reading rebar notes on plans and quotes

Once you know the convention, contractor shorthand decodes itself.#4 @ 18” O.C. means #4 bar at 18 inches on center — exactly the grid this guide computes. E.W. means “each way” (both directions of the grid); T&B means top and bottom mats on a thick slab. Cont. marks continuous bars in footings, and #4 dowels @ 48” O.C. is the standard vertical tie between footing and wall. When a quote says “546 LF of #4,” you can now run the bar-count formula backward: 546 ft ÷ 2 directions ÷ 19.5 ft per bar = 14 bars per direction — a 20×20 slab at 18-in spacing, and you know instantly whether the quote matches your slab.

Ordering from a supplier: what to say

Suppliers speak in weight and standard lengths, so translate your numbers before you call. For the 20×20 @ 18-in example the order is not “28 bars” — it is ~600 linear feet of #4 (546 ft plus the 10% allowance the calculator folds in, rounded to a sensible number), which the yard will sell as 30 twenty-foot sticks — 28 for the grid plus 2 for cuts and laps — weighing ~401 lbs. Ask for stick count and weight, confirm the price per pound or per stick, and ask whether they cut to length (most will, for a small fee — worth it on big slabs, where hauling 30 full-length sticks is its own project). Epoxy-coated bar costs more and needs touch-up paint at every cut; plain black bar is the default for residential work.

Common mistakes

Measuring spacing edge-to-edge instead of on center. Spacing is center-to-center; edge-to-edge shortchanges the grid. Skipping edge clearance. Bars need ~3 in of concrete cover at the edges (ACI minimum for ground contact) — concrete protects steel from corrosion, and steel at the face rusts, expands, and spalls the concrete off. Counting only one direction. A slab grid has two axes; every number above doubles. Forgetting dowels. Walls-to-footing dowels and slab thickening bars are not in the grid math — add them separately. Ordering exact footage. A 20-ft stick yields one 19.5-ft bar; a 29.5-ft bar needs two sticks and a splice. Plan cuts and laps into the 10%. Swapping bar size without checking weight. Moving a driveway from #4 to #5 at 18 in jumps the 546-ft example from 365 lb to 570 lb — same grid, 56% more steel.

Run your exact slab through the rebar calculator — bars, linear feet, pounds, tons, and the waste-adjusted order in one result.