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Construction & Home
Rebar Calculator — Linear Feet, Spacing & Number of Bars for a Slab
How much rebar for a slab, footing or driveway grid at any spacing
A rebar calculator turns a slab's footprint and your chosen spacing into the three numbers you actually need at the yard: how many bars run each direction, the total linear feet of steel, and how many 20-foot sticks to buy. Order short and the grid has gaps where the concrete will crack; order long and you've paid for bars that go back in the truck. This tool builds the grid, adds a lap-splice and waste allowance, and always rounds up to whole bars — because rebar is sold in fixed lengths, not by the inch.
The layout is a simple grid. Bars running the length of the slab are spaced across its width, and bars running the width are spaced along its length. The count each way is the same formula:
• Rows (length-wise bars) = floor(width ÷ spacing) + 1 • Columns (width-wise bars) = floor(length ÷ spacing) + 1
The + 1 matters — a fence with 4 sections has 5 posts, and a grid with 8 gaps has 9 bars. Skip it and every estimate comes up one bar short in each direction.
Spacing is given in inches but the slab is in feet, so convert first: 16 in = 16 ÷ 12 = 1.333 ft. Sixteen inches on center is the most common spacing for residential slabs and driveways; 12 in is used for heavier loads, 18–24 in for light-duty pads.
Once you have the counts, the linear feet of steel is just the bars times the run they cover:
Linear feet = (rows × length) + (columns × width)
Work a 10 ft × 10 ft slab at 16 in on center: floor(10 ÷ 1.333) + 1 = 8 rows and 8 columns, so 8 × 10 + 8 × 10 = 160 linear feet before overlap. Real bars have to be tied together where they meet, and the tie-off (a lap splice, typically 40 bar diameters) eats length — so add about 10% for laps and waste. That 160 ft becomes roughly 176 ft, then divide by the stick length you buy (20 ft is standard) and round up: ceil(176 ÷ 20) = 9 sticks.
This calculator also reports the grid spacing in feet so you can chalk lines on the subgrade, and an optional price per foot or per stick for a quick material cost. A waste factor (default 10%) covers laps, cut-offs and the odd miscut. Combined with a concrete pour, it tells you both the steel and the mud in one sitting.
📖 See also: How to Calculate a Tip (and Split the Bill)
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Rows (bars running length-wise) = floor(width ÷ spacing) + 1 • Columns (bars running width-wise) = floor(length ÷ spacing) + 1 • Inches to feet: feet = inches ÷ 12 • Linear feet of rebar = (rows × length) + (columns × width) • With waste/laps: total = linear feet × (1 + waste% ÷ 100) • Sticks to buy = ceil(total linear feet ÷ stick length) • Default spacing 16 in = 1.333 ft · standard stick length 20 ft · default lap/waste 10%
📰 Formula
• Rows (bars running length-wise) = floor(width ÷ spacing) + 1 • Columns (bars running width-wise) = floor(length ÷ spacing) + 1 • Inches to feet: feet = inches ÷ 12 • Linear feet of rebar = (rows × length) + (columns × width) • With waste/laps: total = linear feet × (1 + waste% ÷ 100) • Sticks to buy = ceil(total linear feet ÷ stick length) • Default spacing 16 in = 1.333 ft · standard stick length 20 ft · default lap/waste 10%
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Forgetting the + 1 bar — a grid with 8 gaps has 9 bars, just like a fence with 4 sections has 5 posts.
- Leaving spacing in inches instead of dividing by 12 (16 in is 1.333 ft, not 16 ft).
- Swapping which dimension the bars run along — length-wise bars are spaced across the width, not the length.
- Skipping the lap-splice/waste allowance, so tied overlaps leave you short of steel.
- Rounding sticks down — rebar comes in fixed lengths, so always round up to whole sticks.
💡 Tips
- 16 inches on center is the residential default; drop to 12 in for driveways and loads, open to 18–24 in for light pads.
- Buy 20 ft sticks when you can — fewer lap splices than 10 ft bars means less tying and less wasted overlap.
- Bump the waste factor to 12–15% on big slabs where many bars must be lapped end to end.
- Keep rebar about 3 inches in from each slab edge and roughly mid-depth, supported on chairs, for it to do its job.
- Add the waste before rounding to whole sticks, not after, so the lap allowance isn't lost in the rounding.
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<iframe src="https://www.calcnimbus.com/embed/rebar-calculator" width="100%" height="500" frameborder="0" style="border:1px solid #eee;border-radius:12px"></iframe>
❓ Frequently asked questions
How do I calculate how much rebar I need for a slab?
Build a grid: rows = floor(width ÷ spacing) + 1 and columns = floor(length ÷ spacing) + 1, with spacing in feet. Then linear feet = rows × length + columns × width. Add about 10% for laps and divide by your stick length, rounding up.
What is the standard rebar spacing for a concrete slab?
16 inches on center is the most common spacing for residential slabs, patios and driveways. Heavier loads use 12 inches; light-duty pads can go to 18 or 24 inches. Tighter spacing means more bars and more linear feet.
Why do I add one bar to the rebar count?
The grid has a bar on each edge plus one at every spacing line in between, so the count is the number of gaps plus one. A run with 8 gaps has 9 bars — the same reason a fence with 4 sections needs 5 posts.
How many linear feet of rebar are in a 10x10 slab?
At 16-inch spacing a 10 ft by 10 ft slab needs 8 bars each way, so 8 × 10 + 8 × 10 = 160 linear feet before overlap. With a 10% lap allowance that is about 176 feet, or 9 sticks of 20 ft rebar.
How long is a stick of rebar?
Rebar is most commonly sold in 20-foot sticks, with 10-foot lengths also widely stocked for easier transport. This calculator divides your total linear feet by the stick length you choose and rounds up to whole sticks.
What is a lap splice and why does it use extra rebar?
Where two bars meet end to end they must overlap and be tied so the load transfers — that overlap is a lap splice, usually about 40 bar diameters long. The lapped length is extra steel, which is why the calculator adds a waste factor.
What size rebar should I use for a slab?
#3 (3/8 in) and #4 (1/2 in) bar cover most residential slabs and driveways, with #4 the common driveway choice. Bar size changes strength and lap length but not the grid layout, so the linear-feet math here works for any size.
Can I use this rebar calculator for a footing?
Yes. Treat the footing as a long, narrow slab: set the length and width, and a narrow footing usually runs just 2 to 4 continuous bars its full length. The grid math then gives the bar count and linear feet the same way.
Should I use rebar or wire mesh for my slab?
Welded wire mesh is fine for thin, lightly loaded slabs like sidewalks; rebar on a grid is stronger and standard for driveways, structural slabs and footings. This calculator sizes the rebar grid once you have settled on bar reinforcement.