How Many Bags of Concrete Do I Need?

The yields, and they're unusually clean

Bag	Yields	Per cubic yard
40 lb	0.30 cu ft	90 bags
60 lb	0.45 cu ft	60 bags
80 lb	0.60 cu ft	45 bags

A cubic yard is 27 cubic feet, and 27 ÷ 0.60 = 45 exactly. Worth memorising — it's the fastest sanity check available on a concrete order.

Note how much less volume a bag makes than its weight suggests. Most of that weight is aggregate.

Read the weight, not the bag count

One cubic yard in 80 lb bags is 3,600 pounds. Nearly two tons — to buy, transport, lift, and mix one bag at a time.

That's the number that should decide the job. Ready-mix delivered by truck is usually cheaper and always easier somewhere around the one-yard mark, and often well below it once you price your own weekend.

Bags are right for: post holes, small pads, steps, repairs. Bags are wrong for: a driveway.

People generally discover which category they're in on the day, with a mixer running.

Waste isn't spillage. It's the ground

5–10% is normal, and the big contributor isn't what you'd guess.

Spillage and mix left in the barrow account for a little. The excavation accounts for most of it: a slab specified at 4 inches that actually averages 4½ needs 12% more concrete than the arithmetic says — and nothing about the site tells you in advance.

On a rough subgrade, the high end of that range isn't pessimism.

You cannot pause a pour

This is the part that makes concrete different from every other material on this site.

Concrete begins setting as soon as it's mixed. A slab poured in two sessions an hour apart has a cold joint through it — a permanent plane of weakness where the two pours never bonded.

So running short isn't an inconvenience here. It's a structural defect.

Buy the extra bag. Unopened bags usually go back to the store; a cold joint can't be undone.

The same logic applies to help: you need enough mixing capacity and enough hands to pour the whole thing in one session. Work that out before you buy, not after.

More water is weaker concrete

The water figure in the calculator is for planning how much to have on hand — roughly 3 quarts per 80 lb bag. It is not an instruction.

Mix to the ratio on the bag. Extra water makes concrete easier to work and measurably weaker, and it's the single most common way a home pour ends up under strength.

If it feels stiff, mix it longer rather than wetter.

Leftover bags don't keep

Cement absorbs moisture from the air. A bag left in a damp garage over a winter can set solid inside its own sack — or worse, look perfectly fine and cure weak.

Store what's left sealed, dry and off the floor (a pallet or shelf, not the concrete). And treat a bag that's gone hard as rubbish rather than a bargain. Weak concrete costs far more than a bag does.

Shapes that aren't rectangles

The calculator does length × width × thickness. For anything else:

• Cylinder (post hole, pier): π × radius² × depth
• Irregular: break it into rectangles and add them

Print the order sheet — it has a per-section table and a checklist whose first item is is this really a bag job?

Sources: Concrete Toolkit — bag yield · MyConcreteCalc — bags to yards