How to Calculate Rebar Quantity for Concrete
Bar spacing and grid dimensions, not concrete volume, drive the count
Rebar count is a spacing calculation, not a volume calculation
Unlike concrete, rebar quantity doesn't come from a volume formula — it comes from the reinforcement design: how far apart the bars are spaced (say, every 12 or 18 inches) in each direction across a slab or footing, per the structural drawing or local code requirement for that application.
Counting bars in a grid
Bar spacing is measured centre to centre. To count the bars in one direction, take the slab dimension, subtract the edge clearance (the clear distance from the slab edge to the first bar) at both ends, divide what is left by the spacing, and add one: bars = (usable width ÷ spacing) + 1. The extra bar is there because a grid always has one more bar than spaces.
Bars counted across the width run along the length, and bars counted across the length run along the width. Each bar is as long as the usable dimension it spans.
When the span isn't a whole number of spacings
If the usable width divided by the spacing isn't a whole number, the count depends on how you round. A 10 ft span at 18 in is 6.67 spacings: rounding down gives 7 bars, about 20 in apart, and rounding up gives 8 bars, about 17 in apart. The Rebar Calculator rounds down before adding one.
This guide doesn't say which rounding is correct. The spacing and bar count on the structural drawings govern; if the drawings give a maximum spacing, check that the calculator's count doesn't exceed it.
Total lineal feet, then stock lengths
Multiply each direction's bar count by its run length and add the two. Where a run is longer than one stock length, bars are lapped: add one lap length for each splice. Divide the total by the stock length (commonly 20 ft, though this varies by supplier) and round up. The Rebar Calculator also adds 5% for off-cuts before it divides.
Clearance, laps and bar size come from the drawings
Edge clearance, lap length, bar size and whether a second (top) mat is needed are design values, not estimating choices. Take them from the structural drawings or specification. The calculator's defaults (3 in edge clearance, 2 ft laps) are starting values to replace with your own.
Bar sizes are numbered by diameter in eighths of an inch, so a #4 bar is about 1/2 in across and a #5 about 5/8 in.
Worked Example
A 20 ft 6 in × 10 ft 6 in slab, one mat of bars at 12 in on centre each way, 3 in edge clearance, 20 ft stock lengths and a 5% allowance. Both usable spans divide exactly by the spacing, so the rounding question doesn't arise.
Usable size: 20.5 − 2 × 0.25 = 20 ft long; 10.5 − 2 × 0.25 = 10 ft wide
Bars running lengthwise (spaced across the 10 ft width): 10 ÷ 1 + 1 = 11 bars, each 20 ft
Bars running widthwise (spaced across the 20 ft length): 20 ÷ 1 + 1 = 21 bars, each 10 ft
Total: 11 × 20 + 21 × 10 = 220 + 210 = 430 lineal ft (no run is longer than 20 ft, so no laps)
Bars to buy: 430 × 1.05 = 451.5 ft ÷ 20 ft = 22.6 → 23 bars
Tie points: 11 × 21 = 231 grid intersections
Assumptions & Limitations
- This is a simplified single-mat grid — real structural designs may specify double mats, different top/bottom spacing, or specific lap-splice lengths that a licensed engineer's drawing should always govern.
FAQ
- Why do you add one bar when counting rebar?
- Dividing the width by the spacing counts the spaces between bars. A grid always has one more bar than spaces, because there is a bar at each end.
- Does slab thickness change how much rebar I need?
- In the calculator's single-mat method, the count comes from the slab's length and width, the spacing and the edge clearance. Thickness affects the concrete volume, not the bar count.
- How many 20 ft bars should I buy?
- Divide the total lineal feet, including any laps, by 20 and round up. The Rebar Calculator adds 5% for off-cuts first: 430 ft becomes 451.5 ft, or 23 bars.
Related Calculators
Related Guides
Sources
CRSI, Reinforced Concrete Terminology
Definitions of bar number (approximately the bar diameter in eighths of an inch), spacing (centre to centre), lap splice and concrete cover.
CRSI, Reinforced Concrete Terminology — reference