EstimatorKitFree concrete slab calculator
A slab take-off, not just a volume
Give the slab its length, width and thickness and this returns everything the pour needs in the units it is bought in: tons of stone base, cubic yards and 80 lb bags for the field and the thickened edge, rolls of vapor sheeting after the 6 inch laps, sheets of welded wire or sticks of rebar with their splices, form boards and stakes, and the linear feet of control joint that decide where the slab cracks. The turndown is worked out as a ring, so the four corners are counted once rather than twice, and the panel says so the moment the pour outgrows a single truck.
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- Sub-base, steel, sheeting and forms
- Joint layout included
- Flags a second truckload
4 in on a compacted base, no sheeting: an outdoor slab wants to drain, not to be sealed underneath.
Laps are 6 in — welded wire is lapped one full square, and the common residential sheet is a 6 × 6 grid. Wire lying on the subgrade does nothing: it has to be lifted into the upper third of the slab.
Concrete for 192 sq ft at 4 in
2.75 cu yd · 120 bags
- Field of the slab
- 64 cu ft
- Over-depth at 0.5 in
- 8 cu ft
- Perimeter to form
- 56 lf
- Excavate to
- 8 in
The over-depth allowance is 13% of the field volume here — worth comparing against whatever percentage you were about to add by habit.
| Item | Order | Working |
|---|---|---|
| Stone sub-base | 4 tons | 2.96 yd³ — 4 in compacted, spread 5 in loose, at 2,700 lb/yd³ |
| Welded wire | 10 × 3.5 × 7 ft | 19.5 sq ft each after the 6 in lap, not 24.5 |
| Form lumber | 5 × 12 ft 2×4 | 56 lf of edge, 4 lf left over |
| Stakes | 23 | 3 ft centers plus the four corners |
| Control joints | 28 lf | 2 × 2 panels of 8 × 6 ft, cut 1 in deep |
Before you buy form boards: a 2×4 measures 3.5 in, not 4, so a slab formed off the top of one finishes 0.5 in thin. The board that really reaches 4 in is a 2×6. Set the boards proud of the grade, or dig the difference, or order for what you are actually building — form to the shallower board on a 4 in order and 13% of the load is left in the drum with the driver waiting.
Where this slab will crack
Joints every 10 ft at a factor of 30, giving 4 panels of 8 × 6 ft. At 1.33 : 1 the panels stay inside the 1.5 : 1 shape limit. Cut them 1 in deep, which is a contraction joint has to cut a quarter of the way through the slab to make the crack follow it, and cut early: conventional wet-cut sawing happens between 4 and 12 hours after placing, and early-entry saws start at 1 to 4.
Spacing, lap and joint figures are published guidance reproduced here as reference. What is required on your slab is decided by the local building department and, on anything structural, an engineer.
How to take off a concrete slab
Three decisions, and two of them have to be made before anything is dug.
Set the thickness from what will stand on it
Four inches carries people and furniture — a patio, a walkway, a shed floor. Five is the usual driveway. Six is where a loaded truck, an RV or a trailer jack goes, because those are point loads rather than spread ones. The preset buttons move the thickness, the turndown and the sheeting together, since in practice those three decisions are one decision about what the slab is for.
Decide the base, the edge and the sheeting before you dig
Those three set the excavation depth, and the panel reports it: slab thickness plus stone base in the field, plus the turndown depth around the perimeter. The turndown is worked out as a ring with the corners counted once, because the perimeter-times-width shortcut bills you for each corner twice. Polyethylene belongs under anything heated or finished and does not belong under an open patio, where it holds water against the underside instead of letting it drain away.
Read the take-off, then mark the joints before the pour
The table gives rolls, sheets, sticks, boards, stakes and linear feet of saw cut, each in the unit it is sold or measured in. Mark the joint lines on the forms while they are still visible: after the pour you are looking at a flat gray field with a saw running, and a joint that misses the grid by a foot is a crack that ignores it. The panel sizes and the cut depth come out of the same table.
Technical specifications
| Presets | Patio 4 in on 4 in of stone, shed floor 4 in with a turndown, driveway 5 in on 6, garage or RV pad 6 in |
|---|---|
| Stone sub-base | Tons and yards at 2,700 lb/yd³, ordered at the loose depth that compacts to the one you asked for |
| Turndown geometry | Ring area, perimeter × width less the four corner blocks — default 12 in wide by 12 in deep |
| Over-order allowance | Entered as average over-depth in inches, not a percentage; 1/2 in by default, and shown as its own line |
| Vapor sheeting | ASTM E1745 Class A at 10 mil, joints lapped 6 in; rolls worked out from strips across the short dimension |
| Reinforcement | Welded wire lapped a 6 in square, so a 3.5 × 7 ft sheet covers 19.5 ft² and not 24.5; or #3, #4 and #5 bars in 20 ft sticks with 40-diameter laps |
| Control joints | Spacing factor 24 to 36 × thickness, cut depth a quarter of the slab, panels held to 1.5 : 1 |
| Form lumber | Surfaced sizes, so a 2×4 forms 3.5 in — boards from 8 to 16 ft, stakes at 3 ft centers |
Frequently asked questions
How thick should a concrete slab be?
Four inches for a patio, walkway or shed floor, five for a driveway that sees cars, six where an RV, a loaded trailer or a delivery truck parks. Thickness is a response to point loads rather than to total weight: a car spreads its mass over four contact patches, while a trailer jack or a jack stand puts a thousand pounds on a few square inches, and that is what punches through four inches. Adding an inch to a 20 by 20 slab is 1.2 cubic yards more concrete, which is the cheapest structural upgrade on the whole job and far cheaper than cutting out a punched-through corner later.
How far apart should control joints be, and how deep?
Two to three times the slab thickness in inches, read as a spacing in feet — so 8 to 12 ft on a 4 inch slab and 12 to 18 on a 6 inch one. Cut a quarter of the way through, which is 1 inch on a 4 inch slab, and cut early: a conventional wet saw goes in between four and twelve hours after placing, an early-entry saw from one hour. Keep each panel within a 1.5 to 1 ratio, because a long thin panel cracks across its own waist no matter what the joints around it are doing.
Is welded wire mesh or rebar better in a slab?
Neither stops a slab cracking — both hold the crack shut once it happens, and they do it at different scales. Welded wire is enough for a residential floor or patio where the concern is a hairline opening up over the years; a rebar grid at 18 inches earns its place under vehicles and where the ground is likely to move. What ruins either is placement: steel lying on the subgrade is doing nothing at all, because it has to sit in the upper third of the slab to be on the tension side of a crack. Chairs cost pennies and are the difference between reinforcement and scrap metal in a floor.
Do I need a vapor barrier under a concrete slab?
Under anything heated, finished or floored, yes; under an open patio, no. Ground moisture climbs through a slab by capillary action and comes out at the top, where it lifts vinyl, clouds epoxy and rots the bottom plate of a stud wall. The specification most often cited is ASTM E1745 Class A at 10 mil, laid with joints lapped 6 inches and taped per ASTM E1643; plenty of houses are built on 6 mil instead, which is the same polyethylene at a thinner gauge and a weaker puncture rating. Outdoors the calculation reverses: sheeting under a patio traps water where the slab was relying on the ground to take it away.
How much extra concrete does a thickened edge take?
More than people expect — a 12 by 12 ft slab at 4 inches holds 48 cubic feet, and a 12 inch wide turndown going 12 inches down adds another 29, which is 61 percent on top. That is the number that quietly turns a one-truck pour into a two-truck pour, and it is missing from every calculator that multiplies area by thickness and stops. It is worked out here as a ring rather than as the perimeter times the edge width, because the shortcut counts all four corner blocks a second time.
Can I pour a slab straight onto dirt?
Onto undisturbed, uniform, well-drained soil people do it all the time; onto fill, clay or a mix of both it is how slabs break. What a slab needs from the ground underneath is not strength so much as sameness — a soft patch beside a hard one is what cracks it, because the slab has to span between them. A compacted layer of open-graded stone gives that uniformity, breaks the capillary path and gives water somewhere to go; its tonnage is the first line of the take-off above, ordered at the loose depth that settles to the depth you asked for.
When can I walk on a new slab, and when can I park on it?
Foot traffic after 24 to 48 hours, vehicles after seven days, and full design strength at 28 days by convention because that is when the test cylinders are broken. The seven-day figure is not arbitrary either: it is the period a slab is normally kept damp, and concrete that dries out early never reaches the strength the mix was designed for. In cold weather every one of those numbers stretches, since the reaction that hardens concrete slows down as the temperature falls and effectively stops near freezing.
About slabs: the edge, the steel and where it cracks
A slab is the one pour where the concrete is the simple part. Underneath and inside it there are four other orders — the stone that makes the ground uniform, the sheeting that stops moisture climbing through, the steel that holds a crack tight, and the boards that keep the edge straight while it sets — and each is bought in a unit of its own. That is the reason this page returns a list rather than a figure. It is also where the money hides: a 12 inch turndown around a modest floor adds more concrete than a whole extra inch of thickness would, and a calculator that multiplies area by depth never mentions it. If the shape you are pouring is a footing, a column, a set of steps or a run of post holes rather than a floor, the concrete calculator handles those geometries and their bag counts.
Every slab cracks. Concrete gives up something like four to eight hundredths of a percent of its length as it dries, which is about an eighth of an inch across a twenty foot bay, and there is nowhere for that movement to go except into a crack. Joints do not prevent it; they choose where it happens, by weakening a line and letting the slab break along it below the surface where nobody looks. Everything else follows from that. Cut a quarter of the way through or the crack wanders off the line. Cut early, because the slab does not wait for the saw. Keep panels squarish, because a long panel cracks across its middle. And put the steel high in the section, since a mesh lying in the mud is on the wrong side of the crack it was bought to close.
What goes under the slab decides how long it lasts. Uniform bearing matters more than strong bearing: a soft patch next to a firm one leaves the slab spanning a gap it was never designed to span, which is why a compacted layer of angular open-graded stone — priced by grade on the crushed stone calculator — is worth more than another inch of concrete. Rounded stone is the wrong material there: pea gravel rolls under load rather than locking together, and belongs in a path or a drain instead. The excavation also produces a pile that has to go somewhere, and the topsoil calculator turns that spoil into yards so you can size the skip or the heap. Once the quantities are settled, the concrete cost calculator adds the delivery, the short-load fee and the finishing labor at your own prices.
Your slab, your numbers, your browser
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — measurements, quantities, the prices your supplier quoted you — is uploaded, logged or kept, which is also why the calculators carry on working on a site with no signal.
The take-off you print or copy is generated on the spot from the fields above it. No version of it is stored, so coming back tomorrow means typing the dimensions again — which is the trade-off for nobody else ever having had them.