Quick reference sizes
An 8×8 ft slab at 4 in thick is about 0.8 yd³ — roughly 45 eighty-pound bags with 10% waste. A 10×10 ft pad at the same depth is about 1.23 yd³ before waste — that is where ready-mix starts to win on labour.
Post holes are a different shape but the same idea: diameter × depth gives volume, then divide by bag yield. Three 12-inch diameter holes 3 ft deep are still bag territory.
60 lb vs 80 lb bags
Eighty-pound bags cover more ground per trip from the truck. Sixty-pound bags are easier to lift if you are working alone. The label on the bag is the yield that matters — brands differ by a few percent.
Round up. Running out mid-pour with concrete setting in the mixer is worse than returning one unused bag.
When to stop buying bags
Past ~1 yd³, compare bag cost plus your time against a ready-mix quote with a short-load fee. Many homeowners underestimate how long hand-mixing 50+ bags takes.
Use the Concrete Bag Calculator for your exact footprint, then the Concrete Cost Calculator if you want to line it up against a truck price.
Use a calculator for this
Concrete Bag Calculator
Bag counts for 60 lb, 80 lb, or 25 kg concrete on small pours.
CalculateConstructionConcrete Calculator
Work out how much ready-mix concrete a slab, footing, or column needs.
CalculateConstructionConcrete Cost Calculator
Ready-mix volume with materials, labour, and pump fees.
CalculateFrequently asked questions
How many bags for a 12×12 slab?
At 4 in thick with 10% waste, about 74 sixty-pound bags or 56 eighty-pound bags. Thicker driveways need proportionally more.
Can I mix different bag brands?
Same strength and type on one pour — do not blend fast-set with standard mix in the same slab.
Does this include the gravel base?
No. Order sub-base gravel separately.