Concrete Slab Calculator
Estimate the cubic yards of concrete, or the number of 60-lb and 80-lb bags, for a flat slab such as a patio, shed base or garage pad.
Calculator
| Cubic yards (exact) | 1.23 yd³ |
|---|---|
| Cubic feet | 33.3 ft³ |
| 80-lb bags of concrete mix | 62 bags |
| 60-lb bags of concrete mix | 82 bags |
How it is calculated
10 ft × 10 ft × 4 in ÷ 12 = 33.3 cubic feet. Divide by 27 to get 1.23 cubic yards, then add 10% for spillage and uneven ground: 1.36 cubic yards.
About this calculator
A concrete slab is the flat poured base under a patio, shed, garage floor or walkway. Its volume is simple geometry: length × width × thickness. The amount you actually order should be a little higher than the exact figure, because forms are never perfectly tight and the ground under the slab is never perfectly level.
Concrete is sold by the cubic yard when a truck delivers it, and by the bag (commonly 40, 50, 60 or 80 lb) at home centers. Bags are convenient for small jobs. For larger jobs a ready-mix truck is usually more practical, but suppliers often set a minimum load or add a short-load fee for small orders, so ask before you book.
Worked examples
| Project | Cubic yards to order (+10% waste) | Rough cost |
|---|---|---|
| a 10x10 ft slab, 4 in thick | 1.36 yd³ | $190 – $272 |
| a 12x12 ft slab, 4 in thick | 1.96 yd³ | $274 – $391 |
| a 10x20 ft slab, 4 in thick | 2.72 yd³ | $380 – $543 |
| a 20x20 ft slab, 6 in thick | 8.15 yd³ | $1,141 – $1,630 |
Buying notes
- Check the yield printed on the bag. This calculator assumes about 0.6 cubic feet per 80-lb bag and 0.45 per 60-lb bag, which is typical, but yields differ by brand and product.
- Gravel base, reinforcement (rebar or wire mesh), forms and finishing tools are not included in the concrete estimate.
- Mixing dozens of bags by hand is hard work. When the bag count gets large, compare it with the price of a ready-mix delivery.
Quick answers by size
Click a size for the exact quantity and cost.
How to measure and use this calculator
- Measure the length and width of the area in feet. For an L-shape, split it into rectangles and add the results.
- Decide the thickness. Patios and shed pads are commonly 4 inches; check local code for anything structural.
- Enter the three numbers. The calculator converts to cubic feet, then cubic yards, and adds waste.
- Compare ready-mix and bagged concrete. Bags are easier for small jobs; a truck is cheaper per yard for larger ones.
Common mistakes
- Measuring thickness from the dirt instead of from the top of the compacted base.
- Forgetting that forms bow outward and the subgrade is rarely perfectly flat, so the pour takes more than the math.
- Ordering exactly the calculated amount. Running short mid-pour leaves a cold joint.
Tips
- Order 5–10% extra: a slightly uneven subgrade can swallow more concrete than the math says.
- Ready-mix is cheaper per yard than bagged mix once you need roughly 1 cubic yard or more; below that, bags are easier.
- Compact the gravel base first. Thickness is measured from the top of the base, not the dirt.
Frequently asked questions
How thick should a concrete slab be?
Patios and shed pads are commonly poured 4 inches thick. Driveways and heavy loads usually call for 5–6 inches or more, so check local building requirements.
How many 80-lb bags make a cubic yard?
With a typical yield of about 0.6 cubic feet per bag, roughly 45 bags make one cubic yard (27 ÷ 0.6). Use the yield printed on your bag.
Should I order extra concrete?
Yes. Allowing 5–10% for an uneven subgrade, spillage and slightly bowed forms is common. This calculator adds 10%.
Does a slab need a gravel base?
Often yes. A compacted gravel base helps drainage and reduces cracking, but requirements depend on soil, climate and local code, so confirm locally.