How Much Rebar Do I Need for a Slab?

A 20 ft × 20 ft driveway with #4 bar at 12 inches on center and 3 inches of edge cover needs about 40 bars — roughly 780 linear feet before waste. With the usual 10% cut-and-lap allowance you are closer to 43 sticks of 20 ft.

That is the number that empties a trailer short. You divide 20 by 1 and load thirty-something bars. The far edge still needs the last stick. The pour waits while someone runs back to the yard for more steel.

The bar-count formula is:

bars = floor((dim in − 2 × cover) ÷ spacing) + 1

Run it once for each direction, then add. Never skip the +1. Add 5–10% for cuts, offcuts, and lap splices. Estimate only — not engineering or building-code advice.

*Sticks include ~10% ordering allowance, rounded up.

How many bars for a 20×20 slab?

Convert feet to inches. A 20 ft side is 240 in. Subtract cover on both edges: 240 − 2 × 3 = 234 in of clear span. Divide by spacing: 234 ÷ 12 = 19.5. Floor that to 19, then add one for the far-edge bar → 20 bars each way.

Twenty plus twenty is 40 bars. Each bar is about 19.5 ft long (the other direction minus cover), so 20 × 19.5 × 2 = 780 linear feet for a single mat. #4 weighs about 0.668 lb per foot, so call it ~520 lb of steel before waste.

Stock is usually sold in 20 ft sticks. 780 ÷ 20 = 39 sticks of perfect net length — but you still need the waste factor. At 10% you order about 43 sticks. That is what goes on the pickup list, not the neat geometry number.

The formula, on a tape

Measure length and width in feet. Pick spacing on center (12 in is the common residential driveway default; light patios sometimes run 16–18 in). Pick edge cover — often 2–3 in for a slab on grade over a base; cast-against-earth cover is a different case and comes from the code or the stamped drawing.

For each direction:

• Clear span (in) = dimension × 12 − 2 × cover
• Bars = floor(clear ÷ spacing) + 1
• Bar length (ft) ≈ the other dimension − 2 × cover ÷ 12

Total linear feet = (bars lengthwise × their length) + (bars widthwise × their length). Divide by stock length and round up. Then add 5–10%.

When a run is longer than one stick, you lap. A common rule of thumb is about 40 bar diameters (#4 ≈ 20 in of lap), but the schedule on the plan wins. Laps eat steel. That is why the order quantity is never the bare grid total.

In Rebar Takeoff Master you enter the slab, spacing, and cover once. It returns bar count, linear feet, and stick count on your phone. Free grid math; Pro unlocks deeper takeoff at $9.99.

Three mistakes that short the mat

Forgetting the +1. Dividing clear span by spacing gives the gaps, not the bars. Twenty feet at 12 in spacing is not “20 bars” by accident of the numbers — it is floor + 1. Drop the +1 and the last edge has no steel. The mat looks fine until you count chairs against the form.

Ignoring cover. Counting bars across the full outside dimension puts steel into the cover zone or shortens the real clear span. Either you order the wrong length or you set bars too close to the edge. Cover is not decoration; it is the number that keeps the grid inside the concrete.

Ordering net linear feet with no waste. Offcuts, wrong cuts, and lap splices on anything longer than stock eat 5–15%. A 780 ft mat ordered as exactly 39 sticks of 20 ft leaves you one bad cut from a hole in the grid while the truck is already on site. Buy the allowance. Leftover stick is cheaper than a second run.

What size rebar for a driveway slab?

#4 (½ in) at 12 to 16 inches on center both ways is the durable residential default for driveways and garage floors that see cars. Light patios and walks sometimes use #3 at 16–18 in — or welded wire mesh — when the base is good and control joints are cut. Anything carrying a wall, column, or heavy truck comes off an engineered drawing, not a blog chart.

Mesh is a different takeoff: sheet size (often 8×20), overlap, and coverage. If your job is mesh instead of bar, run the mesh path in Rebar Takeoff Master rather than forcing a bar grid.

Bags of concrete still matter

Steel and concrete land on the same morning. Once the mat is right, you still need the yardage. A 20×20 at 4 in is about 4.94 yd³ — see how many bags of concrete do I need if you are bagging a small pad, or call the plant for a driveway. Sibling tools on the EBN Build Calculators hub cover fence posts, mix, and the rest of the jobsite math.

These counts are estimates only — not engineering or building-code advice. Spacing, cover, bar size, and lap length change with load, frost, and local rules. Measure twice. Order the waste. A spare stick beats a cold joint waiting on steel.

Estimate only — not engineering or building-code advice.