Work out the steel for a slab grid before you order. Enter the slab size, bar spacing and bar size and get bar counts each way, total linear feet, 20 ft sticks and total rebar weight.
Rebar Calculator
Two-way grid for a slab or flatwork pour. Bars run in both directions at the spacing you set.
You need
Allow extra length for lap splices, bars lap 40–60 bar diameters where they join (24–30 in for #4). Order sticks, not pieces, and cut on site.
A rebar calculator counts the bars in a two-way grid from the slab size and the spacing: bars each way equals the width in inches divided by the spacing, plus one. It then totals linear feet, converts that into 20 ft sticks, and multiplies by the bar’s weight per foot. Enter your slab above for the full steel list.
How to Calculate Rebar for a Slab
Rebar turns a plain slab into a reinforced one: the grid holds cracks tight, spreads loads and keeps sections level when the ground moves. The count is pure geometry, a grid at 18 inches on-center across a 20 ft slab needs a bar every 18 inches in both directions, but ordering mistakes are common because suppliers sell 20 ft sticks while drawings show a grid. This rebar calculator bridges the two: grid in, sticks and pounds out.
The Formula
Total linear feet = (bars × length) + (bars × width)
Sticks = total linear feet ÷ 20 ft, rounded up · Weight = linear feet × lb per ft for the bar size
The “plus one” bar is the one most estimates miss: a grid needs a bar at both edges, so the count is always spaces plus one. Spacing is measured on-center, from the middle of one bar to the middle of the next, not clear distance between bars.
Worked Example: 20 × 20 ft Slab, #4 Bars at 18 in On-Center
Width in inches is 240. 240 ÷ 18 = 13.3 spaces, round up to 14, plus 1 = 15 bars each way. Total steel = (15 × 20) + (15 × 20) = 600 linear ft. At 0.668 lb/ft for #4, that is 401 lb of steel, or 30 sticks exact, 33 sticks with the 10% lap-and-cut allowance. The calculator above reproduces these numbers.
Standard Rebar Sizes and Weights
| Bar size | Diameter | Weight | Typical use |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft | Light slabs, stirrups, dowels |
| #4 | 1/2 in | 0.668 lb/ft | Slabs, driveways, footings, the default |
| #5 | 5/8 in | 1.043 lb/ft | Walls, heavier slabs, beams |
| #6 | 3/4 in | 1.502 lb/ft | Structural slabs and columns |
| #8 | 1 in | 2.670 lb/ft | Heavy structural work |
Weights are the standard published weights per ASTM A615 bar sizes. If your engineer specified a size, use it, do not trade down a size to save money, because strength drops faster than weight.
Why the Lap and Cut Allowance Matters
Bars join end to end with lap splices, and every lap consumes length without adding coverage: a #4 lap is roughly 24–30 inches of doubled bar. Cuts at edges and around openings waste more. The default 10% extra covers typical laps on a rectangular grid; raise it to 15% on pours with many openings, steps or curved edges, and drop it to 5% only on a simple rectangle cut from full sticks with a planned cut list.
How to Measure and Calculate: Step by Step
- Measure the slab length and width in feet, inside the forms.
- Take the bar size and spacing from your drawing or local code, 18 in on-center is common for residential slabs.
- Enter size, spacing and bar size above; note bars each way and total linear feet.
- Check the sticks figure includes the lap allowance before you order.
- Plan bar supports (chairs or dobies) so the grid sits at the specified height, usually mid-slab or the lower third.
Worked Example 2: 12 × 24 ft Garage Slab, #4 at 16 in On-Center
Across the 12 ft width: 144 ÷ 16 = 9 spaces + 1 = 10 bars of 24 ft. Along the 24 ft length: 288 ÷ 16 = 18 spaces + 1 = 19 bars of 12 ft. Total = (10 × 24) + (19 × 12) = 468 linear ft = 313 lb of #4, or 24 sticks exact and 26 with 10% extra. Note the long direction needs more bars, grids are rarely square even when slabs are.
Common Quantities at a Glance: Cheat Sheet
| Job / quantity | Result |
|---|---|
| 10 × 10 ft slab, 18 in o.c., #4 | 15 bars · 150 LF · 100 lb |
| 20 × 20 ft slab, 18 in o.c., #4 | 30 bars · 600 LF · 401 lb |
| 20 × 20 ft slab, 12 in o.c., #4 | 42 bars · 840 LF · 561 lb |
| 24 × 24 ft slab, 18 in o.c., #5 | 34 bars · 816 LF · 851 lb |
| One 20 ft #4 stick | 13.4 lb |
Practical Tips From the Job
Tie the grid at every second or third intersection, not every one, the ties hold position, not strength. Keep bars at least 3 inches clear of the ground on chairs; steel lying in the dirt does no work and rusts. Cut and bend the day before the pour, and stack sticks by length so the crew is not hunting sizes while the truck waits.
Common Mistakes to Avoid
- Forgetting the plus-one bar at the far edge of the grid.
- Ordering linear feet as if suppliers sell cut pieces, they sell 20 ft sticks.
- Ignoring lap length, then running short on the last row.
- Letting the grid sink to the bottom of the slab during the pour.
- Spacing bars by eye instead of marking the form edges.
What It Costs and How to Order Smart
Steel cost is weight-driven: total pounds multiplied by your local per-pound or per-stick price. For the 20 × 20 example, 33 sticks of #4 is the order line, get the per-stick price from two suppliers, because rebar pricing varies more by region than concrete does. Add chairs, tie wire and a bolt cutter or bender rental to the real budget; they are small lines that always get forgotten.
Who Uses This Calculator?
Builders use this calculator at take-off to convert a drawing grid into a stick order. Contractors check a supplier quote against it before accepting delivery. Engineers sketching a slab design use the weight figure for early cost models. Architects confirming a detail can test how a spacing change moves the steel tonnage, and homeowners adding a garage slab get the same count the yard will quote from.
Rebar Calculator FAQs
How much rebar do I need for a 20×20 slab?
At 18 inches on-center with #4 bars, a 20 × 20 ft slab needs 15 bars each way, 30 bars total. That is 600 linear feet of steel, about 401 lb, which you order as 30 × 20 ft sticks, or 33 sticks once a 10% lap and cutting allowance is added.
What rebar spacing is best for a concrete slab?
Eighteen inches on-center is the common residential grid for 4–6 inch slabs; 12 inches is used for heavier loads, poor soil or engineered designs, and 24 inches appears only on light-duty work. Your drawing or local code sets the spacing, this calculator counts bars for any spacing you enter.
Do I need rebar in a 4 inch concrete slab?
Not always. A 4 inch patio on good compacted ground often uses welded wire mesh or fiber instead. Rebar earns its cost where the slab carries vehicles, sits on fill or expansive soil, or must limit crack width. When in doubt, the grid is cheap insurance compared with replacing a cracked slab.
How much does rebar weigh?
Weight depends on bar size: #3 is 0.376 lb per foot, #4 is 0.668, #5 is 1.043, #6 is 1.502 and #8 is 2.670 lb per foot. One 20 ft stick of #4 therefore weighs about 13.4 lb, and the 600 ft grid in the worked example totals about 401 lb.
How long should rebar laps be?
A common rule is 40–60 bar diameters: roughly 24–30 inches for #4 bars and 30–36 inches for #5. The exact lap is set by your engineer or code based on concrete strength and bar coating. Laps are why this calculator adds a cutting-and-lap allowance on top of the exact grid length.
What is the difference between rebar and wire mesh?
Rebar is individual steel bars tied into a grid; mesh is a factory-welded sheet of thinner wires. Mesh controls surface cracking on light slabs and is faster to place, while rebar adds real bending strength and is specified for structural and vehicle slabs. The Wire Mesh Calculator on this site sizes sheets and rolls.
How high should rebar sit in a slab?
For slabs on grade, the grid usually sits in the lower-middle of the slab depth, about 2 inches of cover from the bottom on a 4–6 inch slab, held on chairs or dobies. Steel resting on the ground or pushed to the very bottom adds almost no strength, so supports are not optional.
Can I use rebar in footings too?
Yes, footings typically run two or three continuous horizontal bars lengthwise rather than a grid, with vertical dowels tying into the wall or column above. Enter the footing run as a long narrow grid here for a close estimate, and check the Footing Calculator for the concrete volume that goes with it.
How accurate is this rebar calculator?
The bar count is exact geometry for a rectangular two-way grid: spaces rounded up, plus one edge bar. What varies on site is cutting waste and lap length, which is why the allowance field exists. For irregular shapes, split the pour into rectangles, calculate each, and add the results.
Related Calculators
Sources used for standards and conversions: National Ready Mixed Concrete Association (NRMCA) guidance on ordering and yield; American Concrete Institute (ACI) practice for mix and placement; Portland Cement Association (PCA) design and control guidance; US manufacturers’ printed yields (80 lb bag = 0.60 ft³). Depths, sizes and spacings set by your local building code govern wherever they differ from the general figures here.
Disclaimer: Results are estimates for planning and ordering. Confirm quantities with your supplier before ordering, and have structural work, footings, walls, columns, beams and anything load-bearing, checked by a licensed engineer.