Rebar Calculator
How much rebar do I need for a 20x20 slab?
With #4 rebar at 18 in on center and 3 in edge clearance, a 20 × 20 ft slab takes 14 bars each way (28 bars, each 19.5 ft): 546 linear feet, 28 sticks of 20 ft, about 365 lb of steel and roughly 64 chairs. At 12 in spacing it takes 40 bars, 780 linear feet, 40 sticks and about 521 lb.
This rebar calculator estimates the steel for a single grid of reinforcing bar in a concrete slab. Enter the slab length and width in feet, the bar spacing, the edge clearance, the bar size (#3, #4 or #5) and whether you buy 20 ft or 10 ft sticks.
It returns bars in each direction, total linear feet with lap splices, sticks to buy, steel weight and an approximate chair count. Use it alongside the [concrete calculator](/tools/concrete-calculator) to plan the whole pour.
Enter your details
How to Use the Rebar Calculator
- 1
Enter the slab size
Type the slab length and width in feet, then the edge clearance in inches (the gap between the bar ends and the forms, 3 in by default).
- 2
Choose the grid
Pick the bar spacing (12, 16, 18 or 24 in on center), the bar size (#3, #4 or #5) and the stick length you can buy and haul (20 ft or 10 ft).
- 3
Read the results
The calculator shows the sticks to buy, total linear feet with laps, bars running each way and their length, steel weight and an approximate chair count. Results update as you type.
How the rebar calculator works
Each bar stops short of the slab edge by the clearance at both ends, so a bar running the length of the slab is the length in inches minus twice the clearance. Bars running the length are spread across the width at the chosen spacing, starting at one edge: the count is the clear width divided by the spacing, rounded down, plus one. The same is done the other way.
When a bar run is longer than one stick, pieces are joined with a lap splice of 40 bar diameters: 15 in for #3, 20 in for #4 and 25 in for #5. Each lap adds that length to the run. Weight is the total linear feet times the standard weight per foot: 0.376 lb for #3, 0.668 lb for #4 and 1.043 lb for #5.
Sticks are counted the way you would cut them. Each run uses whole sticks plus one end piece, and as many end pieces as fit are cut from each stick. Short runs are cut several to a stick. Chairs are estimated at about one every 3 ft each way across the grid, never more than the number of bar crossings.
Bars one way = ⌊(other side ft × 12 − 2 × clearance in) ÷ spacing in⌋ + 1
Bar length (in) = side ft × 12 − 2 × clearance + laps × 40 × bar diameter
Weight (lb) = total linear ft × lb per ft (0.376, 0.668 or 1.043)Worked example: a 20 x 20 ft slab with #4 at 18 inches
Switch to 10 ft sticks and each 19.5 ft run needs three pieces joined by two 20 in laps, for a 22.83 ft run. That raises the total to 639.3 linear feet, 66 sticks and about 427 lb, which is why 20 ft sticks are usually worth hauling if you can.
- Clear run each way: 20 × 12 − 2 × 3 = 234 in (19.5 ft).
- Bars each way: 234 ÷ 18 = 13, rounded down, plus 1 = 14 bars.
- Each 19.5 ft bar fits inside a 20 ft stick, so there are no laps and each bar uses one stick: 14 + 14 = 28 sticks.
- Linear feet: 28 bars × 19.5 ft = 546 ft (166.4 m).
- Weight: 546 × 0.668 = 364.7 lb, about 365 lb (165 kg).
- Chairs: 19.5 ÷ 3 = 6.5, rounded up to 7, plus 1 = 8 each way, so 8 × 8 = 64 chairs.
Rebar for common slab sizes
All figures use #4 bar, 3 in edge clearance and 20 ft sticks. Bars are listed as "along the length + along the width". Weight is the steel placed, not offcuts.
| Slab | Spacing | Bars | Linear ft | 20 ft sticks | Weight (lb) | Chairs |
|---|---|---|---|---|---|---|
| 10 × 10 ft | 12 in | 10 + 10 | 190 | 10 | 127 | 25 |
| 10 × 10 ft | 18 in | 7 + 7 | 133 | 8 | 89 | 25 |
| 12 × 12 ft | 12 in | 12 + 12 | 276 | 24 | 184 | 25 |
| 12 × 12 ft | 18 in | 8 + 8 | 184 | 16 | 123 | 25 |
| 10 × 20 ft | 18 in | 14 + 7 | 270 | 14 | 180 | 40 |
| 20 × 20 ft | 12 in | 20 + 20 | 780 | 40 | 521 | 64 |
| 20 × 20 ft | 18 in | 14 + 14 | 546 | 28 | 365 | 64 |
| 20 × 24 ft | 18 in | 16 + 14 | 664 | 35 | 444 | 72 |
| 24 × 24 ft | 18 in | 16 + 16 | 805 | 44 | 538 | 81 |
| 30 × 30 ft | 18 in | 20 + 20 | 1,247 | 80 | 833 | 121 |
| 30 × 40 ft | 18 in | 27 + 20 | 1,698 | 97 | 1,134 | 165 |
Rebar sizes, weights and lap lengths
The bar number is its diameter in eighths of an inch. The same 20 × 20 ft grid at 18 in uses 546 linear feet in any size, but weighs about 205 lb in #3, 365 lb in #4 and 570 lb in #5.
| Bar size | Diameter | Weight per ft | 20 ft stick | 10 ft stick | Lap (40 × dia) |
|---|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.52 lb | 3.76 lb | 15 in |
| #4 | 1/2 in | 0.668 lb | 13.36 lb | 6.68 lb | 20 in |
| #5 | 5/8 in | 1.043 lb | 20.86 lb | 10.43 lb | 25 in |
Edge cases the simple grid misses
- Thickened edges and footings: perimeter grade beams and footings usually get their own continuous bars, often two or more along the edge. Add those separately; the calculator covers the field grid only.
- Bends and dowels: L-shaped corner bars, dowels into an existing slab and bars tied into a retaining wall are not included.
- Lap length: 40 bar diameters is a common rule of thumb for slab work, not a universal code value. Plans, local code or an engineer may require longer laps, especially for higher-strength steel or structural members.
Placing rebar: tips
- Set the grid on chairs or concrete blocks (dobies) near mid-depth rather than pulling it up during the pour, and tie bars at crossings with wire ties.
- Many patios, walks and shed pads use welded wire mesh or fiber-reinforced concrete instead of rebar. Mesh is quicker to lay; rebar is stiffer and easier to keep at the right height.
- Compact the gravel base well before setting chairs. Rebar controls crack width but does not make up for a soft or uneven subgrade.
When to call a professional
A rebar grid in a patio or shed slab is common DIY work. Foundations, footings, garage and driveway slabs, suspended slabs, retaining walls and anything carrying a building or vehicle load are structural: bar size, spacing, cover and laps should come from stamped plans, your local building code or a structural engineer, and the steel is often inspected before the pour. Use this calculator to check quantities and budget for steel, not to design reinforcement.
Features
- Bars in each direction at 12, 16, 18 or 24 in on center
- Lap splices of 40 bar diameters on runs longer than a stick
- Sticks to buy for 20 ft or 10 ft rebar, rounded up
- Steel weight for #3, #4 and #5 bar in lb and kg
- Approximate chair count at about one per 3 ft each way
Frequently Asked Questions
How far apart should rebar be in a concrete slab?
Residential slab grids are commonly spaced 12 to 24 inches on center, with 18 inches a frequent choice for patios and driveways using #3 or #4 bar. Closer spacing gives more crack control and uses more steel. Structural slabs and footings should follow your plans or local code rather than a rule of thumb.
How long should a rebar lap splice be?
This calculator uses 40 bar diameters, a common rule of thumb for slab work: 15 inches for #3, 20 inches for #4 and 25 inches for #5. Codes and engineers may require longer laps depending on the steel grade, concrete strength and where the splice falls, so use the lap length on your plans if you have one.
How much does a 20 ft stick of rebar weigh?
A 20 ft stick of #3 rebar weighs about 7.5 lb, #4 about 13.4 lb and #5 about 20.9 lb, based on standard weights of 0.376, 0.668 and 1.043 lb per foot. A 10 ft stick weighs half as much.
Where should rebar sit in a slab?
For a typical 4 inch slab on grade, rebar is usually placed near the middle of the slab depth, supported on chairs or concrete blocks so it stays put during the pour. Rebar lying on the ground under the concrete does very little. Keep the bar ends a few inches back from the edges so they stay covered.
Do I need rebar or wire mesh for a patio?
Many patios and walkways use welded wire mesh, fiber-reinforced concrete or no reinforcement at all, with control joints to manage cracking. Rebar is stiffer and easier to hold at the right height, so it is often chosen for driveways, thicker slabs and poor soils. Check local practice and code for your project.
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