Estimate rebar for a rectangular slab, wall, or footing. Enter the project dimensions, bar spacing, stock length, and bar size to get total linear feet, whole stock bars to buy, approximate weight, and a field lap-splice length.
From slab size to bars, spacing, and weight
Every mode here runs one grid: subtract the edge clearance from each side of the slab, divide what's left by your bar spacing to count bars in each direction, add up their runs, pad for waste, then round up to whole stock bars. A 20-by-12-foot slab at the 3-inch cover, 12-inch spacing, #4 , and 20-foot-stock defaults leaves a 19.5-by-11.5-foot grid — 13 bars running the 19.5-foot length and 21 running the 11.5-foot width, so 13 × 19.5 + 21 × 11.5 = 495 of steel raw. The 10 waste pad lifts that to 544.5 , about 364 of #4, which rounds up to 28 twenty-foot bars. The table and sections below are that same grid applied to real slabs, bar sizes, and the wall/footing mode.
Bar count comes almost entirely from the on-center spacing, not the slab shape. 12-inch spacing is the default for a denser mat; widening to 18 cuts the steel by roughly a third.
Changing the bar size leaves the bar count and total length identical — it only raises the weight you haul and the lap length at each splice.
Slab mode lays a two-way grid; wall/footing mode is a single-plane layout that stacks horizontal courses up a height and runs verticals along the length. Switch modes for a wall or a strip footing.
This counts the grid you specify for ordering. It does not size the bar, set the spacing, or fix the cover for structural loads — engineered drawings do that.
Example: a wall and a footing in wall/footing mode
- Switch to wall/footing mode for a vertical layout.
- A 30-foot-long, 8-foot-tall wall with #5 at 12-inch spacing both ways counts 9 horizontal courses (8 ÷ 1 , plus one) and 31 vertical bars (30 ÷ 1 , plus one).
- Their runs total 9 × 30 + 31 × 8 = 518 ; the 10 waste pad lifts that to 569.8 feet, about 594 pounds of #5, which rounds up to 29 twenty-foot bars.
- A shallow strip footing uses the same mode with the depth entered as the height: a 40-foot footing only 2 deep drops to 3 courses and 41 verticals — 222.2 feet, or 12 bars.
How much rebar by slab size
Every row below comes straight from this calculator at the 3-inch cover, 12-inch spacing, #4 , 10 waste, and 20-foot-stock defaults, so the chart and the tool always agree. Change the spacing or the bar size and the numbers move with you.
| Slab (length × width) | Bars (length + width) | Rebar length, 10% waste | Weight (#4) | 20-ft bars |
|---|---|---|---|---|
| 10 × 10 | 11 + 11 | 229.9 | 154 | 12 |
| 12 × 12 | 13 + 13 | 328.9 | 220 | 17 |
| 20 × 12 | 13 + 21 | 544.5 | 364 | 28 |
| 24 × 24 | 25 + 25 | 1,292.5 | 863 | 65 |
| 30 × 40 | 41 + 31 | 2,677.4 | 1,789 | 134 |
Spacing, not slab shape, is what moves the count. Widening the same 20-by-12-foot slab from 12-inch to 18-inch on-center spacing cuts the steel by about a third — from 544.5 and 28 bars down to 370.15 and 19 bars. Lighter residential slabs often specify 18 ; 12 is common where a plan calls for a denser mat. Weights are ASTM A615 nominal values for a #4 ; switch bar size in the next section to change them.
Which bar size to enter, and what it changes
The bar size does not change how many bars you need or how far they run — the slab and the spacing set that. It changes the weight you haul and the lap length at each splice. This table holds the 20-by-12-foot slab fixed at 12-inch spacing and swaps only the bar:
| Bar size | Weight per foot | Weight for the 20 × 12 slab | Lap at 40 diameters |
|---|---|---|---|
| #3 | 0.376 | 205 | 15 |
| #4 | 0.668 | 364 | 20 |
| #5 | 1.043 | 568 | 25 |
| #6 | 1.502 | 818 | 30 |
All four order the same 28 twenty-foot bars for that slab, because the grid is identical — only the tonnage and the splice overlap grow. #4 (½-inch) and #5 (⅝-inch) cover most residential slabs and footings; #3 suits light mats, and #6 shows up in heavier structural work. The per-foot figures are the ASTM A615 nominal unit weights.
Slab mode vs. wall and footing mode
Slab mode lays a two-way grid and counts bars in both directions across the slab. Wall and footing mode is a single-plane layout: the vertical-course spacing sets how many horizontal bars stack up the wall height (or footing depth), and the horizontal spacing sets how many vertical bars run along the length. The worked example above sizes a 30-foot wall this way, and the same mode handles a strip footing when you enter the footing depth as the height. Neither mode places steel for structural demand — each one counts the grid you specify and totals its length, pieces, and weight.
Waste, stock length, and lap splices
Three inputs fine-tune the order. Waste adds a percentage to the raw grid length for cuts, offcuts, and site loss; 10 is the default and a common allowance, dialed lower for a clean rectangular pour or higher for a cut-heavy layout. Stock length is the length of each stick you buy — most yards carry 20-foot bars — and the tool rounds the padded total up to whole sticks so the last bar is never a fraction. Lap splice is where two bars overlap end to end: the tool multiplies the bar diameter by a lap factor (40 diameters by default), so a #5 (0.625-inch diameter) laps 40 × 0.625 = 25 . Treat that as a contractor's field shorthand, not a code figure — ACI 318 ties a real Class B tension splice to the bar's development length, which depends on concrete strength, cover, and bar spacing and is usually longer.
What this estimate is not
This is an ordering takeoff, not a structural design. It counts bars for a grid or wall you specify and totals their length, pieces, and weight; it does not size the bar, choose the spacing, or set the cover for the loads, soil, or seismic demand on your project — those come from an engineer's stamped drawings. It also skips what a bar-bending schedule handles: hooks, bends, chairs and supports, corner and edge bars, two-mat top-and-bottom reinforcement, openings, and curved or stepped slabs. Use it to build a supplier list and a budget, then confirm the size, spacing, and splices against your plan before the steel is delivered.
Frequently asked questions
How do I estimate the total cost of rebar for my project?
Use the stock-bar count from the calculator, then multiply by your local supplier price per bar. The tool intentionally leaves pricing out because rebar prices vary by region, bar size, coating, delivery, and order size.
How do I find the total length of rebar my project requires?
Add up the length of every bar in your grid to get the total rebar length. The yard charges for a full stick even when you need only part of the last one.
How does this calculator figure out rebar grid dimensions?
In slab mode, it subtracts edge clearance from the slab length and width, then counts bars in each direction from the on-center spacing. In wall / footing mode, it counts horizontal courses across height and vertical bars along length.
What method does this calculator use to find total rebar length?
The calculator sums the run of every bar in the grid to find a total length. Multiply that length by the weight per foot of your chosen bar size to get an estimated rebar weight. The grid math alone does not give you weight without that extra step.
How do I calculate bar spacing, total quantity, coverage area, and weight from my slab dimensions?
It returns bar count, coverage area, and estimated weight at once. For slabs with odd shapes, split the area into rectangles, run each section through the tool, and add the results together.