Enter the size of your slab, the bar spacing and the bar size. The calculator lays out the grid both ways and tells you how many bars to buy, the total length, the weight and the number of tie points.
Waste and cost
For slabs that carry structural loads, follow the bar size and spacing on your plans or local code.
How the rebar grid is worked out
Rebar in a slab usually runs both ways in a square grid. The calculator keeps the bars a set distance from the edges, then fits as many as the spacing allows in each direction.
- Bars running the long way = (slab width − 2 × edge gap) ÷ spacing, plus 1
- Bars running the short way = (slab length − 2 × edge gap) ÷ spacing, plus 1
- Where a bar is longer than one stick, two pieces overlap. The calculator uses 40 bar diameters for each overlap, a common rule of thumb.
Worked example
A 20 ft × 10 ft slab with #4 bars at 12 inches, 3 inches from the edges:
- 10 bars run the long way, each 19.5 ft
- 20 bars run the short way, each 9.5 ft
- Total: 195 + 190 = 385 ft of bar
- With 5 percent for cutting: 21 bars of 20 ft
- Weight: 385 × 0.668 = about 257 lb, with 200 places to tie the bars together
Rebar sizes and weights
| Bar | Diameter | Weight per foot | Weight of a 20 ft bar |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.5 lb |
| #4 | 1/2 in | 0.668 lb | 13.4 lb |
| #5 | 5/8 in | 1.043 lb | 20.9 lb |
The bar number is its diameter in eighths of an inch, so #4 is 4/8, or half an inch.
Spacing and placement
- Many residential slabs, such as patios and driveways, use #3 or #4 bars 12 to 18 inches apart. Slabs that carry structural loads should follow the plans.
- Keep bars about 3 inches in from the edges so concrete covers them.
- Rebar only works when it sits inside the concrete, not on the ground. Lift it onto plastic chairs or concrete blocks so it ends up near the middle of the slab.
- Tie the bars where they cross with wire, so the grid doesn’t move during the pour.
Rebar or wire mesh?
Welded wire mesh is lighter and quicker to lay, and it’s common in thin slabs to help hold cracks together. Rebar is stronger and better where a slab will carry heavy loads, such as a driveway for trucks. If you’re not sure, follow local practice or ask your supplier.
Frequently asked questions
How much rebar do I need for a 10 × 10 slab?
With #4 bars at 12 inches and a 3 inch edge gap, you need 10 bars each way, about 190 ft in total. That’s 10 bars of 20 ft with a little extra, weighing about 127 lb.
How far apart should rebar be in a slab?
For many residential slabs, 12 to 18 inches apart in both directions. Closer spacing makes a stronger slab but uses more steel.
How much does #4 rebar weigh?
0.668 lb per foot, so a 20 ft bar weighs about 13.4 lb.
How much should rebar overlap?
A common rule of thumb is 40 times the bar diameter, which is 20 inches for #4 bars. Structural work should follow the lap length on the plans.
Do I need rebar in a concrete patio?
Not always. Many 4 inch patios use wire mesh or fiber-reinforced concrete instead. Rebar is worth adding where the slab will carry vehicles or sits on soft ground.
Results are estimates for planning. Bar weights are standard values. See How We Calculate for the formulas and values we use.