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Rebar Calculator
Lay out a rebar grid for a concrete slab. Enter the slab size, the bar spacing in each direction and the edge clearance to get the number of bars each way, the total length, the stock bars to buy, the weight and the cost.
Includes the lap and waste allowance.
- Bars running lengthwise
- 10
- Bars running widthwise
- 20
- Stock bars to buy
- 22
- Weight
- 282.9 lb
- Rebar cost
- $198.00
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Enter the slab length and width.
- Enter the spacing for bars in each direction and the edge clearance.
- Choose the bar size and the stock length sold near you.
- Keep about 10% for laps and offcuts, then read the totals.
Worked examples
A 20 × 10 ft slab with #4 bars 12 in apart both ways, 3 in edge clearance and 10% for laps
- Bars running lengthwise
- 10
- Bars running widthwise
- 20
- Total rebar length
- 423.5 ft
- Stock bars to buy
- 22
- Weight
- 282.9 lb
- Rebar cost
- $198.00
A 12 × 12 ft patio with #3 bars 18 in apart and no lap allowance
- Bars running lengthwise
- 8
- Bars running widthwise
- 8
- Total rebar length
- 184 ft
- Stock bars to buy
- 10
- Weight
- 69.18 lb
- Rebar cost
- $60.00
Bar sizes and weights
US bar numbers are the diameter in eighths of an inch. #3 is ⅜ in and weighs 0.376 lb per foot; #4 is ½ in at 0.668 lb per foot; #5 is ⅝ in at 1.043 lb per foot; #6 is ¾ in at 1.502 lb per foot. Stock lengths are usually 20 ft, with 10 ft pieces common at home centers.
Where two bars meet end to end they must overlap. A lap of about 40 bar diameters is a common figure — 20 in for #4 — which is what the lap allowance covers.
Spacing and placement
Residential slabs and driveways are often reinforced with #3 or #4 bars at 12–18 in each way, held on chairs so the steel sits in the middle to upper third of the slab rather than on the ground, with about 3 in of concrete between the steel and the soil or slab edge.
These figures are planning estimates. Local building codes and a qualified professional govern the actual design. Structural slabs, footings and walls need an engineer’s reinforcement schedule.
Questions people ask
How much rebar do I need for a 20×10 slab?
At 12 in spacing with 3 in edge clearance: 10 bars 19.5 ft long and 20 bars 9.5 ft long, 385 ft in total. With 10% for laps, 423.5 ft, which is 22 stock bars of 20 ft.
How much does #4 rebar weigh per foot?
0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb.
How far apart should rebar be in a slab?
12 to 18 in each way is typical for residential slabs and driveways. The right spacing depends on slab thickness, load and soil, so follow your plans or local code.