Field Guide
How Much Rebar Do I Need? Slabs, Footings & Laps
Rebar is laid out as a grid in slabs and as continuous bars in footings. Count the bars from the spacing, add splices where runs are longer than a bar, and convert to stock bars and pounds.
1. How to Calculate Rebar (The Math Explained)
Here's the exact process this calculator uses, with a 12 × 24 ft driveway, #4 bar at 18 in and 3 in edge clearance as the example.
-
1
Take off the edge clearance
Bars stop short of the forms so they're fully covered by concrete.
Lengthwise run = 24 ft − 2 × 3 in = 23.5 ft Crosswise run = 12 ft − 2 × 3 in = 11.5 ft -
2
Count the bars each way
Divide each clear dimension by the spacing and add one.
Lengthwise bars = 11.5 × 12 ÷ 18 = 7.7 → 7 + 1 = 8 Crosswise bars = 23.5 × 12 ÷ 18 = 15.7 → 15 + 1 = 16 -
3
Add lap splices, then total
The 23.5 ft runs are longer than a 20 ft bar, so each needs one splice with a 20 in (40 × ½ in) overlap.
Length = 8 × (23.5 + 1.67) + 16 × 11.5 = 385 ft Bars = 385 × 1.05 ÷ 20 = 20.2 → 21 bars of #4 Weight = 385 × 0.668 = 257 lbs
2. Bar Sizes & Weights
US rebar is numbered by its diameter in eighths of an inch, so #4 is 4/8 = ½ in. Weights are the standard nominal values.
| Bar | Diameter | lb / ft | 20 ft bar |
|---|---|---|---|
| #3 | 3/8 in | 0.376 | 7.5 lbs |
| #4 | 1/2 in | 0.668 | 13.4 lbs |
| #5 | 5/8 in | 1.043 | 20.9 lbs |
| #6 | 3/4 in | 1.502 | 30.0 lbs |
3. Placing Rebar
Keep it off the ground. Set the grid on chairs or concrete blocks (dobies) so it ends up inside the slab. Rebar lying on the dirt does almost nothing.
Tie the crossings. Wire ties at the intersections keep the grid from shifting while the concrete is placed; many crews tie every other crossing.
Stagger the splices so laps in neighboring bars don't line up across the slab.
Structural work: follow your plans
Footings, foundations, retaining walls and structural slabs carry real loads. Bar size, spacing, lap length and cover come from your plans, an engineer or the local building code; use this calculator for quantities once those are set.
4. Frequently Asked Questions
How much rebar do I need for a 10×10 slab?
With #3 bar at 18 in on center and 3 in clear of the edges, each bar runs 9.5 ft and you need 7 bars each way: 7 × 9.5 × 2 = 133 ft, about 50 lbs. With 5% waste that's 7 bars of 20 ft (cut in half).
How far apart should rebar be in a slab?
Slabs on grade commonly use bars 12 to 24 in apart each way, closer for heavier loads. Driveways and garage slabs often use #4 bar at 12 to 18 in. Your plans or local code set the actual spacing.
How much should rebar overlap?
Lap splices are commonly around 40 bar diameters: about 15 in for #3, 20 in for #4 and 25 in for #5. Codes and engineers specify anywhere from about 30 to 60 diameters depending on the concrete, bar and location, so use the value on your plans if you have one.
How much does rebar weigh per foot?
Standard weights are #3: 0.376 lb/ft, #4: 0.668 lb/ft, #5: 1.043 lb/ft and #6: 1.502 lb/ft. A 20 ft #4 bar weighs about 13.4 lbs.
How many rebar chairs do I need?
Plan on a chair about every 3 to 4 ft in each direction, so the grid stays up off the ground and ends up embedded in the slab instead of lying on the dirt. This calculator uses one every 3 ft each way.
What size rebar do I need for a footing?
Residential continuous footings commonly use two #4 bars running the length of the footing, but the size and number depend on the load, soil and local code. Use the footing setting with the bar count from your plans.
Rebar or wire mesh for a slab?
Welded wire mesh mainly controls shrinkage cracks in thin slabs like patios and walks. Rebar adds more strength and is common in driveways, garage slabs and anything carrying vehicles. Both only work if they end up in the middle of the slab, not on the ground.