Concrete Slab Cost Calculator
Price a concrete slab from your own rates: ready-mix and the short-load fee, the gravel base, forms, reinforcement, placing and finishing, ending with the cost per square foot.
What a Concrete Slab Actually Costs to Pour
Enter the slab dimensions and your own rates and this works out the yardage to order, the gravel underneath, the form perimeter and the rebar in the grid, then puts your prices on all of it and divides by the area. What it will not print is a national average cost per square foot. That figure blends regions, thicknesses, finishes and new work against replacement, and quoting it to a customer means quoting somebody else’s costs. Watch two lines in particular: the short-load fee below your supplier’s minimum, and a pump where the truck chute will not reach the forms.
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QUANTITIES
How to Price a Concrete Slab
A concrete slab costs the ready-mix, plus what you do to the ground under it, plus the forms and the steel, plus getting the concrete from the truck into the forms and finishing it. Work out the cubic yards first, because that is the one line that comes straight out of the dimensions. Then price the rest separately, because on a small residential pour the concrete is often under half the invoice and a cost per square foot copied off a national average is somebody else’s job, not yours.
1. Get the cubic yards
Length times width times thickness in feet, divided by 27. A 20 by 20 slab at 4 inches is 400 x 0.333 = 133 cubic feet, which is 4.94 cubic yards. The calculator does it, but knowing the arithmetic is how you sanity-check any number a supplier or a tool gives you.
2. Add the over-order allowance
Ten per cent is the usual figure. Sub-grade is never perfectly flat and the truck cannot bring you the last quarter yard later: running short means a cold joint or a second delivery, both of which cost more than the extra concrete.
3. Check the short-load threshold before you quote
Suppliers charge a surcharge below a minimum load. Both the threshold and the fee are on their price sheet. This is the item that makes two small pours on separate days cost noticeably more than one larger pour, and it is worth combining a slab with a footing or a walkway when the schedule allows.
4. Price what goes underneath
Excavation depth, the compacted gravel base, the vapour barrier and hauling the spoil away. Gravel is the same arithmetic as the concrete: area times base thickness in feet, divided by 27. On a sloping or clay site this line can rival the concrete itself.
5. Price the forms and the steel
Forms are priced along the perimeter, so a slab with steps, curves or a thickened edge carries more form per square foot than a plain rectangle. For a rebar grid, count bars in both directions: the number of bars in each direction is one more than the number of gaps, which is the step people drop.
6. Add placing, finishing and access
Placing and finishing are labour against the surface area. Access is separate: a chute reaches roughly the length of the truck, and past that you are into buggies or a pump, and a pump carries a minimum charge that lands on a small job as hard as on a big one.
7. Divide by the area last
The cost per square foot is an output, not an input. It is the number you compare against your own past jobs to see whether this one is priced like they were.
Worked Example: a 20 x 20 Patio Slab
20 by 20 feet, 4 inches thick, 4 inch compacted gravel base, rebar grid at 16 inches both ways, 10% over-order. Quantities only — put your own rates against them.
| Item | Working | Result |
|---|---|---|
| Slab area | 20 x 20 | 400 sq ft |
| Concrete, measured | 400 x (4 ÷ 12) ÷ 27 | 4.94 cu yd |
| Concrete, to order | 4.94 x 1.10 | 5.43 cu yd |
| Gravel base | 400 x (4 ÷ 12) ÷ 27 | 4.94 cu yd |
| Form perimeter | (20 + 20) x 2 | 80 linear ft |
| Rebar, one direction | floor(20 ÷ 1.33) + 1 = 16 bars x 20 ft | 320 ft |
| Rebar, both directions | 320 x 2 | 640 linear ft |
| Short-load check | 5.43 cu yd against your supplier’s minimum | usually under it |
Worked Example: a 24 x 24 Driveway Slab
Same method, a job that carries vehicles: 5 inches thick, 6 inch base, rebar at 18 inches, and no truck access to the back of the pour.
| Item | Working | Result |
|---|---|---|
| Slab area | 24 x 24 | 576 sq ft |
| Concrete, measured | 576 x (5 ÷ 12) ÷ 27 | 8.89 cu yd |
| Concrete, to order | 8.89 x 1.10 | 9.78 cu yd |
| Gravel base | 576 x (6 ÷ 12) ÷ 27 | 10.67 cu yd |
| Form perimeter | (24 + 24) x 2 | 96 linear ft |
| Rebar, both directions | 17 bars x 24 ft, both ways | 816 linear ft |
| Placement | chute will not reach | pump, at its minimum charge |
Cubic Yards per 100 Square Feet
Multiply by your slab area in hundreds of square feet. This is the whole of the concrete arithmetic in one line.
| Thickness | Cubic yards per 100 sq ft | Typical use |
|---|---|---|
| 4 in | 1.23 | Patios, shed bases, walkways, interior slabs |
| 5 in | 1.54 | Driveways and anything carrying a vehicle |
| 6 in | 1.85 | Heavier loads, garage and workshop floors |
| 8 in | 2.47 | Structural slabs and heavy equipment bases |
Common Slab Sizes at 4 Inches
Measured volume before any over-order allowance.
| Slab | Area | Concrete |
|---|---|---|
| 10 x 10 ft | 100 sq ft | 1.23 cu yd |
| 12 x 12 ft | 144 sq ft | 1.78 cu yd |
| 16 x 20 ft | 320 sq ft | 3.95 cu yd |
| 20 x 20 ft | 400 sq ft | 4.94 cu yd |
| 24 x 24 ft | 576 sq ft | 7.11 cu yd |
| 30 x 40 ft | 1,200 sq ft | 14.81 cu yd |
The Lines That Decide Whether the Job Pays
None of these come out of the yardage, and all of them come out of your margin when they are left off the quote.
Excavation and spoil
Digging is only half of it. The soil has to go somewhere, and on a suburban plot with no room for a pile that means a trailer or a skip and the trips to empty it.
Access to the forms
Work out the truck route before you price the pour, not on the morning. A back garden with a side gate is a wheelbarrow job or a pump job, and the difference between those two is most of a day.
Control joints
Cut them early and to about a quarter of the slab depth. Cutting late is what produces the random crack the customer calls you about, and that call is unpaid work on a job you already closed.
Curing and the weather
Blankets in the cold, retarder and covering in the heat, and a pour that gets cancelled because of rain still costs you the day. On a driveway the cure also decides when the customer can park, which is a promise worth making carefully.
Washout
There is no legal free place to wash out a truck or a mixer. Somebody pays for it and it should be in the quote.
Where Estimates Go Wrong
Ordering exactly the calculated yardage
Sub-grade is never dead flat, and a slab that comes out 3% over the calculation leaves you short at the far end. A short pour finishes as a cold joint or waits for a second truck at a second delivery charge.
Forgetting the short-load fee on a small pour
Below the supplier’s minimum load the surcharge can be a large fraction of a small order. Quoting a two-yard pour at the per-yard rate is how that job loses money.
Pricing the pump as an afterthought
A pump has a minimum charge whether you use half an hour of it or a full day. On a small back-garden slab it can be one of the largest single lines, and it has to be decided before the quote goes out.
Treating the gravel base as optional
It is the cheapest insurance on the job and the first thing cut to win a bid. A slab poured on unprepared ground is a callback with your name on it two winters later.
Quoting a national cost per square foot
That figure blends regions, thicknesses, finishes, new work and replacement. Your own cost per square foot, out of your own rates, is the only one that describes the job you are about to do.
Concrete Slab Cost — Common Questions
How much does a concrete slab cost?
It is the ready-mix plus the base, the forms, the steel, the placing and the finishing, and on a small residential slab those together often exceed the concrete. Take the yardage from the calculator, put your supplier’s delivered price against it, then add each of the other lines separately. Two items catch people out: the short-load fee below your supplier’s minimum, and a pump where the chute cannot reach the forms.
How many cubic yards is a 20x20 slab?
At 4 inches thick, 4.94 cubic yards before any over-order allowance, or about 5.4 yards ordered at the usual 10%. The arithmetic is 400 square feet times a third of a foot, divided by 27.
What does a concrete driveway cost to pour?
The same lines as any slab, with three differences that raise it: the thickness goes to 5 or 6 inches because it carries vehicles, the base is deeper, and the reinforcement is usually a rebar grid rather than fibre or mesh. Access is often easier on a driveway than on a back-garden patio, which pulls the other way.
Should I include the gravel base in the quote?
Yes, as its own line. Showing it separately makes it much harder for a customer to compare your quote against one that quietly leaves it out, and it gives you something to point at when they ask why yours is higher.
How much rebar does a slab need?
That is an engineering question for anything structural or retaining. For the quantity at a given grid spacing, the calculator counts bars in both directions and remembers that each direction has one more bar than it has gaps.
Is a short-load fee negotiable?
Sometimes, and it is worth asking, but plan as if it is not. The reliable way around it is to combine pours so the order clears the supplier’s minimum in one delivery.
Why does the tool not show me an average price per yard?
Because a delivered yard costs different money in different markets and different months, and a figure printed here would be stale within a season. Your supplier’s current price sheet is a phone call away and it is the only correct number for your quote.
What is a reasonable over-order allowance?
Ten per cent on a straightforward rectangular slab. Go higher where the sub-grade is rough, where the forms are complicated, or where a second delivery would be genuinely difficult to arrange.
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