Rebar Calculator for a Concrete Slab
Estimate the reinforcing steel for a rectangular slab. Enter the slab size and your bar spacing to see how many bars run each way and the total linear feet.
How to calculate rebar for a slab
- Enter the slab length and width in feet.
- Set the center-to-center bar spacing in inches, often 12, 16 or 18.
- Read off the bars each way, total bars and total linear feet to order.
Examples
20 by 10 ft slab at 12 in spacing
Length 20 ft, width 10 ft, spacing 12 in
11 + 21 bars, 32 total, 430 linear feet
Frequently asked questions
How is rebar spacing measured?
Spacing is the center-to-center distance between parallel bars. A 12 inch grid puts bars one foot apart in both directions, forming a square mat across the slab.
Why is there one more bar than the span divided by spacing?
A mat needs a bar on both edges, not just the gaps between them. So the count is the span divided by spacing, rounded down, plus one for the closing edge bar.
What spacing should I use?
Patios and walkways often use 16 to 18 inch grids, while driveways and structural slabs use 12 inches or tighter. Follow your engineer or local code for the right spacing.
Does this account for edge clearance or overlaps?
No. The grid assumes a bar on each edge and full-length bars. Add extra for lap splices, typically about 40 bar diameters, when bars are spliced end to end.
How do I convert linear feet to a number of sticks?
Divide the total linear feet by the length of the rebar you buy, often 20 feet, and round up. Then add a margin for laps, bends and cutting waste.
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