Volume first — then bags
Estimating bags is really estimating volume, then dividing. Every bag prints its yield — the finished cubic feet it makes once mixed — right on the label: 0.60 ft³ for an 80 lb bag, 0.45 for a 60, 0.30 for a 40. Work out the cubic feet of your pour, divide by the yield of the bag you're buying, and round up. That's the whole calculation. The reason a calculator helps isn't the arithmetic; it's that the volume step is where the two easy mistakes hide.
The first is mixing units. Slab and footing thickness gets measured in inches, but length and width in feet — pour four inches of concrete and it's a third of a foot deep, not four. Multiply straight through without converting and your volume comes out twelve times too big. This calculator keeps thickness in inches and does the conversion for you, but if you're checking by hand, that's the number to watch.
The second is rounding the wrong way. You can't buy three-quarters of a bag, so every partial bag rounds up to a whole one — which is why the waste allowance matters more for bagged concrete than for ready-mix. A pour that pencils out to 61.2 bags isn't 61 bags; it's 62, and that's before you account for what spills off the mixing board or disappears into a subgrade that isn't dead level. Ten percent over the net volume is the common starting point; open, uneven, or over-excavated ground wants more.
Where the bag count jumps
Slab thickness moves the bag count the most, because it multiplies the whole area — going from a 4-inch patio to a 6-inch driveway slab is 50% more concrete over the same footprint. Post holes are the opposite trap: people size the concrete to the post, but the concrete fills the hole, minus the post that displaces it, so an over-dug hole quietly doubles the bags. And yields are near-identical across the big brands — Quikrete and Sakrete both publish 0.60 ft³ for an 80 lb bag — but a fast-setting or high-early product can differ, so it's worth a glance at the bag you're actually holding.
Post depth is deliberately not selected here. Quikrete’s Setting Posts in Concrete guide says ⅓–½ of the above-ground post length plus 6 inches of gravel, while local frost, soil, wind, and product instructions may require more. The post-hole mode keeps depth user-entered and applies that number directly; the related Fence Calculator adds a clearly labeled conservative planning baseline when the fence layout itself is being estimated.
One judgment call the numbers can't make for you: when to stop bagging. As a rule of thumb, not a spec, once a pour crosses about a cubic yard — call it 45 to 60 bags — the mixing time and the pallet cost start losing to a short-load ready-mix delivery. Run your total here first; if it's climbing toward that range, price a truck before you buy the pallet. The longer walkthrough — including the worked 10×10 slab and why post depth is conservative — is in How Many Bags of Concrete Do You Actually Need.