Question 1

How many bags of concrete do I need for a 10×10 slab?

Accepted Answer

A 10×10 ft slab at 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards. With a 10% buffer, you need roughly 37 cubic feet of concrete. That works out to about 46 bags of 80lb mix, or 62 bags of 60lb mix. At this volume, ordering ready-mix is worth pricing out — it's typically cheaper than 46 bags once you factor in your time.

Question 2

What's the difference between 60lb and 80lb concrete bags?

Accepted Answer

Both are the same concrete mix — the only difference is how much you're lifting. An 80lb bag yields about 0.60 cubic feet; a 60lb bag yields about 0.45 cubic feet. For small jobs, 60lb bags are easier to manage solo. For larger slabs, 80lb bags reduce the number of bags you have to move and mix. Either works.

Question 3

How thick should a concrete slab be?

Accepted Answer

Walkways and patios: 4 inches is standard. Driveways with regular passenger car traffic: 4 to 5 inches. Driveways that see trucks or heavy equipment: 5 to 6 inches. Shed floors and non-load-bearing slabs: 3.5 to 4 inches. Footings and foundations vary by local code — check with your municipality before pouring anything structural.

Question 4

When should I use ready-mix concrete instead of bags?

Accepted Answer

Once you're above about 1 cubic yard (27 cubic feet), ready-mix is usually worth pricing. Mixing bagged concrete by hand is slow and tiring at scale, and the consistency suffers as you rush later batches. Ready-mix arrives already mixed and you just place it. For post holes and small footings under half a yard, bagged concrete — especially fast-setting mix — is the practical choice.

Question 5

How do I calculate concrete for post holes?

Accepted Answer

Post hole volume is a cylinder: π × radius² × depth. In practice, a 10-inch diameter hole 2 feet deep is about 1.1 cubic feet — roughly 2 bags of 80lb mix per hole. The calculator handles this automatically. If you're using fast-setting concrete, you can pour it dry into the hole and add water without mixing — it cures in 20 to 40 minutes.

Question 6

Should I add extra concrete for waste?

Accepted Answer

Yes — we build in a 10% buffer automatically. Concrete sticks to tools, spillage happens, and forms are rarely perfectly square. The 10% buffer covers normal losses. On post holes especially, the soil shifts and holes are never perfectly cylindrical, so the actual volume consumed is usually higher than the geometric calculation.

Question 7

What's the formula for concrete volume?

Accepted Answer

For slabs: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet. For cylindrical post holes: π × (diameter in inches ÷ 24)² × depth (ft) = cubic feet. Divide cubic feet by 27 for cubic yards. Divide by bag yield (0.45 for 60lb, 0.60 for 80lb) for bag count. Round up and add 10%.

Question 8

Can I mix bagged concrete with just water?

Accepted Answer

Yes — bagged concrete (like Quikrete or Sakrete) comes dry and just needs water. Use the minimum water needed to make it workable: too much water weakens the finished concrete. The mix should be thick and hold its shape when squeezed, not runny. Mix in a wheelbarrow or tub with a hoe, or use a rented electric mixer for larger volumes.

Bag Weight	Yield (cu ft)	Bags per Cu Yd	Best For
40 lb	0.30	90	Small repairs, tight spaces
60 lb	0.45	60	Post holes, smaller footings
80 lb	0.60	45	Slabs, driveways, large footings

Concrete Calculator

Calculate Concrete Needed

How the Concrete Calculator Works

Bag Yield Reference

Worked Examples

Pro Tips

Concrete & Finishing Supplies

Frequently Asked Questions