A Free Tool · Slab Rebar Grids · Linear Feet & Stock Bars
How much rebar do you actually need?
Enter the length and width of a slab, pick your on-center spacing, and get how many
bars run in each direction, the total linear feet of rebar, and how many 20-foot stock
bars to put on your shopping list. The numbers count a grid of bars running both ways,
hold the end bars in from the edges, and add a lap-and-waste factor so you don't come up
short at a splice.
Bars each direction, linear feet & stock bars·12″ / 16″ / 18″ / 24″ spacing·20-foot stock, lap/waste built in
Read this first
This is a layout and quantity estimate, not engineering. The rebar size, the grid
spacing, the concrete cover, and the lap-splice length for a structural slab are all
governed by the engineered design and your local building code — not by a default in
a calculator. Use this to estimate how many 20-foot bars to buy once you already know the
spacing your project calls for, and confirm that spacing against your plan or your
inspector. Always buy a little extra; running short at a splice is worse than a leftover
offcut.
The calculator
Estimate rebar for a slab grid
Enter the slab length and width in feet, choose your on-center spacing, and you'll get the number of bars running each direction, the total linear feet, and the number of 20-foot stock bars to buy — with edge clearance and a lap/waste factor applied.
12–18″ is typical for residential slabs; set by the structural design and code.
How far the first and last bar sit in from each edge. About 3″ is common.
Covers lap splices and offcuts. 10% is a reasonable default.
Rebar is commonly sold in 20-foot lengths; 10 and 40-foot also exist.
Bars running the length
Bars running the width
Total linear feet (raw)
Linear feet + lap/waste
20-foot stock bars to buy
The math, honestly
How the bar count is figured
A slab is reinforced with a grid of bars running in both directions,
so the count is done twice. For each direction, take the slab dimension in inches,
subtract the edge clearance from both ends, divide by the spacing, round down,
and add one: floor((dimension×12 − 2×clearance) / spacing) + 1.
The +1 counts the very first bar sitting at the starting edge.
Then it's just length. The bars that run the length are each as long
as the slab is long, and the bars that run the width are each as long
as the slab is wide. Total linear feet is
(bars across width × length) + (bars across length × width).
Add your lap-and-waste factor, then divide by the stock bar length and round up:
ceil(linear feet × (1+waste) / stock length).
Why the waste factor matters: a slab is almost always wider than a
single 20-foot bar, so bars get spliced end to end and every lap
consumes extra steel — a common rule of thumb is roughly 40 bar diameters of
overlap. Add normal offcuts and the 10% default is reasonable for a simple grid;
complex layouts may need more.
Bars per direction by spacing
How many bars run in one direction across a slab edge of a given length, at each common
on-center spacing, using a 3-inch edge clearance — the same formula the calculator
uses. Tighter spacing means more bars and more steel.
Slab edgeone dimension, ft
12″ o.c.bars
16″ o.c.bars
18″ o.c.bars
24″ o.c.bars
8 ft
8
6
6
4
10 ft
10
8
7
5
12 ft
12
9
8
6
16 ft
16
12
11
8
20 ft
20
15
14
10
Counts use floor((feet×12 − 6) / spacing) + 1 with a 3-inch
clearance at each edge. These are the bars in a single direction; a full grid runs bars
both ways, so the slab needs a count for its length and a count for its width.
Common slab sizes
Worked examples for a few common slabs on a 12-inch grid with a 3-inch clearance, a 10%
lap/waste factor, and 20-foot stock bars — so these match what the calculator gives
you. Square slabs use the same bar count in each direction.
Slab12″ grid
Bars each way
Linear feetraw
20-ft bars+10% waste
10 × 10 ft
10 / 10
200
11
12 × 12 ft
12 / 12
288
16
12 × 20 ft
20 / 12
480
27
20 × 20 ft
20 / 20
800
44
"Bars each way" is bars running the length / bars running the width. Linear feet is
(bars×length) + (bars×width); the bar count is that figure times
1.10, divided by 20, rounded up. The 10 × 10 row is the worked example used to
verify this tool.
Reading the result well
A bar count is only useful if you act on it sensibly. Four things worth knowing before
you buy.
Spacing comes from the design, not the calculator
This tool lets you compare 12, 16, 18, and 24-inch grids, but it does not tell you which one your slab needs. The correct spacing — along with the bar size and concrete cover — is set by the structural design and your local building code. Pick the spacing first from your plan, then use the calculator to count bars.
Account for laps on wide slabs
A 20-foot bar only spans 20 feet. Any slab wider than that needs bars spliced end to end, and each lap splice — commonly around 40 bar diameters of overlap — eats extra steel. The default 10% waste factor covers laps plus normal offcuts for a simple grid; a slab with many splices, curves, or odd dimensions may justify bumping it to 15%.
Edge clearance and concrete cover are different
Edge clearance is how far the end bars sit in from the slab edge, which is what changes the bar count here. Concrete cover is the depth of concrete around the steel that protects it from rust — commonly about 3 inches against the ground. Both are governed by code; the 3-inch clearance default is a layout convenience, not a spec for either.
Buy whole bars — and don't under-buy
You can't buy a fraction of a bar, so the calculator rounds the stock count up. Keep the extra on hand until the mat is tied; a leftover bar is cheap insurance and offcuts rarely splice cleanly into the next run. If a slab dimension is right at a spacing boundary, round up rather than risk laying one bar short.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These are background definitions, not
engineering specifications — follow your local building code and any engineered
plan for the real thing.
Rebar
Short for "reinforcing bar" — the ridged steel bar embedded in concrete to carry tension, which concrete on its own handles poorly. In a slab it's usually laid as a crossing grid and tied at the intersections.
Rebar grid (mat)
The crossing layout of bars running in both directions across a slab, tied where they cross. Because it runs both ways, the steel count is figured twice — once for the bars running the length and once for the bars running the width.
On center (o.c.)
The spacing between bars measured center-to-center, given in inches. A "16-inch o.c." grid puts a bar every 16 inches. Smaller numbers mean tighter spacing, more bars, and more steel. The right spacing comes from the structural design and code.
Edge clearance
How far the first and last bar in a run sit in from the slab edge. This calculator uses it to find the usable width before counting bars: spacing divides into the dimension minus twice the clearance, rounded down, plus one. Roughly 3 inches is a common default.
Concrete cover
The thickness of concrete surrounding the steel on all sides, protecting it from corrosion and fire. It is not the same as edge clearance; for slabs cast against soil it is commonly around 3 inches on the ground side, set by code.
Lap splice
The overlap where one bar continues another end to end, so load transfers across the joint. A common rule of thumb is about 40 times the bar diameter of overlap. Laps consume extra steel, which is part of why a waste factor is added.
Lap / waste factor
The extra rebar bought above the raw linear-feet figure — typically around 10% — to cover lap splices, offcuts, and measurement slack. Like an overage, it's cheap insurance against the costlier problem of coming up short.
Bar size (#3, #4)
Rebar is sized in eighths of an inch of diameter: a #3 bar is 3/8 inch (0.375 in), a #4 bar is 1/2 inch (0.50 in). Size changes the weight and the lap length but not the linear-feet count. The correct size is set by the structural design and local code.
Frequently asked
For a 10 ft × 10 ft slab on a 12-inch grid with a 3-inch edge clearance, you get 10 bars running each direction: (120 − 6) ÷ 12 = 9.5, rounded down to 9, plus 1 = 10. That's 10 bars × 10 ft plus 10 bars × 10 ft = 200 linear feet. Adding a 10% lap-and-waste factor brings it to 220 linear feet, which is 11 bars of 20-foot rebar (220 ÷ 20 = 11). Wider spacing, like 16 or 18 inches, cuts the count. Try it in the calculator.
Twelve to eighteen inches on center is typical for residential slabs, with 16 inches a very common default. Lighter-duty pads sometimes go to 18 or 24 inches; slabs carrying heavier loads tighten toward 12 inches. The exact spacing — along with bar size and concrete cover — is set by the structural design and your local building code, not by a rule of thumb. This tool lets you compare 12, 16, 18, and 24-inch grids so you can see how the bar count changes.
Set the first bar one edge-clearance distance in from the slab edge, then place each following bar at the chosen on-center spacing until you reach the far edge, again leaving a clearance. Do this in both directions so the bars cross to form a grid, then tie the intersections with wire and set the mat on chairs so the steel sits at the right height in the slab. The calculator counts bars the same way: usable width divided by spacing, rounded down, plus one.
Two different things share the word "clearance." Edge clearance is how far the end bars sit in from the slab edge — a few inches, often around 3, which this calculator uses to count bars. Concrete cover is how much concrete surrounds the steel on all sides to protect it from rust; for slabs cast against the ground it's commonly about 3 inches on the soil side. Both are governed by code and the structural design, so treat the defaults here as a layout estimate, not a spec.
A common rule of thumb for lap splices is about 40 times the bar diameter — roughly 20 inches for a #4 bar — but the real number depends on bar size, concrete strength, and code. Because a slab is usually wider than a single 20-foot bar, you'll splice bars end to end, and every lap consumes extra steel. That's what the waste factor accounts for: this calculator defaults to 10% to cover laps plus normal offcuts.
Rebar size is given in eighths of an inch of diameter: a #3 bar is 3/8 inch (0.375 in) and a #4 bar is 1/2 inch (0.50 in). #3 is lighter and easier to bend and is often used in light residential slabs and walkways; #4 is the common choice for driveways, footings, and slabs carrying more load. Bar size does not change the linear-feet math — the number of bars and their length is the same — but it changes the weight and the lap length, and the correct size is set by the structural design and local code.
Divide your total linear feet (with the waste factor already added) by the length of the stock bar you can buy, then round up to a whole bar. Rebar is commonly sold in 20-foot lengths, though 10-foot and 40-foot bars exist too. For 220 linear feet at 20-foot stock, that's 220 ÷ 20 = 11 bars. Rounding up matters because you can't buy a fraction of a bar, and offcuts from one bar rarely splice cleanly into the next.
No. This is a layout and quantity estimate, not engineering. Rebar size, grid spacing, concrete cover, and lap-splice length for a structural slab are governed by the engineered design and your local building code. Use this tool to estimate how many 20-foot bars to put on your shopping list once you already know the spacing your project requires — and confirm that spacing against your plan or your inspector.