Rebar Calculator
This rebar calculator works out a rectangular slab grid: how many bars run each way at your spacing, how long they are after the edge cover, the total length and the weight in lb and kg – for US bar sizes #3 to #18 or metric bars.
Example: Units: US (ft, in) · Slab length (m or ft) 20 · Slab width (m or ft) 15 · Bar spacing, center to center (cm or in) 12 · Edge cover (cm or in) 3 · Bar size: #4 (0.668 lb/ft) · Layers (mats): 1 layer → Result: 35 bars · 582.5 ft (177.55 m) in total · 389.1 lb (176.5 kg). Source: Wikipedia, Rebar – US sizes (bar #n = n/8 inch up to #8; nominal mass #3 0.376 lb/ft … #18 13.60 lb/ft) and European sizes (EN 10080 Table 6: 6 mm 0.222 kg/m … 40 mm 9.86 kg/m) (read 15.10.2026). Updated: .
How it is calculated
A slab grid needs bars in both directions. A 20 × 15 ft slab with #4 bars at 12 in and 3 in edge cover takes 15 bars along the length and 20 across the width – 35 bars, 582.5 ft of rebar and 389.1 lb of steel.
How the count works
- Bars running along the length are spaced across the width: ⌊(width − 2 × cover) ÷ spacing⌋ + 1.
- Each of them is as long as the slab minus the cover at both ends.
- The same again for the other direction; with two mats everything doubles.
- Weight = total length × nominal mass: #4 weighs 0.668 lb/ft, #5 1.043 lb/ft, a 10 mm bar 0.617 kg/m.
Examples (3 in edge cover)
| Slab | Spacing | Bar | Layers | Bars | Length | Weight |
|---|---|---|---|---|---|---|
| 20 × 15 ft | 12 in | #4 | 1 | 35 | 582.5 ft | 389.1 lb |
| 20 × 15 ft | 18 in | #3 | 1 | 24 | 398 ft | 149.6 lb |
| 30 × 20 ft | 12 in | #5 | 2 | 100 | 2,350 ft | 2,451 lb |
| 10 × 10 ft | 12 in | #4 | 1 | 20 | 190 ft | 126.9 lb |
US bar numbers give the diameter in eighths of an inch up to #8 (#4 = 1/2 in); #9 to #18 are larger. Metric bars follow EN 10080 with the diameter in millimeters. For the concrete itself use the concrete calculator; the weight of other steel sections is in the steel weight calculator.
What to add on site
Bars come in stock lengths (often 20 ft or 6 to 12 m), so long runs need lap splices – commonly 40 or more bar diameters, set by the engineer. Add hooks, chairs and a few percent of waste on top of the result.
Frequently asked questions
How do I calculate how much rebar I need?
Count the bars each way – ⌊(span − 2 × cover) ÷ spacing⌋ + 1 – and multiply by their length. A 20 × 15 ft slab with #4 at 12 in and 3 in cover needs 35 bars and 582.5 ft.
How much does #4 rebar weigh?
0.668 lb per foot; a 20 ft bar weighs 13.36 lb. #3 weighs 0.376 lb/ft and #5 1.043 lb/ft.
What spacing is used for slab rebar?
That is set by the design. Driveways and patios are often built with #3 or #4 at 12 to 18 in; structural slabs follow the engineer’s drawings.
What do the rebar numbers mean?
Up to #8 the number is the diameter in eighths of an inch: #3 = 3/8 in, #4 = 1/2 in, #8 = 1 in. Larger sizes are close to that rule but not exact.
Does the calculator include laps and waste?
No. It gives the clean grid. Add lap splices where bars are joined, hooks and a waste allowance.
Sources and legal basis
As of: · Editorial policy
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