Rebar Calculator
Your estimate
- Total rebar length
- 780 ft237.7 m
- Rebar weight
- 521 lb236 kg
- Bars along the length
- 20
- Bars along the width
- 20
A rebar grid for a slab or patio, with the number of stock bars to buy.
Updated 2026-10-20 · Based on: bag yields printed by concrete-mix makers (0.30–0.60 ft³ per bag) and a 1.54 dry-volume factor.
How to use the Rebar Calculator
- Enter the slab length and width.
- Choose the spacing (12 in or 18 in is common for slabs) and the edge cover (2–3 in).
- Pick the bar size and stock length (20 ft in the US, 6 or 12 m elsewhere).
- Read the number of sticks to buy and the weight.
Formula
Bars along the length = (width − 2 × cover) ÷ spacing + 1, and the same the other way.
Total length = bars × bar length. Bars longer than the stock length are lapped about 40 bar diameters.
Worked example
With the values preset in the calculator above:
- Length
- 20 ft
- Width
- 20 ft
- Bar spacing
- 12 in
- Edge cover
- 3 in
- Bar size
- #4 (½ in)
- Stock bar length
- 20 ft
- Extra for waste
- 5 %
- Stock bars to buy
- 42
- Bars in the grid
- 40
- Total rebar length
- 780 ft (237.7 m)
- Rebar weight
- 521 lb (236 kg)
- Bars along the length
- 20
Rebar weight per foot
- #3 (⅜ in): 0.376 lb/ft
- #4 (½ in): 0.668 lb/ft
- #5 (⅝ in): 1.043 lb/ft
- #6 (¾ in): 1.502 lb/ft
Frequently asked questions
How much rebar do I need for a 20×20 slab?
At 12 in spacing with 3 in edge cover: 40 bars, 780 ft in total, about 42 sticks of 20 ft (with 5 % extra).
What spacing should rebar be in a slab?
12–18 in on center both ways is typical for driveways and patios with #3 or #4 bar. Structural slabs follow the engineer’s drawings.
Does a 4 inch slab need rebar?
Patios and walkways often use wire mesh or fibers instead. Driveways and garage floors usually get #3 or #4 rebar on chairs.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.