How Much Rebar Do I Need? — Bar Count, Length and Spacing Guide
Quick answer: count bars along each span (span ÷ spacing + 1 per direction), add both directions for total length, then add ~10% for splice overlaps. A 10×20 driveway at 16-in spacing needs about 16 sticks of 20-ft #4 rebar.
Why the count matters before you pour
Concrete is brilliant in compression and nearly useless in tension. The moment a slab has to carry weight across an unsupported span — a car driving over the middle of a driveway, a footing holding up a wall — the bottom of the concrete wants to crack and pull apart. Rebar is the steel skeleton that absorbs that tension, and it only works if it runs in both directions, in a grid, everywhere the slab is stressed. Buy too little and you have a slab with gaps in the mat — cracks will find those gaps first. Buy too much and you've wasted money on steel the concrete never needed. The count math is simple, but skipping the +1 edge bar or the splice allowance is where DIY pour budgets quietly go wrong.
Spacing rules: how close is close enough?
Spacing is the first decision, and it is load-driven:
- 12 in on center — driveways, garage slabs, anything with vehicle weight. This is the tightest residential standard and gives the stiffest mat.
- 16-18 in on center — house slabs, patios, sidewalks. The default for most poured residential work; strong enough with #4 bar at 3-4 in of slab thickness.
- 24 in on center — light-duty paths, shed floors, thin utility pads where you're mainly controlling shrinkage cracks.
The spacing is measured center-to-center, and the first bar sits one spacing in from the edge — hence the +1: a 10-ft span at 16-in spacing holds (10 ft − 6 in edge) ÷ 16 in = 7.1 → 7 gaps → 8 bars.
Bar sizes: #3, #4 or #5?
- #3 (3/8 in) — light slabs, thin pads, patios that see only foot traffic. Cheaper and easier to bend, fine when the load is modest.
- #4 (1/2 in) — the workhorse. Driveways, house slabs, footings. This is what most residential suppliers quote when you say "rebar for a slab."
- #5 (5/8 in) — heavier footings, retaining walls, structural beams. Only reach for it when the engineer or the code calls for it.
Bar diameter drives your splice overlap: overlap at least 30 bar diameters — about 12 in for #3, 15 in for #4, and 19 in for #5. That's the extra length you must add when a bar doesn't reach the full span.
The count math, step by step
- Measure the slab in feet (length and width) and pick your edge clearance — 3 in is the standard.
- Bars along the length = floor((width − 2 × edge) ÷ spacing) + 1. Each of these bars runs the full length.
- Bars across the width = floor((length − 2 × edge) ÷ spacing) + 1. Each runs the full width.
- Total rebar length = (bars along × length) + (bars across × width).
- 20-ft sticks = ceil(total ÷ 20), then add 10% for splices.
Worked example — a 10×20 driveway at 16-in spacing with 3-in edges: along the length, (10 − 0.5) ÷ 1.33 = 7.1 → 7 + 1 = 8 bars; across, (20 − 0.5) ÷ 1.33 = 14.6 → 14 + 1 = 15 bars. Total = 8 × 20 + 15 × 10 = 310 ft → 16 sticks of 20 ft with the splice allowance. That's the number to call your supplier with.
Common mistakes that weaken the slab
- Rebar on the bottom with no chairs. Bricks and rocks work in a pinch but shift during the pour. Use plastic chairs or wire supports to hold the bar 2-3 in up from the ground — a bar lying on the gravel does nothing but rust.
- Splices all in one line. If every overlap lands on the same row, you've created a continuous weak seam. Stagger the laps so no cross-section is all splice.
- Forgetting the edge bar. The +1 bar at each edge is what controls the corner cracks that appear in the first winter.
- Cutting bars to save money. A continuous 20-ft bar is stronger than two 10-ft bars spliced in the middle of a driveway. Position splices away from the heaviest traffic path when you can.
- Using the wrong size "because it was on sale." #3 in a driveway carrying a truck is a structural risk, not a bargain.
FAQ
Structural slabs and anything load-bearing (footings, driveways, patios with vehicles) should be reinforced. Small pads like a 4x4 post base can skip it. Wire mesh is weaker than rebar but better than nothing.
12-in spacing on center is typical for driveways and heavy loads; 16-18 in works for most patios and house slabs; 24 in suits light-duty paths. The closer the spacing, the stronger the mat - and the more bars you buy.
Keep rebar 3 in from the edges and 2-3 in from the top surface (concrete cover). The cover protects the steel from moisture and keeps it from rusting and spalling the concrete.
Overlap bars by at least 30 bar diameters (about 15 in for #4 rebar) at splices, and stagger the splices so they do not all line up in one row. Tie the overlap with tie wire where bars cross.
#3 (3/8 in) for light slabs and patios, #4 (1/2 in) for driveways and footings, #5 (5/8 in) for heavy structural work. Your engineer or local building code should dictate size - spacing alone is not enough.