How to use the rebar calculator
- Enter the slab length and width.
- Enter the bar spacing on centre from your plans.
- Enter the clearance between the bar ends and the slab edges.
- Enter the stock bar length you will buy, commonly 10 or 20 ft.
- Keep extra for laps where bars join and for offcuts.
Confirm bar size and spacing with your local code or an engineer before ordering. Decoratoradvice estimates quantity, not structural design.
Rebar formulas
Clear length = slab length − 2 × edge clearance
Clear width = slab width − 2 × edge clearance
Bars running lengthwise = (clear width ÷ spacing, rounded down) + 1
Bars running widthwise = (clear length ÷ spacing, rounded down) + 1
Total length = lengthwise bars × clear length + widthwise bars × clear width
Stock bars = total length × (1 + extra) ÷ stock length, rounded up
Worked example
A 20 × 20 ft slab, bars at 12 in, 3 in clearance, 20 ft stock, 10% extra:
Clear length and width: 20 ft − 6 in = 19 ft 6 in
Bars each way: 19.5 ÷ 1 = 19.5, rounded down to 19, + 1 = 20
Total: 20 × 19.5 + 20 × 19.5 = 780 ft
With 10% extra: 858 ft ÷ 20 = 42.9, so 43 stock bars
Common bar sizes
In the US, the bar number is the diameter in eighths of an inch.
| Bar | Diameter | Weight per foot |
|---|---|---|
| #3 | ⅜ in | 0.376 lb |
| #4 | ½ in | 0.668 lb |
| #5 | ⅝ in | 1.043 lb |
Your plans or local code set the bar size, spacing and clearance. This calculator counts material for the layout you enter; it does not size reinforcement.
Tips
- Set bars on chairs or blocks so they sit inside the slab, not on the ground.
- Tie the grid at intersections so it does not shift when concrete is placed.
- Where bars must join, the overlap length is set by your plans; increase the extra allowance if you have many laps.
- Work out the concrete at the same time with the concrete calculator, or the volume alone with the cubic yard calculator.
Common mistakes
- Forgetting the +1. Twenty spaces need twenty-one bars.
- Measuring bars to the slab edge. Bars stop short of the edge by the clearance.
- No allowance for laps on slabs longer than the stock bar.
Frequently asked questions
How much rebar do I need for a 10 × 10 slab?
At 12 in spacing with 3 in clearance: 10 bars of 9 ft 6 in each way, 190 ft in total, or 11 bars of 20 ft with 10% extra.
How far apart should rebar be in a slab?
It depends on the slab thickness, the load and the soil. Your plans or local code set the spacing; this calculator counts bars for whatever spacing you enter.
Do I need rebar in a concrete patio?
Some patios use rebar, some use wire mesh or fibres, and some use none. Follow your design or a local contractor's advice for your soil and use.
How many bars come in a ton?
At 0.668 lb per foot, a ton (2,000 lb) of #4 bar is about 2,994 ft, or roughly 150 bars of 20 ft.
Does rebar go in block walls?
Reinforced block walls use vertical bars in grouted cores, as specified in the plans. See the concrete block calculator for block counts.
Key takeaways
- Bars each way = clear dimension ÷ spacing, rounded down, plus one.
- Measure bar lengths inside the edge clearance.
- Add extra for laps and offcuts, then divide by the stock length.
- Bar size and spacing come from your plans or local code.