Concrete
Concrete slab calculator
The yards including the thickened edge that most orders miss, the base and barrier underneath, what each reinforcement actually does — and where to cut the joints, drawn on your own slab.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-16 · Nothing you type leaves your browser
What are you pouring?
The walls land on the perimeter, so the perimeter is thickened to carry them. That edge is concrete nobody counts.
12' 6", 12.5 and 12-6 all work
Blank uses 4 in, set by the use.
Eight per cent covers an uneven subgrade and the bit left in the chute.
Read as 20' × 20' = 400 ft², 80 ft of perimeter
The same job as mesh and more of it, and it stays where it is put because it is stiff enough to stand on.
What it does not do: Prevent cracking. Nothing prevents cracking — reinforcement decides what a crack does next.
Used only to pick a regional labour rate. It stays in your browser.
You need
7.5 yd³
400 ft² × 4 in = 4.94 yd³ + 1.98 yd³ of edge · +8% waste · rounded up to the quarter yard
The thickened edge is 29% of the concrete
Length × width × thickness gives 4.94 yd³ and misses the 1.98 yd³ that goes into the perimeter. It is the commonest reason a pour runs short — the edge gets dug and never gets ordered.
Where to cut
2 × 2 panels of 10 × 10 ft, with the rebar grid underneath.
Shopping list
- Ready-mix concrete, ordered in quarter-yard steps
4.94 yd³ of slab + 1.98 yd³ of thickened edge + 8% waste7.5 yd³ - What the thickened edge adds
12 in wide and 12 in deep around 80 ft of perimeter. Length × width × thickness misses this entirely.+29% - Compacted base, 4 in
Ordered loose at 5.9 yd³, because it loses about a fifth of its volume under the plate8.6 tons - #3 (3/8 in) rebar at 18 in, grid both ways
Or 6×6 W1.4/W1.4 welded wire mesh. On chairs at mid-depth, or it does nothing.560 ft - Control joints at 10 × 10 ft
2 × 2 panels, cut 1 in deep within 6–18 hours40 ft to cut - Form lumber
Around the perimeter, braced. Reusable if you are careful pulling it.80 ft
What each reinforcement actually does
On your 400 ft². They are not four grades of the same thing.
- Nothing—
Works for a small, thin, unloaded pad on ground that does not move — a shed floor for a mower, a pad under a condenser. Not: Hold a crack together once it forms, which on anything larger it will.
- Fibre in the mix$72
Controls plastic shrinkage cracking in the first hours, while the concrete is still soft. It is genuinely good at that and it costs very little. Not: Replace structural steel. It does nothing for a crack that opens in year three, and no code lets it substitute for rebar under a vehicle load.
- Welded wire mesh$140
Holds a crack tight once it has formed, so the slab keeps transferring load across it — but only if it sits at mid-depth, on chairs. Not: Anything at all lying on the ground, which is where it ends up on most residential pours. Mesh walked into the base is decoration.
- Rebar grid on chairs — yours$288
The same job as mesh and more of it, and it stays where it is put because it is stiff enough to stand on. Not: Prevent cracking. Nothing prevents cracking — reinforcement decides what a crack does next.
What it costs
National average. Concrete, base, reinforcement, forms, barrier and the work to excavate, form, pour and finish.
$14.35 per ft². Concrete is $1,238 of it and the work $3,760 — finishing a slab is a skill with a time limit on it, which is why it is the larger half and why a cheap quote is usually a fast one.
Worth knowing
- The thickened edge adds 1.98 yd³ — 29% more concrete than length × width × thickness gives you. It is the commonest reason a slab pour runs short, because the edge is dug but never added to the order.
- Cut the joints 1 in deep — a quarter of the slab — within 6 to 18 hours of the pour, and no more than 10 ft apart. That is 40 ft of cutting on this slab. Joints do not stop a slab cracking; they decide where it cracks, and a slab always cracks.
On this page — 7 sections
The concrete that length by width by thickness misses
The short version
- The thickened edge is real concrete. On a shed or garage slab it is a tenth to a fifth more than the field volume, and it is the usual reason a pour runs short.
- A slab always cracks. Joints do not prevent it; they decide where, and only if they are cut in time.
- Fibre and steel are not two grades of the same thing. One works in the first hours, the other for thirty years.
- Mesh on the ground does nothing. It only works at mid-depth, on chairs.
Almost every slab that carries something — a shed, a garage wall, a retaining kerb — has a deeper band of concrete around its perimeter. The excavation is dug for it, the forms are set for it, and then the concrete is ordered from length times width times thickness, which does not include it.
That is how a pour ends with the truck empty and eight feet of slab still to fill, at which point the options are all bad: a cold joint through the middle of a new slab, or a second delivery with its own minimum charge.
yd³ = (area × thickness ÷ 12 + perimeter × edge width ÷ 12 × extra depth ÷ 12) ÷ 27
- extra depth
- how far the edge goes BELOW the slab, not its total depth
- edge width
- how far in from the form the thickening runs, typically 12 in
- perimeter
- 2 × (length + width)
Worked through — A 20 × 20 ft shed base at 4 in: the field is 400 × 0.333 = 133 ft³, or 4.94 yd³. The 12 in wide edge goes 12 in deep, so 8 in below the slab: 80 ft of perimeter × 1 ft × 0.667 ft = 53 ft³, another 1.98 yd³. That is 40 per cent more than the field before waste — on a small slab the edge is not a detail.
A slab always cracks; joints decide where
Concrete shrinks as it dries, by roughly half a per cent of its length. A slab is restrained by friction against the ground at every point, so it cannot shrink freely — and something has to give. It gives by cracking.
A control joint is a deliberate line of weakness cut into the surface so the crack forms underneath it, straight and tidy, instead of wandering across the slab. The rule is a ratio: panels no more than 2.5 times the slab thickness in feet, which is ten feet for a four-inch slab and fifteen for a six-inch one.
And the panels want to be roughly square. A long thin panel shrinks much more along its length than across it, and it cracks across its own middle whatever the joints around it are doing — which is why the calculator flags any layout past a ratio of 1.5.
American Concrete Institute, ACI 302.1R, Guide to Concrete Floor and Slab Construction · checked 2026-09-15
2.5×
Thickness, in feet
maximum joint spacing
¼
Of the slab depth
how deep to cut
6–18 h
The window to cut
shorter in hot weather
1.5:1
Worst panel shape
past this it cracks anyway
Fibre, mesh and rebar are three different jobs
These get sold as a ladder — a little reinforcement, more reinforcement, most reinforcement — and they are not. They act at different times and on different problems, and knowing which is which is the difference between paying for something useful and paying for a roll of wire that ends up in the base.
| Option | What it does | What it does not |
|---|---|---|
| Nothing | Works for a small, thin, unloaded pad on ground that does not move — a shed floor for a mower, a pad under a condenser. | Hold a crack together once it forms, which on anything larger it will. |
| Fibre in the mix | Controls plastic shrinkage cracking in the first hours, while the concrete is still soft. It is genuinely good at that and it costs very little. | Replace structural steel. It does nothing for a crack that opens in year three, and no code lets it substitute for rebar under a vehicle load. |
| Welded wire mesh | Holds a crack tight once it has formed, so the slab keeps transferring load across it — but only if it sits at mid-depth, on chairs. | Anything at all lying on the ground, which is where it ends up on most residential pours. Mesh walked into the base is decoration. |
| Rebar grid on chairs | The same job as mesh and more of it, and it stays where it is put because it is stiff enough to stand on. | Prevent cracking. Nothing prevents cracking — reinforcement decides what a crack does next. |
| Fibre | Steel on chairs | |
|---|---|---|
| When it acts | The first hours, while it is soft | For the life of the slab |
| Against | Plastic shrinkage cracking | Drying shrinkage and load |
| Cost per ft² | about $0.18 | about $0.72 |
| Replaces the other? | No | No |
| Under a vehicle | Not adequate alone | Required |
| Fails when | Nothing — it is cheap insurance | It is laid on the ground instead of chaired |
What goes under it decides whether it lasts
A slab does not span. It is not a beam — it spreads load sideways into whatever is beneath it, which means the ground is part of the structure and any soft spot in it is a crack in the concrete.
Four inches of compacted crushed aggregate does two things: it gives a uniform surface to bear on, so the slab is supported evenly rather than bridging between high spots, and it gives water somewhere to go instead of sitting under the concrete and freezing. It is the same base rock a patio uses, priced by the gravel calculator.
Inside a building, the vapour barrier goes on top of that stone and directly under the concrete. Not under the stone — that is a common and expensive inversion, because it leaves the stone as a reservoir between the barrier and the slab.
Under the slab, in order
- Topsoil and anything organic removed. Organic material rots and leaves a void.
- Subgrade compacted and graded to the fall the slab will have.
- Compacted aggregate base, 4 in for a patio and 6 for heavy vehicles, laid in lifts.
- Vapour barrier directly under the slab if it is inside — lapped 6 in, taped, turned up at the edges.
- Reinforcement on chairs at mid-depth, tied, and someone tasked with keeping it there.
- Isolation joint material wherever the slab meets a wall, a column or an existing slab.
- Forms braced properly. Concrete is heavy and a blown form is not recoverable.
The base is crusher run, ordered by the ton with a compaction allowance — about a fifth more loose volume than the finished depth. The gravel calculator gives the tonnage and the delivery.
Gravel calculatorFive ways this goes wrong
01Ordering from length × width × thickness
It misses the thickened edge, which on a shed or garage slab is a tenth to a fifth of the concrete. The truck leaves and there is still slab to fill.
Instead — Add perimeter × edge width × the extra depth below the slab, then waste, then round to the quarter yard.
02Cutting the joints the next day
The window is six to eighteen hours and it is shorter in heat. Past it, the slab has already cracked where it chose, and the saw cut is decoration.
Instead — Plan the grid before the pour and have the saw ready. On a hot day that may mean cutting the same evening.
03Laying mesh on the base
It ends up below the crack it was supposed to hold together, which is the same as not buying it.
Instead — Chairs at mid-depth, or spend the money on fibre and an extra inch instead.
04Putting the vapour barrier under the stone
The stone then sits between the barrier and the slab holding water against the concrete — the opposite of the intention.
Instead — Barrier directly under the slab, lapped and taped, turned up at the perimeter.
05Letting it dry instead of curing it
Concrete gains strength by reacting with water, not by drying out. A slab left in the sun for its first days can lose a large share of the strength that was paid for.
Instead — Keep it damp — cover, sheet or spray — for several days, and longer in heat.
How the calculator works it out
- 01Decide the thickness from the useFour inches for a patio or a garage floor, five to six where vehicles heavier than a car park on it. It is set by the load, not by the budget.
- 02Work out the field volumeLength × width × thickness, divided by 27 for cubic yards. This is the part every calculator does.
- 03Add the thickened edgePerimeter × edge width × the EXTRA depth below the slab. On a shed or garage slab it is often a tenth to a fifth more concrete.
- 04Add waste and round to the quarter yardEight per cent covers an uneven subgrade and what stays in the chute. Plants sell in quarter-yard steps.
- 05Choose reinforcement by what it doesFibre for the first hours, steel on chairs for the next thirty years. They are not two grades of the same thing.
- 06Lay the joints out before the pourPanels no more than 2.5 × the thickness in feet, kept roughly square, cut a quarter deep within 6–18 hours.
Joint spacing, panel shape and cut depth follow the concrete institute’s guidance on slabs; thicknesses and reinforcement by use follow residential practice; the base is priced from the same aggregate survey the gravel calculator uses. Everything is on the sources page, dated.
Two assumptions worth stating. The thickened edge is counted only for the depth it goes BELOW the slab, because the top part of it is already in the field volume — counting the whole edge depth is the opposite error and over-orders. And the joint grid is divided evenly rather than cut at the maximum and left with an odd strip, because equal panels crack better than one big one and a remainder.
Nothing you type is sent anywhere — the whole calculation runs in your browser.
The words on a slab quote
Thickened edge and control joint are the two that decide the order and the outcome.
- Thickened edge
- A deeper band of concrete around the perimeter, or under a wall line, that carries a load the field of the slab does not. It is dug as part of the excavation and forgotten as part of the order.
- Control joint
- A deliberate weakness cut into the slab so it cracks in a straight line where you put it. It does not prevent cracking; nothing does.
- Panel ratio
- The long side of a joint panel divided by the short side. Past about 1.5 the panel cracks across its own middle no matter how well the joints were cut.
- Plastic shrinkage cracking
- Fine cracks that open in the first hours while the concrete is still soft, caused by the surface drying faster than the body. It is what fibre in the mix is genuinely good at preventing.
- Drying shrinkage
- The slow contraction of concrete over months as it loses water. It is what control joints are for, and it is the reason a slab that looked perfect in June has a line across it by Christmas.
- Chairs
- Small supports that hold reinforcement at mid-depth while the concrete goes around it. Without them, steel sits on the ground and contributes nothing.
- Welded wire mesh
- A grid of light wire in sheets or rolls. It works at mid-depth and it is worthless on the base, which is where it ends up on most residential pours.
- Vapour barrier
- Polythene sheet directly under an interior slab, lapped and taped. It stops ground moisture rising through the concrete for the life of the building, and it cannot be added later.
- Subgrade and sub-base
- The soil, and the compacted aggregate laid on it. The slab spans nothing — it spreads load into what is underneath, so anything soft below is a crack above.
- Short load fee
- What a ready-mix plant charges for delivering less than its minimum load. It is billed by the yard below the minimum and it is the biggest surprise on a small pour.
- Curing
- Keeping the slab damp while it gains strength, for several days. It is not drying — concrete hardens by chemical reaction with water, and a slab allowed to dry out early never reaches the strength it was paid for.
- Isolation joint
- A compressible strip where the slab meets something that does not move with it — a wall, a column, an existing slab. Without it, the two push against each other and the slab cracks at the contact.
Common questions
- How do I calculate concrete for a slab?
- Length × width × thickness in feet, divided by 27 for cubic yards, plus the thickened edge if there is one, plus about eight per cent waste, rounded up to the quarter yard. The step people miss is the edge: a 20 × 20 ft shed slab with a 12 inch thickened perimeter carries about fifteen per cent more concrete than the field volume alone.
- How many yards of concrete for a 20x20 slab?
- A flat 20 × 20 ft slab at four inches is 4.94 cubic yards before waste, so about 5.5 yards ordered. Put a 12 inch thickened edge around it for a shed or garage and it goes to roughly 6.25 — the extra is entirely in the perimeter, and it is dug whether or not it is ordered.
- How thick should a concrete slab be?
- Four inches for a patio, a walkway, a garage floor or a shed base. Five to six inches where vehicles heavier than a car park on it, with a rebar grid. Thickness is set by the load, and the base underneath matters at least as much — a six inch slab on soft ground still cracks.
- Do I need rebar in a 4 inch slab?
- It depends on what drives on it. A patio or a shed floor is fine with fibre in the mix and well-cut joints. Anything a vehicle uses wants a rebar grid on chairs at mid-depth, because reinforcement does not stop cracks — it holds them tight so the slab keeps carrying load across them, which is the whole point.
- Is fiber concrete as good as rebar?
- No, and they are not competing. Controls plastic shrinkage cracking in the first hours, while the concrete is still soft. It is genuinely good at that and it costs very little. Replace structural steel. It does nothing for a crack that opens in year three, and no code lets it substitute for rebar under a vehicle load. Fibre and steel do different jobs at different times in the life of a slab, and a driveway wants both.
- Does welded wire mesh do anything?
- Holds a crack tight once it has formed, so the slab keeps transferring load across it — but only if it sits at mid-depth, on chairs. Anything at all lying on the ground, which is where it ends up on most residential pours. Mesh walked into the base is decoration. If it is not going on chairs — and on most residential pours it is not — the money buys more by going into fibre and an extra inch of thickness.
- How far apart should control joints be?
- No more than 2.5 times the slab thickness in feet: 10 ft for a 4 inch slab, 15 ft for a 6 inch. Keep the panels roughly square — past a ratio of about 1.5 the panel cracks across its own middle — and cut a quarter of the depth.
- When should control joints be cut?
- Between 6 and 18 hours after the pour. Too early and the saw ravels the edge; too late and the slab has already chosen where to crack. In hot weather the window closes fast, which is why a Saturday pour and a Monday cut is the commonest way to end up with a random crack across a new slab.
- How much does a concrete slab cost?
- Usually $6 to $14 per square foot poured and finished, including the base and the reinforcement. The concrete itself is a smaller share than people expect — forming, placing and finishing is the larger half, and it is skilled work with a time limit on it.
- Do I need a vapour barrier under a concrete slab?
- Under anything inside a building, yes, and directly under the slab rather than under the stone. Moisture rises through concrete for the life of the building; it is what makes flooring adhesive fail and coatings peel years later. It costs very little and it cannot be retrofitted.
- What happens if I pour a slab without a base?
- The slab spans nothing — it spreads load into whatever is beneath it. On soft or uneven ground that means differential settlement and cracks that step. Four inches of compacted crushed aggregate gives the slab a uniform surface to bear on and somewhere for water to go.
- Can I pour a concrete slab myself?
- Forming, basing and ordering are all reasonable DIY. Finishing is the hard part, because concrete does not wait: once it starts to set you have a fixed window to float, edge, trowel and cut, and the window is shorter in hot weather. On anything over a few hundred square feet, most people who do it once hire it out the second time.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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