Free rebar calculator
Estimating reinforcing steel for a slab or footing comes down to laying out a two-way bar grid: trim the slab by the concrete cover, count the bars at your chosen spacing in each direction, and total the length they run. This rebar calculator returns the bar count, total linear feet, number of standard 20 ft sticks to buy, and the steel weight — updated live, as you type.
On this page15 sections
| Bar size | Weight | Est. cost |
|---|---|---|
| #3 | 442 lb | $582 |
| #4 (selected) | 785 lb | $905 |
| #5 | 1,226 lb | $1,357 |
| #6 | 1,765 lb | $2,003 |
Cost uses the waste-adjusted linear feet at regional plain-steel estimates (#3 ≈ $0.45, #4 ≈ $0.70, #5 ≈ $1.05, #6 ≈ $1.55 per ft). Epoxy or galvanized bar costs more. Confirm with your supplier.
Spacing and cover should follow your local code or engineer. How accurate is this?
Results are estimates. Consult a professional.
How the rebar calculator works
Rebar is laid in a two-way grid: a set of bars running one direction and a second set crossing them, both spaced on center. To estimate the steel, the calculator first trims the slab by the concrete cover you leave around the edges, then counts how many bars fit at your spacing in each direction, multiplies each set by the length it runs, and adds the two totals to get the linear feet of rebar. Finally it divides by the 20-foot length rebar is stocked in to tell you how many sticks to buy.
What the result tells you
Two numbers do the work. The linear-feet figure is the true quantity of steel in the slab — what a weight estimate and a price are built on. The 20-foot-bar count is what you carry out of the yard, because rebar is sold in sticks, not by the foot. The calculator shows both, plus the total weight, so you can order sticks and check the haul weight at the same time.
What goes into your rebar estimate
A rebar estimate is built from four inputs. The slab size and spacing set the grid; the edge clearance trims it; the bar size sets the weight. Get each right and the count is right.
Spacing — how tight the grid is
Spacing is the on-center distance between parallel bars, measured center to center. Tighter spacing means more bars and more steel: dropping from 18 inches to 12 inches adds roughly half again as many bars in each direction. Residential slabs commonly use 12, 16 or 18 inches depending on the load.
Edge clearance — the cover at the perimeter
Concrete cover keeps the steel set in from every face so it does not rust or break out. For slabs cast against the ground, ACI 318 calls for about 3 inches of cover where the concrete meets earth, and 1.5 to 2 inches elsewhere. The calculator subtracts the clearance from both sides of each dimension before counting bars, so the grid never runs to the very edge.
Bar size — the weight per foot
Bar size sets the weight but not the count: the grid layout is the same whether you use #3 or #6. A #4 bar weighs about two-thirds of a pound per foot; a #6 bar weighs over twice that. The calculator multiplies your total linear feet by the published weight for the size you pick.
A worked example using the rebar calculator
Maria is reinforcing a 20 ft × 30 ft slab on grade with a 12-inch grid of #4 rebar, leaving 3 inches of cover at the edges. She wants the linear feet, the number of 20-foot sticks to buy, and the steel weight.
Step 1 — Trim the slab by the cover
Take 3 inches (0.25 ft) off each side: effective length = 30 − 0.5 = 29.5 ft, effective width = 20 − 0.5 = 19.5 ft.
Step 2 — Count the bars each way
Lengthwise bars are spaced across the 19.5 ft width: floor(19.5 ÷ 1) + 1 = 20 bars, each 29.5 ft long. Widthwise bars are spaced across the 29.5 ft length: floor(29.5 ÷ 1) + 1 = 30 bars, each 19.5 ft long.
Step 3 — Add up the linear feet
(20 × 29.5) + (30 × 19.5) = 590 + 585 = 1,175 linear feet of rebar.
Step 4 — Convert to 20 ft sticks and weight
1,175 ÷ 20 = 58.75, rounded up to 59 sticks with no waste — or 65 sticks once a 10% allowance for laps and off-cuts is added. At 0.668 lb/ft the steel weighs 785 lb.
Rebar size chart (#3 to #6)
US rebar is numbered by diameter in eighths of an inch — a #4 bar is 4/8 inch, or half an inch. These are the four sizes used in most residential slabs and footings, with the ASTM A615 diameter and weight per foot.
| Bar size | Diameter (in) | Weight (lb/ft) | Typical use |
|---|---|---|---|
| #3 | 0.375 | 0.376 | Light slabs, sidewalks, small footings |
| #4 | 0.500 | 0.668 | Driveways, patios, most residential slabs |
| #5 | 0.625 | 1.043 | Heavier slabs, foundation walls, footings |
| #6 | 0.750 | 1.502 | Structural footings, columns, heavy loads |
Diameters and weights per ASTM A615/A615M. Bar number = diameter in eighths of an inch.
How much rebar do I need by spacing?
If you just want a ballpark before you measure, this table gives the linear feet for common slab sizes at typical spacings, using a 3-inch edge clearance. These are bare totals — add 10% for laps and waste before you order.
| Slab size | 12 in spacing | 16 in spacing | 18 in spacing |
|---|---|---|---|
| 10 ft × 10 ft | 190 LF | 152 LF | 133 LF |
| 12 ft × 12 ft | 276 LF | 207 LF | 184 LF |
| 20 ft × 20 ft | 780 LF | 585 LF | 546 LF |
| 20 ft × 30 ft | 1,175 LF | 891 LF | 803 LF |
| 24 ft × 24 ft | 1,128 LF | 846 LF | 752 LF |
Linear feet of a two-way grid with 3 in edge clearance, before waste. Tighter spacing means more steel; divide by 20 and round up for stock-bar count.
When to use this rebar calculator
Reach for it whenever a slab or footing needs a reinforcing grid and you have to buy the steel — over-ordering ties up money, under-ordering means a second trip with the truck or, worse, a gap in the grid.
- Slabs on grade — patios, shed and garage floors, driveways and equipment pads reinforced with a bar grid.
- Footings and grade beams — where continuous bars run the length and tie into the grid.
- Foundation walls — laying out the vertical and horizontal bar spacing before ordering.
- Comparing spacing options — to see how much steel 12 in versus 18 in spacing costs before you commit.
Waste factor and lap splices: how much extra to order
Order the exact calculated linear feet and you will come up short on a real job. Bars on runs longer than a stock length have to be lapped — overlapped and tied where one bar meets the next — and that overlap is steel that does not show up in the bare grid math. Cut ends, miscut bars and breakage add a little more.
The 10% rule
For most slabs, add 10%. On a small slab where every bar fits within a single 20-foot stick with no splicing, you can trim that toward 5%. On large slabs with many lapped runs, 10% is the floor and some estimators go to 15%. The calculator's waste field defaults to 10% and adjusts up to 20%.
Why laps matter
A lap splice is typically dozens of bar diameters long — often around 30 to 40 times the bar diameter, so a couple of feet for a #4 bar — and there is one at every joint on a long run. On a slab whose longest side exceeds 20 feet, the lap length compounds quickly, which is exactly what the waste allowance is there to cover.
Rebar definitions
Rebar calculator questions
What spacing should I use for a slab?
For residential flatwork, 12 to 18 inches on center is typical: 12 inches for driveways and slabs carrying more load, 16 to 18 inches for patios and lighter slabs. Anything structural should follow an engineer's or local code's specified spacing rather than a rule of thumb.
Why does the calculator add a bar (the "+1")?
It is a fence-post count. A grid that has N gaps between bars needs N+1 bars to close both ends — the same reason a 100-foot fence with posts every 10 feet needs 11 posts, not 10. Leaving the +1 off undercounts the steel by one bar in each direction.
Does the bar size change how much rebar I need?
Not the count or the length — the grid is identical whether you use #3 or #6. Bar size only changes the weight and the price per foot, so it affects the haul weight and cost, not the number of sticks.
How accurate is this rebar calculator?
The grid math is exact. Trimming the slab by the cover, dividing each span by the spacing, adding one and multiplying by the run length gives the precise linear feet for a clean rectangular two-way grid. The weight uses the published ASTM A615 figure for the bar size, so if your measurements are right, the steel total is right.
The stock-bar count and any cost are planning figures. Lap splices, the actual cut pattern your fabricator uses, chairs and supports, and the offcuts left over all shift the real purchase a little, which is exactly why the waste factor exists. Use the linear-feet and weight numbers as the dependable core, confirm spacing and cover against your local code or engineer, and order to the high side — a few spare sticks always beat a gap in the grid.
Bar diameters and weights per foot follow ASTM A615/A615M. Cover and spacing guidance follows ACI 318, "Building Code Requirements for Structural Concrete." The grid bar-count method matches published material calculators such as Inch Calculator's rebar tool.Frequently asked questions about the free rebar calculator
About this rebar calculator
This rebar calculator runs entirely in your browser — nothing you enter is sent anywhere, and the bar count, linear feet, stick count, and weight recompute instantly as you adjust the slab dimensions, spacing, clearance, or bar size. The grid math follows the standard residential method and ASTM A615 bar weights, so the steel total is dependable once your measurements are right.
It's part of our construction calculators collection — pair it with the concrete and concrete block calculators to plan a full pour. Browse every tool in the full calculator directory.