Concrete Slab Calculator
Cubic yards and bags of concrete for a slab, patio, or pad.
4 in is standard for patios and walkways, 5 to 6 in for driveways.
You need
4.07 cubic yards
Waste included. Order this amount from a ready-mix supplier.
- Volume with waste
- 110.0 cu ft
- Volume before waste
- 100.0 cu ft
- 80 lb bags
- 184 bags
- 60 lb bags
- 245 bags
Estimates only. Verify quantities before ordering.
How this is calculated
Volume = length (ft) x width (ft) x thickness (in) / 12, giving cubic feet. Divide by 27 to get cubic yards, then add your waste percentage. Bag counts assume an 80 lb bag yields 0.60 cu ft and a 60 lb bag yields 0.45 cu ft.
Enter the length and width of your slab in feet and the thickness in inches, and this calculator returns the volume of concrete you need in cubic yards, plus the equivalent number of bagged concrete mixes. Ready-mix suppliers sell by the cubic yard, so that is the number to give them when you call.
A waste factor is built in because some concrete is always lost to uneven subgrade, spillage, and over-excavation. 10% is a sensible default for a typical slab on grade. If your base is well compacted and formed precisely, you can drop it to 5%. For rough or sloped ground, use 15%.
Concrete for common slab sizes
Every row includes a 10% waste factor. Order the cubic-yard figure from a ready-mix supplier, or use the bag count for hand mixing.
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 8 x 8 ft, 4 in (shed pad) | 0.87 cubic yards | 40 bags |
| 10 x 10 ft, 4 in (patio) | 1.36 cubic yards | 62 bags |
| 12 x 12 ft, 4 in | 1.96 cubic yards | 88 bags |
| 10 x 20 ft, 4 in (parking pad) | 2.72 cubic yards | 123 bags |
| 20 x 20 ft, 4 in | 5.43 cubic yards | 245 bags |
| 24 x 24 ft, 6 in (garage) | 11.73 cubic yards | 528 bags |
Choosing a mix strength
Concrete is specified by compressive strength in psi at 28 days. A 3,000 psi mix suits most residential flatwork: patios, walkways, and shed pads. Driveways and garage floors are usually 4,000 psi, both for the extra strength and because higher-strength mixes are more resistant to freeze and thaw damage and to de-icing salts. Bagged mixes from a home center are typically around 4,000 psi already.
Air entrainment matters more than most people realize anywhere that freezes. Entrained air creates microscopic bubbles that give freezing water somewhere to expand into, and without it a slab exposed to winter will scale and flake at the surface within a few years. If you order ready-mix in a cold climate, ask for it specifically; it is standard for exterior work but worth confirming on the ticket.
Resist the urge to add water on site. Water beyond the design mix is the single fastest way to weaken concrete, and a soupy mix that spreads easily can lose a substantial share of its strength along with gaining shrinkage cracks. If the mix is too stiff to place, the answer is a plasticizer from the truck rather than a hose.
Reinforcement: mesh, fiber, or rebar
Reinforcement does not stop concrete cracking; it holds the crack closed and keeps the two sides working together. Welded wire mesh is the traditional choice for residential slabs and needs to sit in the middle third of the slab thickness, supported on chairs. Mesh lying on the ground because someone walked it flat during the pour contributes essentially nothing.
Fiber reinforcement is mixed into the concrete and distributed throughout, which controls the fine shrinkage cracking that appears as the slab cures. It is convenient and cheap, and it does not replace steel where structural capacity is needed. For a patio it is often enough on its own; for a driveway it is usually specified alongside mesh or bar rather than instead of it.
Rebar is used where loads are heavier or spans longer, typically a grid of number 3 or number 4 bar tied at intervals and supported off the subgrade. Whatever you use, the subgrade beneath it does more work than the reinforcement does: 4 inches of compacted gravel over undisturbed soil prevents far more cracking than any amount of steel laid over soft fill.
Control joints and curing
Concrete shrinks as it cures and will crack. Control joints decide where. Cut or tool them to about a quarter of the slab depth, spaced in feet at roughly two to three times the slab thickness in inches, so a 4 inch slab wants joints every 8 to 12 feet. Keep the panels between joints close to square, since long thin panels crack across the middle regardless of the joint spacing.
Timing matters for tooled joints, which go in while the concrete is still plastic, and for saw cutting, which is usually done within 6 to 18 hours. Cut too early and the edges ravel; too late and the slab has already cracked where it wanted to.
Curing is about keeping water in, not letting concrete dry out. Concrete gains strength through hydration, a chemical reaction that needs moisture, and a slab left to dry in sun and wind can lose a significant fraction of its potential strength in the first week. Cover with plastic, wet burlap, or a curing compound and keep it damp for at least three days and preferably seven. This costs almost nothing and is the most commonly skipped step in a DIY pour.
Common questions
How thick should a concrete slab be?
4 inches is the standard for patios, walkways, and shed pads. Driveways that carry cars should be 5 to 6 inches, and slabs that will support heavy vehicles or equipment are often 6 inches or more with reinforcement. Local code may set a minimum, so check before you pour.
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag yields about 0.60 cubic feet, so a full cubic yard (27 cubic feet) takes about 45 bags. That is why bagged concrete only makes sense for small pours: beyond roughly one cubic yard, ready-mix delivered by truck is usually cheaper and far less work.
Should I order ready-mix or buy bags?
As a rule of thumb, use bags for anything under about 1 cubic yard (a small pad or a few fence posts) and ready-mix for anything larger. Mixing 40 or more bags by hand is exhausting and makes it hard to finish the slab before the first batches start to set.
Do I need rebar or wire mesh in a slab?
For most residential slabs 4 inches thick, welded wire mesh or fiber-reinforced mix controls cracking well. Driveways and thicker structural slabs usually call for #3 or #4 rebar on a grid. Reinforcement does not stop concrete from cracking, but it holds the cracks tightly together.