Section 1
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#4 is standard for residential slabs.
20 ft is standard retail stock.
3 in where concrete meets earth.
Applied where a run exceeds stock length.
Covers offcuts and bends.
Enter your local supplier rate.
This rebar calculator works out how much rebar you need for a slab, footing, wall or round pad — total linear feet, sticks to buy, weight, lap splice allowance, tie wire and cost. It accounts for edge cover and splices, which is where most estimates fall short.
The short answer
For a two-way grid, count the bars in each direction and multiply by their length: (Width ÷ spacing + 1) × Length + (Length ÷ spacing + 1) × Width.
A 24 × 24 ft slab with #4 bar at 18 inches on center needs 16 bars each way — about 752 linear feet, or 40 sticks of 20 ft rebar, weighing roughly 500 lb.
On this page
How to use this rebar calculator
Choose what you’re reinforcing, enter the dimensions and spacing, and the rebar calculator returns total linear feet along with the number of sticks to buy, total weight, tie points and cost. Four modes cover most residential work:
- Slab grid — a two-way mat for a garage pad, patio, shed floor or driveway
- Footing — continuous longitudinal bars along a run, with optional transverse bars
- Wall — vertical and horizontal bars in a retaining or foundation wall
- Round slab — a grid inside a circle, where every bar is a different chord length
Bar size, stock length, edge cover, lap splice multiplier, waste and price sit behind Advanced options. Two of those change the answer significantly and are worth understanding — cover and laps both get their own sections below.
The rebar grid formula
A slab mat is two sets of parallel bars at right angles. Count each set, multiply by the bar length, and add them. The refinement most estimates skip is subtracting edge cover — rebar doesn’t run to the edge of the concrete, it stops 3 inches short on each side.
Clear length = Length − (2 × cover)
Clear width = Width − (2 × cover)
Bars running lengthwise = (Clear width ÷ spacing) + 1
Bars running widthwise = (Clear length ÷ spacing) + 1
Total feet = (bars lengthwise × clear length) + (bars widthwise × clear width)
Worked example — 24 × 24 ft slab, #4 at 18 in on center
Total tie points: 16 × 16 = 256 intersections, needing roughly 300 ft of tie wire.
Rebar sizes and weights
US bar sizes are numbered in eighths of an inch — a #4 bar is 4/8 inch, or half an inch, in diameter. Metric equivalents are labelled by nominal millimetre diameter.
| Size | Diameter | Metric | lb per ft | 20 ft stick | Typical use |
|---|---|---|---|---|---|
| #3 | 3/8 in | 10M | 0.376 | 7.5 lb | Patios, walkways, light slabs |
| #4 | 1/2 in | 13M | 0.668 | 13.4 lb | Garage slabs, driveways, footings |
| #5 | 5/8 in | 16M | 1.043 | 20.9 lb | Heavy slabs, retaining walls |
| #6 | 3/4 in | 19M | 1.502 | 30.0 lb | Structural footings, columns |
| #7 | 7/8 in | 22M | 2.044 | 40.9 lb | Commercial and structural work |
| #8 | 1 in | 25M | 2.670 | 53.4 lb | Heavy structural elements |
Weight matters more than people expect. A 24 × 24 ft slab mat in #4 is about 500 lb of steel — manageable. The same mat in #5 is 780 lb, and it costs proportionally more. Going up a bar size is a real decision, not a free upgrade.
What rebar spacing should you use?
Spacing has more effect on quantity than bar size does. Halving the spacing roughly doubles the linear feet.
| Application | Bar size | Spacing | ft per 100 ft² |
|---|---|---|---|
| Patio, walkway | #3 | 24 in o.c. | ~100 ft |
| Shed floor | #4 | 18 in o.c. | ~133 ft |
| Garage slab | #4 | 18 in o.c. | ~133 ft |
| Driveway | #4 | 16–18 in o.c. | ~133–150 ft |
| RV pad, heavy vehicles | #4–#5 | 12 in o.c. | ~200 ft |
| Foundation wall | #4–#5 | 16 in o.c. | ~150 ft |
| Retaining wall | #5 | 12 in o.c. | ~200 ft |
These are common residential practice, not code. Spacing for any structural element — retaining walls above about 4 ft, anything holding up a building, anything on expansive soil — comes from an engineer’s design, and the difference is not cosmetic.
Lap splices — the part most calculators miss
Rebar comes in fixed lengths, usually 20 ft. Any run longer than that needs two bars overlapped, and the overlap has to be long enough to transfer force from one bar to the next. That lap is dead length you have to buy but never see.
Lap = Multiplier × bar diameter
#4 bar at 40d = 40 × 0.5 in = 20 in
#5 bar at 40d = 40 × 0.625 in = 25 in
#6 bar at 40d = 40 × 0.75 in = 30 in
40 bar diameters is a common residential rule of thumb for tension splices in normal-weight concrete, with a 12 inch absolute minimum. Actual required lap length under ACI 318 depends on concrete strength, bar coating, cover and spacing, and can be shorter or considerably longer. For structural work, use the number on the drawings rather than a rule of thumb.
Never line up every splice in the same plane across a slab. That creates a continuous weak line exactly where you added steel to prevent one. Offset alternate bars so splices are distributed — the calculator gives you the total lap footage, but staggering them is on you.
Cover and placement
Rebar only works where it sits, and cover — the distance from steel to the concrete surface — is what protects it from corrosion.
| Condition | Minimum cover |
|---|---|
| Concrete cast against earth | 3 in |
| Formed concrete exposed to earth or weather (#6 and larger) | 2 in |
| Formed concrete exposed to earth or weather (#5 and smaller) | 1.5 in |
| Interior slabs not exposed to weather | 3/4 in |
Vertically, slab reinforcement belongs in the middle third of the depth — for a 4 inch slab, roughly 1.5 to 2 inches from the bottom. Hold it there with chairs or dobies at about 3 ft centers in both directions. The calculator estimates chair count from the tie points.
Steel lying on the vapor barrier at the bottom of a slab contributes almost nothing structurally, because a slab in bending needs the steel away from the neutral axis. Pulling bars up as the concrete goes in — “hooking” — does not reliably position them either. Buy the chairs. They cost a fraction of the rebar and determine whether any of it works.
Rebar needed by slab size
#4 bar at 18 inches on center, 3 inch edge cover, including laps and 5% waste.
| Slab size | Bars each way | Linear feet | 20 ft sticks | Weight |
|---|---|---|---|---|
| 8 × 10 ft | 6 × 7 | 120 ft | 6 | 80 lb |
| 10 × 12 ft | 7 × 8 | 175 ft | 9 | 117 lb |
| 12 × 20 ft | 8 × 14 | 350 ft | 18 | 234 lb |
| 16 × 20 ft | 11 × 14 | 440 ft | 22 | 294 lb |
| 20 × 20 ft | 14 × 14 | 560 ft | 28 | 374 lb |
| 24 × 24 ft | 16 × 16 | 846 ft | 43 | 565 lb |
| 30 × 40 ft | 20 × 27 | 1,900 ft | 95 | 1,269 lb |
Notice the jump from 20 × 20 to 24 × 24. Runs pass 20 ft, so lap splices kick in and the footage climbs faster than the area does. That step is invisible in most rebar calculators and is a common reason people come up short.
Footings, walls and round slabs
Continuous footings
A strip footing usually carries two or three longitudinal bars running the full length, sometimes with transverse bars or stirrups at intervals. Quantity is simply rows × total run, plus laps.
Linear feet = Number of bar rows × Total run + laps
96 ft perimeter footing, 2 rows of #4
= 2 × 96 = 192 ft + 8 splices × 20 in
= ~206 ft before waste
Walls
Walls take horizontal bars up the height and vertical bars along the length, both at the specified spacing. A foundation wall typically also has corner bars and dowels tying it to the footing, which are extra and not counted here.
Round slabs
A circular pad is the awkward one, because every bar in the grid is a different chord length. The calculator works out each chord individually — for a 12 ft circle at 12 inch spacing, that’s 23 bars totalling roughly 210 linear feet. Doing this by hand means measuring each bar off a scaled drawing.
Rebar or welded wire mesh?
Slabs with real loads
Rebar
Stronger, stays where you put it, and simple to inspect before the pour. Now standard for garage slabs, driveways and footings.
Light-duty slabs
Wire mesh
Cheaper and faster on patios and walkways. The problem is placement — sheets that come in rolls curl, and mesh routinely ends up on the ground.
Crack control only
Fiber
Mixed in at the plant. Reduces shrinkage cracking but adds no structural capacity — it does not replace rebar on a load-bearing slab.
Cost is closer than people assume. Mesh is cheaper per square foot, but rebar for a residential slab at 18 inch spacing runs roughly $0.55 per foot, which is around $0.75 per square foot of slab. On a two-car garage that’s a difference of maybe $150 — small next to the concrete bill, and rebar is the more forgiving choice if placement isn’t perfect.
What does rebar cost?
| Item | Typical US price |
|---|---|
| #3 rebar, 20 ft stick | $7–$11 |
| #4 rebar, 20 ft stick | $9–$15 |
| #5 rebar, 20 ft stick | $15–$23 |
| Rebar by weight (bulk) | $0.50–$0.90 / lb |
| Tie wire, 3.5 lb roll | $8–$14 |
| Plastic chairs, per 100 | $25–$45 |
Steel prices move more than most building materials, so treat these as a range and price locally. Buying by the stick at a big-box store costs noticeably more per pound than a rebar yard or steel supplier, and yards will often cut and bend to a schedule for a modest fee.
Rebar estimating mistakes to avoid
- Ignoring lap splices. On any slab over 20 ft this is 5–10% of your total footage.
- Forgetting edge cover. Bars stop 3 inches short of the edge, which changes both count and length.
- Counting bars as area ÷ spacing. You need one more bar than that — a 24 ft run at 18 in gives 16 bars, not 15.
- Lining up all splices in one plane. That’s a designed-in weak line straight across the slab.
- Skipping chairs. Rebar on the ground is decoration, not reinforcement.
- Buying by weight without checking the size. 500 lb of #5 is far less coverage than 500 lb of #4.
- Using rule-of-thumb spacing on structural work. Retaining walls and load-bearing footings need an engineer’s schedule.
Frequently asked questions
How do I calculate how much rebar I need?
For a two-way grid, divide the clear width by the spacing and add one to get the number of lengthwise bars; do the same with the length for widthwise bars. Multiply each count by its bar length and add them. Then add lap splices for any run longer than a stock stick, plus about 5% waste.
How much rebar do I need for a 20 × 20 slab?
With #4 bar at 18 inches on center and 3 inch cover, about 14 bars each direction and roughly 560 linear feet — 28 sticks of 20 ft rebar, weighing about 374 lb. At 12 inch spacing it rises to roughly 800 linear feet.
What size rebar is used in a residential slab?
#4 bar — half an inch in diameter — is the standard for garage slabs, driveways and footings. #3 is used on patios and walkways where loads are light. #5 appears in heavier slabs and retaining walls. Anything structural should follow an engineer’s specification.
How far apart should rebar be in a slab?
18 inches on center in both directions is typical for a residential garage slab or driveway with #4 bar. Patios often go to 24 inches with #3. RV pads and areas taking heavy vehicles drop to 12 inches. Halving the spacing roughly doubles the rebar quantity.
How long should a rebar lap splice be?
A common residential rule is 40 bar diameters, with a 12 inch minimum — 20 inches for #4 bar, 25 inches for #5. Actual required lap under ACI 318 depends on concrete strength, cover, bar spacing and coating, and can differ. Use the figure on the drawings for structural work, and stagger splices rather than aligning them.
How much does rebar weigh per foot?
#3 weighs 0.376 lb per foot, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044 and #8 is 2.670. A 20 ft stick of #4 weighs about 13.4 lb.
How long does rebar come in?
20 ft is the standard stock length at most suppliers and big-box stores. Steel yards also carry 10, 30, 40 and 60 ft lengths, and 10 ft is common in retail for easier transport. Longer sticks reduce splices but are harder to handle and haul.
How much rebar cover does a slab need?
3 inches where concrete is cast directly against earth, 1.5 to 2 inches for formed concrete exposed to weather, and 3/4 inch for interior slabs not exposed to weather. Vertically, slab steel belongs in the middle third of the depth, held on chairs.
Is rebar or wire mesh better for a concrete slab?
Rebar is stronger and far easier to position correctly, which is why it has largely replaced mesh for garage slabs and driveways. Wire mesh is cheaper and adequate for patios and walkways, but it frequently ends up lying on the ground where it contributes very little. Fiber mesh controls shrinkage cracking only and does not replace steel.
How much tie wire do I need?
Roughly 14 inches of wire per tie, so about 1.2 feet per intersection. A 24 × 24 ft slab at 18 inch spacing has 256 intersections, needing around 300 feet — one 3.5 lb roll covers about 340 feet of ties. Tying every intersection is standard on slabs; some crews tie alternate ones on large mats.
Do I need rebar in a 4 inch slab?
For a patio or walkway with no vehicle traffic, fiber or mesh is often adequate. For a garage slab, driveway or anything carrying a vehicle, #4 rebar at 18 inches on center is standard practice and often required by local code. Reinforcement doesn’t prevent cracking — it holds cracks tight when they occur.
Can I use this rebar calculator in metric?
Yes. Switch the toggle in the calculator header to m / mm and every input changes to metres and millimetres. Bar sizes show their metric designations — 10M, 13M, 16M and so on — and results are given in both feet and metres, pounds and kilograms.
Standards and sources
- ASTM A615 — Deformed and plain carbon-steel bars for concrete reinforcement
- ACI 318 — Building code requirements for structural concrete, including cover and development length
- ACI 332 — Residential concrete construction code
- CRSI Manual of Standard Practice — bar sizes, unit weights and placing conventions
- Published supplier pricing for stock rebar lengths and accessories
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How this rebar calculator works out its numbers
Bar counts use the clear dimension after subtracting edge cover from both sides, divided by spacing, plus one. Lap splices are added wherever a single run exceeds the selected stock length, at the chosen multiple of bar diameter, with the number of splices per bar calculated from run length divided by stock length. Unit weights follow ASTM A615 nominal values: 0.376 lb/ft for #3 through 2.670 lb/ft for #8. Tie points are counted as grid intersections, with tie wire estimated at 14 inches per tie and chairs at roughly one per four intersections. Round slab mode calculates each chord individually rather than approximating the circle as a square.
Cost uses the price per foot you enter, so it reflects your local supplier rather than a national average.
This rebar calculator produces quantities, not structural design. Bar size, spacing, cover, splice length and development length for any structural element must come from an engineer’s drawings or your local building code. The spacing figures on this page describe common residential practice and are not a substitute for a design, particularly for retaining walls, load-bearing footings and slabs on expansive soils.