Concrete Pad Calculator
Pouring a concrete slab for a shed, patio, or garage? Enter slab dimensions and thickness to calculate total cubic yards of ready-mix concrete or 80lb bags needed.
Results
Slab volume, plus a margin you will use
A pad is a simple rectangular volume, and the only trick is that the two dimensions you measure are in feet while the one that matters most is in inches.
Formula: cubic yards = length × width × (thickness ÷ 12) ÷ 27
This calculator then adds 10%. That margin is not padding for the sake of it: subgrade is never perfectly flat, forms bow slightly under the weight of wet concrete, and a slab poured 4 inches nominal ends up averaging closer to 4.5 in the low spots. Running short mid-pour is the one failure that cannot be fixed afterwards — a cold joint through the middle of a slab is a permanent weakness.
Thickness by what will sit on it
| Use | Thickness | Reinforcement |
|---|---|---|
| Sidewalk, light pad | 3 to 4 in | Fiber mesh is enough |
| Shed, patio | 4 in | 6 × 6 welded wire mesh |
| Driveway, cars | 5 to 6 in | Mesh, or #3 rebar at 18 in |
| RV, trucks, workshop | 6 in+ | #4 rebar at 12 to 16 in |
Every slab wants a compacted gravel subbase under it — 4 inches is standard, and it is what this calculator estimates. Its job is drainage: water held under a slab freezes, expands and cracks it from below. In cold climates the gravel matters more than an extra inch of concrete would.
Bags or a ready-mix truck
An 80 lb bag yields 0.6 cubic feet, so 45 bags make a cubic yard. A 60 lb bag yields 0.45 cubic feet, or 60 bags per yard. Those numbers are the argument against bagging anything large: a 12 × 10 ft pad at 4 inches is 40 cubic feet, or 1.48 cubic yards before the safety margin and 1.63 after it. That is 67 bags of 80 lb — over 5,300 lb of material to carry, open and mix by hand.
The practical ceiling for bags is about 1 cubic yard, and even that is a long day with a mixer. Above it, order ready-mix. Most suppliers have a 1 yard minimum and charge a short-load fee below 3 to 4 yards, but a truck delivers a single continuous pour, which is both less work and a structurally better slab than sequential batches that skin over between loads.
Control joints, and why slabs crack anyway
Concrete shrinks as it cures, and shrinkage cracks are not a defect — they are certain. What you control is where they go. Cut control joints one quarter of the slab depth deep, spaced in feet at roughly 2 to 3 times the thickness in inches: a 4 inch slab wants joints every 8 to 12 feet. The slab then cracks along the joint, out of sight, instead of wandering across the surface.
Cure it slowly. Concrete gains strength by hydration, not by drying, so keep the surface damp for the first several days — cover it with plastic or mist it. A slab that dries fast in sun and wind reaches noticeably lower strength and crazes on the surface.
Frequently asked questions
How thick should a concrete pad for a shed be?
Four inches over a 4 inch compacted gravel base, with 6 × 6 welded wire mesh. Go to 5 or 6 inches for anything that vehicles drive onto, and 6 inches or more with #4 rebar for an RV pad or a workshop floor.
How many 80lb bags of concrete are in a cubic yard?
Forty-five, since an 80 lb bag yields 0.6 cubic feet and a cubic yard is 27. A 60 lb bag yields 0.45 cubic feet, so it takes 60 of those. That weight is the reason bags stop making sense much above one cubic yard.
When should I order ready-mix instead of bags?
Above about 1 cubic yard. A 12 × 10 ft pad at 4 inches needs 67 bags of 80 lb — more than 5,300 lb to haul and mix. A truck also delivers one continuous pour, which is structurally better than sequential batches that skin over between loads. Expect a 1 yard minimum and a short-load fee under 3 to 4 yards.
Why add 10% to the concrete volume?
Because subgrade is never perfectly level and forms bow under the weight of wet concrete, so a nominal 4 inch slab averages closer to 4.5 in the low spots. Running short mid-pour is the one mistake that cannot be corrected — the cold joint it leaves is a permanent weak line through the slab.