Rebar Calculator
A 20 x 20 slab at 18 inches on center needs 30 bars in a single-layer grid and 660 linear feet with 10% waste. Use this rebar calculator to estimate a simple slab layout from slab length, slab width, spacing, and optional waste. It counts the bars running both directions and totals the linear footage you need before you convert that footage into 20-foot or 40-foot sticks.
Quick answer
Bars running the slab length are spaced across the slab width.
What this tells you
- •Bars running the slab length are spaced across the slab width.
- •Bars running the slab width are spaced across the slab length.
- •The calculator places a bar at both slab edges and rounds partial spacing up to the next full bar.
- •Waste adds extra linear footage for cuts and jobsite loss.
How to Use
- 1Enter the slab length in feet.
- 2Enter the slab width in feet.
- 3Enter the on-center spacing in inches. Common slab grid spacing is 12, 18, or 24 inches.
- 4Add optional waste if you want an ordering buffer.
- 5Click Calculate to see bars running each way, total bars, and total linear feet.
How It Works
Formula
Bars Running Length = ceil(Width(ft) x 12 / Spacing(in)) + 1
Bars Running Width = ceil(Length(ft) x 12 / Spacing(in)) + 1
Total Linear Feet = (Bars Running Length x Length(ft)) + (Bars Running Width x Width(ft))
Total with Waste = Total Linear Feet x (1 + Waste%)This estimate uses one layer of straight bars in each direction. The calculator converts slab dimensions to inches for spacing, rounds up when the spacing does not land evenly, adds an edge bar on both sides, and then totals the length of every bar. Waste is applied after the base linear footage is calculated.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
20 x 20 patio slab at 18 inches on center
The width produces 15 bars running the 20-foot length, and the length produces 15 bars running the 20-foot width. That is 30 bars total. Base footage is (15 x 20) + (15 x 20) = 600 linear feet. Add 10% waste to reach 660 linear feet.
12 x 24 driveway section at 12 inches on center
The 24-foot width needs 25 bars running the 12-foot direction. The 12-foot length needs 13 bars running the 24-foot direction. That gives 38 bars total. Base footage is 612 linear feet, and 5% waste brings the order to 642.60 linear feet.
Single-Layer Slab Grid Examples
Quick rebar totals for common square slab sizes before waste. These examples assume one flat grid with straight bars in both directions.
| Slab size | Spacing | Total bars | Base linear feet |
|---|---|---|---|
| 10 x 10 ft | 12 in OC | 22 | 220 ft |
| 10 x 10 ft | 18 in OC | 16 | 160 ft |
| 20 x 20 ft | 12 in OC | 42 | 840 ft |
| 20 x 20 ft | 18 in OC | 30 | 600 ft |
| 30 x 30 ft | 24 in OC | 32 | 960 ft |
Add waste after you total the base linear feet. Then divide by the stock length you plan to buy and round up to the next whole stick.
How to turn linear feet into rebar sticks
Suppliers usually sell rebar in standard lengths such as 20-foot or 40-foot sticks. After you get the total linear feet with waste, divide by the stock length and round up. For example, 660 linear feet needs 33 pieces of 20-foot rebar or 17 pieces of 40-foot rebar.
That piece count is still a takeoff estimate, not a structural schedule. If your plan calls for lap splices, hooks, bent bars, or cut pieces for thickened edges and beams, the drawing takeoff can be higher than a simple grid estimate.
Common mistakes
- Using slab thickness instead of bar spacing
- Counting only one direction of the grid instead of both directions
- Ordering straight sticks from the base linear feet without adding waste or rounding up the piece count
- Using a one-layer estimate for slabs that call for top and bottom mats, edge beams, or extra reinforcement at openings