Slab 1
Advanced options
Rebar grid returns total linear feet and piece count.
18 in is typical for a residential slab.
Allow 5–10%, more on an ungraded base.
Supply only. Excludes pump and short-load fees.
This concrete slab calculator works out how many cubic yards of concrete a slab needs, plus the rebar or wire mesh to reinforce it, the form perimeter, vapor barrier area and delivered cost. It handles rectangular, L-shaped, round and thickened-edge slabs.
The short answer
Concrete for a slab is length × width × thickness, then divided by 27 to get cubic yards. A 20 × 20 ft slab at 4 inches is 20 × 20 × 0.333 = 133 ft³ ÷ 27 = 4.94 yd³. Add 10% waste and order 5.5 yd³.
Quick version: at 4 inches thick, 81 square feet of slab takes one cubic yard of concrete.
On this page
How to use this concrete slab calculator
Choose your slab shape, enter three measurements, and the calculator returns cubic yards to order along with reinforcement quantities, weight, truck loads and supply cost. Four shapes cover almost every residential slab:
- Rectangle — garage pads, shed floors, patios, driveways, equipment pads
- L-shape — patios wrapping a corner, drives with a parking apron
- Thickened edge — monolithic slabs where the perimeter drops below the plate
- Circle — tank pads, round patios, fire pit bases
Reinforcement type, rebar spacing, waste allowance and your local ready-mix rate sit behind Advanced options. Set it to rebar and the calculator returns total linear feet plus the number of 20 ft sticks; set it to wire mesh and it returns 5 × 10 ft sheet count with laps allowed for.
Outside the US? Flip the header toggle to m / mm and every input switches to metric, with results shown in both cubic yards and cubic metres.
The concrete slab formula
Slab volume is one multiplication and one division. The step people miss is converting thickness from inches to feet before multiplying — do it in inches and your answer comes out twelve times too large.
Cubic yards = Length (ft) × Width (ft) × (Thickness in ÷ 12) ÷ 27
4 in = 0.333 ft · 5 in = 0.417 ft · 6 in = 0.5 ft · 8 in = 0.667 ft
There’s a shortcut worth committing to memory. One cubic yard covers 81 square feet at 4 inches, 65 ft² at 5 inches and 54 ft² at 6 inches. Divide your slab area by that number and you have the yardage in your head — a useful sanity check on any concrete slab calculator before you place an order.
Worked example — 20 × 20 ft garage slab at 4 inches
Batch plants sell in quarter-yard increments, so round up to the next quarter rather than the next whole yard.
L-shaped and round concrete slabs
L-shaped slabs
Split the L into two rectangles, work out each area, add them, then multiply by thickness. The trap is double-counting where the two rectangles meet — draw the split so the sections butt against each other rather than overlap.
Cubic yards = (A₁ × B₁ + A₂ × B₂) × (Thickness ÷ 12) ÷ 27
20 × 12 ft section = 240 ft²
10 × 8 ft section = 80 ft²
320 ft² × 0.333 ÷ 27 = 3.95 yd³
Round slabs
Circular pads use the circle area formula. The usual error is entering diameter where the formula wants radius, which returns four times too much concrete.
Cubic yards = π × (Diameter ÷ 2)² × (Thickness ÷ 12) ÷ 27
12 ft diameter at 4 inches
= 3.1416 × 6² × 0.333 ÷ 27
= 1.40 yd³
Slabs with a thickened edge
This is where most slab calculators fall short and where people most often under-order. A monolithic slab — the standard for garages, sheds and additions in much of the US — isn’t a flat plate. The perimeter drops into a footing that carries the wall load, and that edge holds a surprising amount of concrete.
A 24 × 24 ft slab at 4 inches is 7.1 yd³ as a flat plate. Add a 12 inch wide by 8 inch deep thickened edge and you add roughly 2.3 yd³ — a 32% increase. Order the flat-plate number and you’ll run short with a truck idling in your driveway.
Volume = (Area × Slab thickness) + (Perimeter × Edge width × Edge depth)
24 × 24 ft slab, 4 in thick, 12 × 8 in edge
Plate: 576 × 0.333 = 192 ft³
Edge: 96 ft × 1 × 0.667 = 64 ft³
Less corner overlap ≈ 2.7 ft³
= 253 ft³ = 9.4 yd³
Thickened edge and footing dimensions aren’t a preference. For anything structural, footings must reach below the local frost line — that’s zero in parts of the south and 48 inches or more across the northern states and Canada. Your building department publishes the figure. Use this concrete slab calculator to price and order once you have the required dimensions, not to decide what they should be.
How thick should a concrete slab be?
Thickness drives volume more than any other input, so settle it before you calculate. These are common US residential figures.
| Slab application | Thickness | ft² per yd³ | Typical reinforcement |
|---|---|---|---|
| Walkway, garden path | 3–4 in | 81–108 | Fiber or none |
| Patio | 4 in | 81 | Wire mesh |
| Shed floor | 4 in | 81 | #3 or #4 rebar |
| Garage slab | 4–5 in | 65–81 | #4 rebar @ 18 in |
| Residential driveway | 4–5 in | 65–81 | #4 rebar @ 18 in |
| RV pad or heavy vehicle | 6 in | 54 | #4 rebar @ 12 in |
| Shop floor with equipment | 6–8 in | 41–54 | Engineered |
Going thicker isn’t automatically better. A 6 inch slab on a poorly compacted base still cracks — the sub-base does more work than the extra 2 inches ever will. Spend the money on grading, compaction and reinforcement before you spend it on depth.
How much rebar or wire mesh does a slab need?
The calculator handles both. Which you use depends on the job.
Most residential slabs
Rebar grid
#3 or #4 bar at 12–18 in on center, tied at intersections and set on chairs. Stronger, easier to position correctly, and now more common than mesh.
Patios & light duty
Welded wire mesh
6×6 W1.4/W1.4 sheets, 5 × 10 ft. Cheaper but notoriously ends up on the ground instead of mid-slab, where it does almost nothing.
Crack control only
Fiber mesh
Dosed into the mix at the plant. Controls shrinkage cracking but provides no structural capacity — it does not replace rebar on a load-bearing slab.
Bars across = (Width ÷ spacing) + 1 → × Length
Bars along = (Length ÷ spacing) + 1 → × Width
20 × 20 ft slab at 18 in on center
14 bars each way × 20 ft = 280 ft each direction
= 560 linear feet = 28 × 20 ft sticks
Wire mesh sheets are 5 × 10 ft, so 50 ft² gross. Sheets must lap one full square at joints, so effective coverage is closer to 45 ft². The calculator uses that figure, which is why its sheet count runs slightly higher than area ÷ 50.
Steel belongs in the middle third of the slab depth, held there on chairs at roughly 3 ft centers. Mesh or rebar lying on the vapor barrier at the bottom of a slab contributes almost nothing structurally. Pulling mesh up as you pour — “hooking” — does not reliably position it, and it is the most common reinforcement failure on DIY slabs.
Concrete needed by slab size
At 4 inches thick, before waste. Run your own numbers through the concrete slab calculator above for anything not listed.
| Slab size | Area | Cubic yards | Order with 10% | 80 lb bags |
|---|---|---|---|---|
| 8 × 10 ft shed | 80 ft² | 0.99 | 1.25 | 49 |
| 10 × 12 ft patio | 120 ft² | 1.48 | 1.75 | 74 |
| 12 × 20 ft 1-car drive | 240 ft² | 2.96 | 3.25 | 147 |
| 16 × 20 ft | 320 ft² | 3.95 | 4.5 | 196 |
| 20 × 20 ft 2-car garage | 400 ft² | 4.94 | 5.5 | 245 |
| 24 × 24 ft garage | 576 ft² | 7.11 | 8.0 | 353 |
| 30 × 40 ft shop | 1,200 ft² | 14.81 | 16.5 | 735 |
The bag column is there for scale, not as a suggestion. Anything over about one cubic yard should be ready-mix — 245 bags for a two-car garage is over five pallets and several tons to lift twice.
What goes under a concrete slab
The pour is the last step. Ordering concrete before the base is right is how good concrete ends up in a cracked slab.
- Excavate and grade. Strip topsoil and organic material completely. A slab over buried roots or loose fill will settle unevenly no matter how thick it is.
- Compacted sub-base. 4–6 inches of crushed stone or road base, compacted in lifts with a plate compactor. This layer prevents more cracks than slab thickness does.
- Vapor barrier. 6–10 mil polyethylene, lapped 6 inches and taped, turned up at the edges. Allow around 15% extra area for laps — the calculator gives you the figure.
- Forms. Braced against roughly 4,000 lb per cubic yard of hydraulic pressure. Blown-out forms mid-pour is the most common way a DIY slab goes wrong.
- Reinforcement on chairs. Middle third of the slab depth, chairs at about 3 ft centers.
- Pour, screed, float, cure. Keep it damp or covered for at least seven days. Concrete that dries too fast is permanently weaker.
- Control joints cut within 6–18 hours, at a spacing in feet of roughly 2–3 times the slab thickness in inches. A 4 inch slab wants joints every 8–12 ft.
What does a concrete slab cost?
The concrete slab calculator returns supply-only cost, because that’s the number you control when ordering. Supply is typically a third or less of a finished slab price once grading, forms, reinforcement, labor and finishing are added.
| Cost component | Typical US range |
|---|---|
| Ready-mix concrete supply | $130–$180 / yd³ |
| Short-load fee under ~4 yd³ | $60–$150 |
| #4 rebar, 20 ft stick | $9–$15 |
| Wire mesh sheet, 5 × 10 ft | $12–$20 |
| Concrete pump | $800–$1,500 / day |
| Plain slab poured and finished | $6–$12 / ft² |
Prices vary widely by region and by distance from the batch plant, and most suppliers add a surcharge for Saturday delivery. Get a quote for your ZIP code before budgeting.
Slab calculation mistakes to avoid
- Treating a monolithic slab as a flat plate. A thickened edge can add 30% or more to the volume.
- Forgetting to convert inches to feet before multiplying — an answer twelve times too large.
- Dividing area by 50 for wire mesh. Laps mean effective coverage is closer to 45 ft² per sheet.
- Skipping the waste allowance and running short the moment the sub-base has a low spot.
- Using diameter instead of radius on a round slab — four times too much concrete.
- Rounding down. Batch plants sell in quarter-yard steps; always round up to the next quarter.
- Ordering before checking access. A loaded mixer weighs 60,000 lb or more and will crack a residential driveway.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length by width to get square feet, multiply by thickness in inches divided by 12, then divide by 27 for cubic yards. A 20 × 20 ft slab at 4 inches is 400 × 0.333 ÷ 27 = 4.94 yd³. Add 5–10% waste and round up to the next quarter yard.
How many cubic yards of concrete for a 20 × 20 slab?
At 4 inches thick, 4.94 cubic yards before waste — order 5.5 yd³. At 5 inches it rises to 6.17 yd³, and at 6 inches to 7.41 yd³. It also needs about 560 linear feet of #4 rebar at 18 inch spacing.
How many square feet does a yard of concrete cover?
81 square feet at 4 inches thick, 65 ft² at 5 inches, 54 ft² at 6 inches and 108 ft² at 3 inches. One cubic yard is 27 cubic feet, so coverage is simply 27 divided by the thickness in feet.
How thick should a concrete slab be?
4 inches suits patios, shed floors and most garage slabs. Driveways run 4–5 inches, RV pads and heavy vehicle areas 6 inches, and shop floors with equipment 6–8 inches. Walkways can go down to 3 inches. Anything structural should follow your local building code.
How much rebar does a concrete slab need?
For a residential slab, #4 bar at 18 inches on center in both directions is typical. A 20 × 20 ft slab needs about 560 linear feet, or 28 sticks of 20 ft rebar. Heavier loads use 12 inch spacing, which roughly doubles the quantity.
Do I need rebar or wire mesh in a slab?
Rebar is stronger and easier to position correctly, and it is now standard for garage slabs and driveways. Wire mesh is cheaper and adequate for patios and light-duty slabs, but it frequently ends up lying on the ground where it does almost nothing. Fiber mesh controls shrinkage cracking only and does not replace steel.
How much does a concrete slab cost per square foot?
Poured and finished, a plain concrete slab typically runs $6–$12 per square foot in the US, depending on access, thickness, finish and site prep. Concrete supply alone is roughly $130–$180 per cubic yard, which works out around $1.80–$2.20 per square foot at 4 inches.
How much concrete is in a slab with a thickened edge?
A 24 × 24 ft slab at 4 inches is 7.1 yd³ as a flat plate. A 12 inch wide by 8 inch deep thickened edge adds roughly 2.3 yd³ after allowing for corner overlap, giving about 9.4 yd³ before waste. Edge dimensions must meet your local frost depth requirement.
Can I pour a concrete slab in sections?
A structural slab should be poured in one continuous operation. Stopping and restarting creates a cold joint — a permanent weak line where the first placement stiffened before the next arrived. If a slab is too large for one pour, the break must be a designed construction joint with dowels, not an accident of timing.
How long before you can drive on a new concrete slab?
Walk on it after 24–48 hours, drive a car on it at around 7 days, and wait 28 days before heavy vehicles or equipment. Concrete reaches its specified strength at 28 days. Keeping it damp or covered through the first week matters more to final strength than most people expect.
How far apart should control joints be?
Roughly 2–3 times the slab thickness in inches, expressed as feet. A 4 inch slab wants joints every 8–12 feet; a 6 inch slab every 12–18 feet. Cut them within 6–18 hours of placement and to a quarter of the slab depth. Panels should stay close to square rather than long and narrow.
How much does a cubic yard of concrete weigh?
About 4,050 pounds, or roughly two tons, based on 150 lb per cubic foot. A fully loaded 10 yd³ mixer carries around 40,000 lb of concrete plus 25,000 lb of truck — which is why suppliers ask about driveway access before delivery.
Standards and sources
- ACI 318 — Building code requirements for structural concrete
- ACI 302.1R — Guide to concrete floor and slab construction
- ACI 332 — Residential concrete construction code
- ASTM C94 — Ready-mixed concrete specification
- ASTM A1064 — Welded wire reinforcement, sheet sizes and lap requirements
- Local building department frost depth and footing requirements (vary by jurisdiction)
Related calculators
Concrete Bag CalculatorBag counts for 40, 60 and 80 lb sizes
Rebar CalculatorGrid spacing, linear feet and tie count
Slab Cost CalculatorPoured and finished price per ft²
Cubic Yards CalculatorConvert any volume to yd³
Square Footage CalculatorWork out slab area first
How this concrete slab calculator works out its numbers
Slab volume uses standard geometric formulas with all inputs normalised to feet before multiplication, then divided by 27 for cubic yards. The thickened edge mode adds perimeter × edge width × edge depth to the flat plate volume and subtracts the corner overlap so corners aren’t counted twice. Rebar quantities assume a two-way grid at the selected spacing with one bar past each end, reported as total linear feet and 20 ft sticks. Wire mesh counts assume 5 × 10 ft sheets at 90% effective coverage to allow for laps. Weight is calculated at 150 lb per cubic foot, and truck loads assume a 10 yd³ mixer.
Cost figures use the ready-mix rate you enter and reflect supply only — they exclude pumping, short-load fees, weekend surcharges, forms, reinforcement, labor and finishing.
This concrete slab calculator produces quantities, not structural design. Slab thickness, reinforcement size and spacing, footing depth and frost protection must follow your local building code, and anything structural should be reviewed by a qualified engineer. Frost depth in particular varies widely by jurisdiction and directly changes how much concrete a thickened edge requires.