Rebar Calculator
Bars per direction, linear feet & 20-ft sticks for a slab grid
📐 Slab & grid details
Last updated June 2026
Method: Standard two-way grid geometry. Bars on each axis = floor(usable span / spacing) + 1, where the usable span is the slab dimension minus edge clearance on both sides. Linear feet = bars times the dimension each bar spans, summed for both directions.
Included: Bars running lengthwise and widthwise, total bar count, exact linear feet, linear feet with your chosen waste factor, standard sticks to buy, and the number of grid intersections to tie.
Not included: Lap-splice lengths, bar size and weight, chairs and tie wire, second mats, and slab structural design. Confirm spacing, cover and bar size against engineered drawings or local code.
Rebar calculator: everything you need to know
Pouring a 20 ft x 12 ft concrete slab with rebar on a 12-inch grid needs about 512 linear feet of bar - roughly 13 bars running the long way and 21 running the short way, which works out to around 26 standard 20-foot sticks before waste. Get that number wrong and you either stall the pour waiting on a steel run or pay to truck back a half-bundle of bar you never used. This rebar calculator turns two slab dimensions and a spacing into an exact bar count, total linear feet, and a clean shopping list of 20-foot sticks.
How the rebar grid is calculated
A slab is reinforced with a two-way grid: one set of bars runs the length of the slab, a second set runs the width, and they cross to form a mat. The number of bars on each axis is set by the spacing and how far you keep the steel in from the edges:
bars = floor((dimension − 2 × clearance) ÷ spacing) + 1 The +1 matters: if a 12 ft run is spaced every 12 inches you get 12 gaps but 13 bars, because there is a bar at both ends of the run. Once you have the count for each direction, the total length of steel is:
linear feet = (bars₁ × length) + (bars₂ × width) Divide that by your stick length (20 ft is the US standard) and round up to get the number of bars to order.
Worked example: a 20 x 12 ft slab
Take a 20 ft long, 12 ft wide slab, rebar at 12 inches on center, with 3 inches of edge clearance all around. The usable span is the slab minus 6 inches (3 in on each edge) per axis, so 19.5 ft and 11.5 ft. That gives floor(11.5 / 1) + 1 = 13 bars running the long (20 ft) direction and floor(19.5 / 1) + 1 = 21 bars running the short (12 ft) direction. Linear feet are 13 x 20 = 260 ft lengthwise plus 21 x 12 = 252 ft widthwise, for 512 linear feet. At 20 ft per stick that is 26 sticks exactly, or about 29 sticks with a 10% overage. The grid has 13 x 21 = 273 intersections to tie.
How to measure and use this calculator
You only need a tape measure and your plans. Work through the fields in order:
- Slab length and width: measure the finished slab footprint in feet. For an irregular shape, break it into rectangles and run each one separately.
- Bar spacing: enter the center-to-center spacing in inches from your plans (commonly 12, 16, or 18 inches). The preset buttons cover the usual values.
- Edge clearance: the concrete cover that keeps steel away from the slab edge - about 3 inches against the ground is typical.
- Stick length: the length of the bar your supplier sells, usually 20 ft.
- Waste / overage: nudge the slider up for laps and cuts; 10% is a sensible default.
Hit Calculate rebar and read the big linear-foot number at the top, then the "what to buy" card for the stick count to order.
Who this calculator is for
- DIY slab pourers sizing rebar for a shed floor, patio, or garage slab.
- Homeowners sanity-checking a contractor's material list before paying for it.
- Contractors and estimators who want a fast linear-foot and stick count for a takeoff.
- Landscapers reinforcing driveways, walkways, and equipment pads.
- Anyone buying rebar who needs to translate a grid spacing into bundles to load on the truck.
Key rebar terms explained
- Linear feet: the total running length of bar, regardless of size - the figure you actually buy and price by.
- On center (OC): spacing measured from the center of one bar to the center of the next, the standard way spacing is given.
- Concrete cover / clearance: the thickness of concrete between the steel and the surface or edge, which protects the bar from rust.
- Lap splice: the overlap where two bars join end to end so the load transfers; it consumes extra length, typically 12-24+ inches per splice.
- Mat: a complete two-way layer of crossing bars; some slabs use one mat, heavier ones use two.
- Waste factor: the percentage you add on top of the exact figure for cuts, laps, damage, and miscounts.
- Cubic yard: the unit concrete is sold in (27 cubic feet); use the Concrete Calculator alongside this tool to order the mud that goes around the steel.
Scenario 1: a 10 x 10 ft patio at 16-inch spacing
A small 10 x 10 ft patio at 16-inch (1.33 ft) spacing with 3-inch cover has a 9.5 ft usable span each way. That is floor(9.5 / 1.33) + 1 = 8 bars in each direction. Linear feet are 8 x 10 + 8 x 10 = 160 ft, or 8 sticks of 20-foot bar exactly - call it 9 with overage. Wider spacing means fewer bars, which is why a patio uses far less steel than a structural slab of the same size.
Scenario 2: a 24 x 24 ft garage slab at 12-inch spacing
A two-car garage at 24 x 24 ft on a tight 12-inch grid with 3-inch cover has a 23.5 ft usable span each way, giving floor(23.5 / 1) + 1 = 24 bars per direction. Linear feet are 24 x 24 + 24 x 24 = 1,152 ft, which is 58 sticks exactly or about 64 with a 10% overage. Note how much faster steel adds up on a square slab when both dimensions and the grid are large.
Scenario 3: the same slab, looser spacing
Take that 24 x 24 ft slab and open the grid to 18 inches (1.5 ft). The count drops to floor(23.5 / 1.5) + 1 = 16 bars per direction, for 16 x 24 + 16 x 24 = 768 ft - a third less steel than the 12-inch grid. Spacing is the biggest single lever on rebar quantity, which is exactly why it should come from your engineer or code, not from rounding to save a few sticks.
What changes the result the most
- Spacing: the dominant factor - halving the spacing roughly doubles the bar count.
- Slab size: linear feet scale with both dimensions, so a larger slab grows steel quickly.
- Edge clearance: minor for big slabs, but it can drop a bar on small pours.
- Stick length: changes how many bars to buy and how many laps you need, not the linear-foot total.
- Waste factor: the gap between the exact figure and what you actually order.
Tips for ordering rebar
- Always order about 10% extra. Laps, cuts, and the odd bent bar eat into the exact figure fast.
- Account for lap splices when a bar must run longer than one stick - each splice adds 12-24+ inches.
- Buy chairs or dobies to hold the mat at the right height; bar lying on the ground does almost nothing.
- Get tie wire for the intersections - the calculator gives the intersection count to size that.
- Match bar size to your plans (#3, #4, #5). Size does not change the count, but it changes strength and weight.
Limitations and assumptions
- It models a single rectangular two-way mat; for two mats, run it twice and add the totals.
- The base linear feet assume continuous full-length bars - it does not add lap-splice length automatically (use the waste slider).
- It does not size bar diameter, cover depth, or slab thickness, or check that the design meets code.
- It excludes chairs, tie wire, dowels, and edge/perimeter bars beyond the grid.
- It is a material estimate, not a structural design - follow engineered drawings for anything load-bearing.
Related materials and calculators
Rebar is only one part of a slab takeoff. To finish the job, pair this with a few sister tools:
- Use the Concrete Calculator to find the cubic yards of concrete that go around the steel.
- Use the Square Footage Calculator to confirm the slab area before you lay out the grid.
- Use the Cubic Yard Calculator for any base or fill material under the slab.
- Use the Gravel Calculator to size the sub-base the slab is poured on.
- Use the Mulch and Paint calculators for the rest of the project's material list.
💡 Good to know
The "+1" is where people lose count
A 12 ft run at 12-inch spacing has 12 gaps but 13 bars, because there is a bar at each end. Forgetting the end bar is the most common reason a slab comes up one stick short on each axis.
Spacing decides almost everything
Going from a 12-inch to an 18-inch grid cuts the bar count by about a third. That is a big saving - but spacing is a structural decision, so take it from your plans or code, not from the budget.
Keep the steel off the ground
Rebar only works if it sits inside the slab, usually mid-depth or in the lower third, on chairs or dobies. Bar laid flat on the dirt and "pulled up" during the pour rarely ends up where it should.
⚠️ Common mistakes & edge cases
Forgetting the end bar (+1)
Dividing length by spacing gives the number of gaps, not bars. There is always one more bar than gaps, so use floor(span / spacing) + 1 - or you will short yourself a bar on every line.
Ignoring lap splices on long runs
Any bar longer than your 20-foot stick has to be lapped, and each lap eats 12-24+ inches of steel. The base linear-foot figure does not add this - raise the waste slider on large slabs.
Buying exactly the calculated amount
Cuts, bent bars, and miscounts mean the exact number almost always falls short. Order about 10% extra so a missing few feet never stops a pour that is already on the truck.
Treating it as a structural design
This tool counts bars for the spacing you enter; it does not choose bar size, cover, or spacing. For anything load-bearing, those values must come from engineered drawings or local building code.
❓ Frequently asked questions
How does the rebar calculator work?
It lays out a two-way grid for your slab. Bars running the length are counted across the width and bars running the width are counted across the length, using bars = floor(span / spacing) + 1 on each axis. It then multiplies each bar count by the slab dimension that bar spans to get linear feet, adds the two directions together, and divides by your stick length to get the number of standard 20-foot bars to buy.
What rebar spacing should I use for a slab?
Residential slabs-on-grade commonly use #3 or #4 bar at 12, 16, or 18 inches on center, while patios and sidewalks sometimes use welded wire mesh instead. Driveways and heavier slabs often tighten to 12 inches. Spacing, bar size, and concrete cover should come from your structural plans or local building code - this tool counts bars for the spacing you enter, it does not design the slab.
How do I calculate linear feet of rebar?
Linear feet equals the number of bars in each direction times the length each of those bars runs, summed across both directions. For a 20 x 12 ft slab at 12-inch spacing, that is about 13 bars running 20 ft (260 ft) plus 21 bars running 12 ft (252 ft) for roughly 512 linear feet before waste.
How many 20-foot sticks of rebar do I need?
Divide your total linear feet by the stick length (20 ft is standard, though 10 and 40 ft are sold too) and round up. For about 512 linear feet you would need 26 sticks of 20-foot bar exactly, or around 29 sticks once you add roughly 10% for laps, cuts, and waste.
Why should I order about 10% extra rebar?
Bars almost never line up with your slab perfectly. You lose length to lap splices where two bars overlap (typically 12-24 inches), to end cuts, to bars that bend or get damaged on site, and to simple miscounts. Ordering about 10% extra - more for slabs with many splices - keeps a pour from stalling because you ran a few feet short.
What is edge clearance or concrete cover?
Edge clearance (concrete cover) is the gap between the outer rebar and the edge of the slab so the steel stays fully encased in concrete and protected from rust. A common value for slabs cast against the ground is about 3 inches at the bottom and edges. The calculator pulls your bars in from each edge by this amount before spacing them out.
Should rebar go in one layer or two?
Most residential slabs-on-grade use a single mat of bar placed at the correct height (often mid-slab or in the lower third) on chairs or dobies. Thicker structural slabs, elevated slabs, and heavily loaded slabs may call for top and bottom mats. This calculator sizes one two-way grid; if your plans show two mats, run it twice and add the results.
Does this calculator include lap splices and overlaps?
Not in the exact figure. The base linear-foot number assumes continuous bars at full length. Real bars longer than your stick length must be lapped, and each lap consumes extra steel. The waste/overage slider is there to cover that - bump it up if your slab is large enough that many bars need splicing.
Can I use this for a driveway, patio, or footing?
Yes for any rectangular two-way grid such as a driveway, patio, shed slab, or garage floor. For long, narrow footings you usually run continuous longitudinal bars with stirrups rather than a full grid, so this tool is less suited to footings - count those by length and add stirrups separately.
What rebar size does this assume?
The calculator is size-agnostic: it counts bars and linear feet, which is the same regardless of whether you use #3, #4, or #5 bar. Bar size affects strength and weight, not the grid count. Pick the size from your plans, then use the linear-foot total here to price and order it.
How much does rebar weigh?
Weight depends on bar size. As a rough guide, #3 bar weighs about 0.376 lb/ft, #4 about 0.668 lb/ft, and #5 about 1.043 lb/ft. Multiply your total linear feet by the per-foot weight to estimate total weight for delivery or to convert to tons (divide pounds by 2,000).
Is this an estimate or an exact material list?
It is a planning estimate. The bar counts and linear feet are exact for the grid you describe, but real jobs add laps, cuts, chairs, and tie wire that the base number does not. Treat the with-waste figure as your order quantity and confirm bar size, spacing, and cover against your engineered drawings.
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