Home > Concrete Calculator > Rebar Calculator
Rebar Calculator
How much rebar for your slab? Enter size and bar spacing: get total linear feet and 20 ft bar count instantly.
Estimates only. Bag yields per manufacturer specs (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³). Ready-mix trucks carry ≈ 10 yd³. Always round up and confirm with your supplier.
A 20 by 20 ft slab with bars at 18 inch spacing needs about 560 linear feet of rebar, or 28 twenty-foot bars. Enter your slab size and spacing below for an instant rebar estimate.
How to use the rebar calculator
- Enter the slab length and width in feet.
- Enter the bar spacing in inches (18 inches is typical for slabs).
- Read total linear feet and the number of 20 ft bars.
- Add 10 percent extra for laps and offcuts when ordering.
Formula
Bars each way = (dimension ÷ spacing) + 1. Total length = bars × opposite dimension. Bars of 20 ft = total ÷ 20, rounded up.
Practical tips
- Lap bars 30 times the bar diameter (about 15 inches for #4 bar).
- Use chairs or dobies to hold rebar mid-slab, not lying on the ground.
- 18 inch spacing suits most residential slabs; driveways often use 12 inches.
- Tie intersections with wire so bars do not shift during the pour.
Frequently asked questions
What spacing for rebar in a slab?
Eighteen inches on centre suits patios and light slabs. Twelve inches suits driveways.
How much do you lap rebar?
Lap 30 bar diameters: 15 inches for #4 (½ in) bar, 19 inches for #5.
Mesh or rebar: which is better?
Rebar gives more strength and stays put better. Mesh suits thin, light-duty slabs.
Pair with the slab calculator or explore the hub.
Worked examples
Example 1: 20×20 ft slab, 18 in spacing. 14 bars each way = 560 linear feet = 28 x 20-ft bars.
Example 2: 30×40 ft slab, 18 in spacing. 27 + 21 bars = 1,650 linear feet = 83 x 20-ft bars.
Rebar reference table
Linear feet, grid both ways. Add about 10% for laps and waste.
| Slab | Spacing | Linear feet |
|---|---|---|
| 10×10 ft | 18 in | 140 |
| 20×20 ft | 18 in | 560 |
| 20×20 ft | 12 in | 840 |
| 30×40 ft | 18 in | 1,650 |
More rebar questions
What spacing for a slab?
18 inches on centre suits most residential slabs. Use 12 inches for heavy loads.
Is this a structural design?
No. Material estimate only. Structural work needs an engineer’s design.