Concrete Slab Calculator — Yards, Bags and Rebar
Gives you cubic yards and bags side by side, so you can see at a glance whether to mix it yourself or order a truck. Add a section for an L-shaped slab.
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We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Measure the slab
Length × width. An L-shaped patio is two rectangles — add a section for each rather than averaging, because concrete is expensive to get wrong in either direction.
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Choose the thickness for the load
Four inches carries foot traffic, furniture and a shed. Five to six inches is needed wherever a vehicle will sit, and anything carrying a structure needs a proper footing designed for it, not just a thicker slab.
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Add ten per cent
Sub-bases are never perfectly level, and the low spots swallow concrete. Ten per cent is the trade standard. Running out halfway through a pour leaves a cold joint you cannot remove.
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Decide: bags or a truck
Around one cubic yard is the crossover. Below it, bags are practical. Above it you are mixing forty-five 80 lb bags for every yard, by hand, against a setting clock — order ready-mix instead.
Concrete for a 4 in. slab
With the 10% waste allowance shown in the last column.
| Slab | Area | Bare volume | 80 lb bags | Order with waste |
|---|---|---|---|---|
| 4 × 4 ft | 16 sq ft | 0.20 cu yd | 9 | 0.22 cu yd |
| 8 × 8 ft | 64 sq ft | 0.79 cu yd | 36 | 0.87 cu yd |
| 10 × 10 ft | 100 sq ft | 1.23 cu yd | 56 | 1.36 cu yd |
| 10 × 20 ft | 200 sq ft | 2.47 cu yd | 112 | 2.72 cu yd |
| 12 × 12 ft | 144 sq ft | 1.78 cu yd | 80 | 1.96 cu yd |
| 12 × 20 ft | 240 sq ft | 2.96 cu yd | 134 | 3.26 cu yd |
| 20 × 20 ft | 400 sq ft | 4.94 cu yd | 223 | 5.43 cu yd |
| 20 × 30 ft | 600 sq ft | 7.41 cu yd | 334 | 8.15 cu yd |
There are 45 × 80 lb bags in a cubic yard. Once you are past about 25 bags, ready-mix is both cheaper and far easier.
Thickness by use
| Use | Thickness | Sub-base | Reinforcement |
|---|---|---|---|
| Garden path | 3–4 in. | 4 in. compacted gravel | Fiber or light mesh |
| Patio, shed base | 4 in. | 4 in. compacted gravel | Welded wire mesh |
| Driveway, car parking | 5–6 in. | 6–8 in. compacted gravel | Mesh or #4 rebar grid |
| Garage floor | 6 in. | 6 in. compacted gravel | #4 rebar at 18 in. |
| Footing for a structure | Designed | Undisturbed soil | To the engineer's drawing |
A slab is only as good as what is under it. Concrete on soft or uncompacted ground cracks whatever its thickness.
How much concrete for a 10x10 slab?
A 10 × 10 ft slab at 4 in. thick is 33.3 cubic feet, or 1.23 cubic yards. Add 10% waste and you should order 1.36 cubic yards — call it 1.5, since suppliers rarely split finer than a half.
In bags that is 61 × 80 lb bags. Mixing that by hand is most of a day and the first batch has begun setting before the last is poured, so this is squarely a ready-mix job.
Bags or ready-mix?
The line is about one cubic yard, or 45 × 80 lb bags.
- Under a yard — bags. A mixer hired for the day handles it comfortably and there is no minimum delivery to pay.
- Over a yard — ready-mix, every time. It arrives mixed to a consistent strength, it is cheaper per yard, and it all goes in while it is still workable.
Most suppliers charge a short-load fee under three or four yards. Even with that, one truck usually beats sixty bags.
Get the sub-base right
More slabs fail from what is underneath than from the concrete itself. Four inches of compacted gravel under a patio, six to eight under a driveway, compacted in layers rather than dumped.
Concrete is strong in compression and weak in tension. Give it a soft spot to bridge and it cracks there, however thick it is.
Control joints are not optional
Concrete shrinks as it cures and it will crack. Control joints decide where. Cut them to a quarter of the slab depth — one inch on a four-inch slab — within 24 hours of the pour, in panels no more than about ten feet across.
Without them the slab still cracks, just wherever it likes, in a jagged line across the middle.
Finishing the surface
What you do in the hour after the pour decides how the slab looks for thirty years.
- Screed — drag a straight board across the forms to level it, working the concrete back and forth.
- Bull float — immediately, to push the stone down and bring paste up. Stop there and wait.
- Wait for the bleed water to go — this is the step people rush. Working the surface while water is still sitting on it traps that water and gives you a weak, dusty, flaking finish.
- Edge and cut the joints — run an edger around the perimeter, then cut control joints a quarter of the slab depth.
- Broom finish — drag a stiff broom across for grip. A smooth trowelled finish outdoors is lethal when wet.
Curing is not drying
Concrete does not dry out to gain strength. It hydrates — a chemical reaction that needs water to continue — and if the surface dries too fast that reaction stops before it has finished.
Keep the slab damp for at least seven days: cover it with plastic sheeting, mist it, or spray a curing compound. Concrete cured properly for a week reaches significantly higher strength than the same mix left to dry in the sun, and the surface is far less prone to dusting and scaling.
Pouring in extreme weather needs more care again — above about 90°F the mix stiffens before you can finish it, and below 40°F hydration nearly stops. Early morning in summer, midday in winter.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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area = length × width, summed over every sectionL-shaped slabs total correctly. -
volume = area × thickness ÷ 12Thickness in inches to cubic feet. -
× (1 + waste ÷ 100)An uneven sub-base swallows more than you expect. -
cubic yards = volume ÷ 27Ready-mix is always ordered by the yard. -
bags = volume ÷ bag yield, rounded up0.60 cu ft per 80 lb bag — 45 bags to a cubic yard. -
mesh sheets = area ÷ 42A 5 × 10 ft sheet covers 50 sq ft, less the overlap at the edges.
Common questions
How many bags of concrete in a cubic yard?
Forty-five 80 lb bags, sixty 60 lb bags, or seventy-two 50 lb bags. That is why anything over a yard is a ready-mix job.
How thick should a concrete patio be?
Four inches over a 4 in. compacted gravel base. Go to five or six inches anywhere a vehicle will drive or park.
How much concrete for a 12x12 slab?
At 4 in. thick, 48 cubic feet or 1.78 cubic yards. With 10% waste, order about 2 cubic yards.
Can I pour concrete straight onto soil?
You can, and it will crack. Soil holds water, moves with frost and settles unevenly. Four inches of compacted gravel costs very little and is the difference between a slab that lasts and one that does not.
Do I need rebar or mesh?
Mesh or fiber for a patio or shed base. Rebar at 18 in. centers for a driveway or garage floor. Reinforcement does not stop cracks forming — it holds the slab together once they do.
How long before I can walk on it?
Roughly 24 to 48 hours for foot traffic, seven days before anything heavy, and 28 days to full design strength. Keep it damp for the first few days — concrete cures by hydrating, not by drying out.
What is a short load fee?
A surcharge most suppliers add below three or four cubic yards, because the truck and driver cost the same whatever they carry. Even with the fee, ready-mix usually beats mixing sixty bags by hand.
How do I work out an L-shaped slab?
Split it into rectangles and add a section for each. The volumes are added and the waste applied to the total.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- Quikrete and Sakrete bag yields — cubic feet per bag and bags per cubic yard
- American Concrete Institute ACI 302 — slab thickness, control joint spacing and depth
- International Residential Code R506 — sub-base and slab requirements for residential work
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.