How this calculator works. Volume = area × thickness, plus your waste allowance (10% default — uneven subgrade always eats more than you think). Bag yields are the manufacturer-standard figures printed on Quikrete and Sakrete bags: 0.60 cu ft per 80-lb bag and 0.45 cu ft per 60-lb bag.
Bags or ready-mix truck?
This is the decision that matters more than the math. A cubic yard of concrete is 45 eighty-pound bags — each needing lifting, mixing, and placing before the previous batch stiffens. The practical rule: under half a yard (about 22 bags), bag it; over one yard, order ready-mix; in between, consider renting a mixer or ordering a short-load delivery, and be honest about your back and your helpers.
Slab thickness isn't a place to economize. Four inches is the residential standard for patios and shed pads; driveways need 5–6 inches with reinforcement. Thin slabs crack — and a cracked slab can't be topped up later.
Quick reference
| Project | Typical thickness | 1 cu yd covers |
|---|---|---|
| Patio, walkway, shed pad | 4 in | ≈80 sq ft |
| Driveway, garage floor | 5–6 in | ≈54–65 sq ft |
| Footings | per code (often 8+ in) | ≈40 sq ft |
Buying tips
Ready-mix suppliers quote per yard with a minimum-load fee — get the full quote, not just the per-yard rate. Order a quarter-yard more than calculated: coming up short mid-pour creates a cold joint, a permanent weak line where fresh concrete meets stiffened. For bags, buy on a dry day and store off the ground; a bag that's absorbed moisture is a rock.
Common questions
How many 80-lb bags make a yard?
45 bags. That number alone usually settles the bags-vs-truck question — mixing 45 bags by hand is a brutal day even with two people.
Do I need rebar or mesh?
Patios and pads: wire mesh or fiber-reinforced mix is typically enough. Driveways: rebar grid. Anything structural: check local code. Reinforcement controls cracking; it doesn't prevent it — control joints every 8–10 ft do the rest.
What about post holes?
Different calculation (cylinder volume, minus the post). As a quick rule, a standard 8"-diameter hole 2 ft deep with a 4×4 post takes about two-thirds of an 80-lb bag of fast-setting mix. A dedicated post-hole calculator is coming soon.
Can I pour over an existing slab?
Only with a bonded overlay product and a sound, clean base — regular concrete under 2 inches thick will fail. For cracked slabs, removal is usually the honest answer.