How This Calculator Works
The calculator multiplies your slab's length × width × thickness to get the exact volume, converts it to cubic yards (the unit ready-mix suppliers quote), and then applies your chosen waste factor. It also translates the total into 60 lb and 80 lb bag counts using the standard yields printed on the bag: a 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.60 cubic feet of mixed concrete.
The waste factor matters more than most first-time pourers expect. Subgrades are never perfectly flat, forms bow slightly, and some mix always stays in the mixer or wheelbarrow. Running out of concrete mid-pour forces a cold joint — a visible seam that is structurally weaker — which is why nearly every contractor orders 5–10% extra. Our default of 10% is the safe middle ground for DIY slabs.
Example: 10 ft × 10 ft Patio at 4 Inches
| Measure | Exact | With 10% waste |
|---|---|---|
| Cubic feet | 33.3 ft³ | 36.7 ft³ |
| Cubic yards | 1.23 yd³ | 1.36 yd³ |
| 60 lb bags | 75 | 82 |
| 80 lb bags | 56 | 62 |
At around 62 bags of 80 lb mix (roughly $430 in bags, plus a long day of mixing), a pour this size sits right at the crossover point where calling a ready-mix supplier for ~1.4 yards often costs a similar amount delivered — check both with our concrete cost estimator. For anything over about 1.5 yards, ready-mix nearly always wins on both price and quality.
Tip: Measure thickness at several spots after excavating. A slab planned at 4″ that averages 4.5″ over gravel needs about 12% more concrete — the most common reason DIY pours come up short.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
A 10×10 slab at 4″ thick is 33.3 ft³ of concrete. With 10% waste, that's about 82 bags of 60 lb mix or 62 bags of 80 lb mix. At that quantity, pricing ready-mix delivery for ~1.4 yards is usually worth it — see the cost estimator.
How thick should my slab be?
Patios, sidewalks, and shed bases are commonly 4″. Driveways and garage floors typically run 4–6″ depending on vehicle weight. See our slab thickness guide for recommendations by use, plus rebar and mesh basics.
What does a yard of ready-mix concrete cost?
Typically about $125–$170 per cubic yard delivered, depending on region, mix strength (PSI), and order size. Orders under 4–5 yards usually carry a short-load fee. Estimate your total with the cost estimator.
How do I calculate concrete for post holes or footings?
Round holes use the cylinder formula (πr² × depth). Our footing & post hole calculator handles both round sonotube pours and rectangular footings, including multiple holes at once.
Can I embed this calculator on my website?
Yes — grab the free copy-paste snippet on our embed page. It's popular with contractors, landscapers, and building-supply sites.