Worked example
A 12 inch tube, 48 inches deep: radius 6 inches, so the volume is π × 6² × 48 cubic inches, which is π cubic feet, about 3.14. Divide by 0.60 and one hole wants about 5.24 bags. Buy 6 if you are filling one hole with nothing spilled. Four holes are about 20.94 bags. Add 10% and you are at about 23.04, so the page opens on 24 bags.
What this is not
It is not a frost-depth map, a soil bearing value, or a post size. A deck footing that is too small fails even if you bought the right number of bags for the hole you dug. Ask the building department how deep and how wide the footing has to be before you order concrete.
Joist layout is a different shopping list. See joist spacing basics and the board calculator for the lumber on top of the frame.
Common questions
How many 80 lb bags are in a 12 inch by 4 foot tube?
A 12 inch diameter tube, 48 inches deep, holds pi cubic feet, about 3.14. An 80 lb bag of ordinary concrete mix that yields 0.60 cubic feet covers that hole in about 5.24 bags, so you buy 6 bags for one hole with no waste. Four holes at 10 percent waste come to about 23.04 bags, so you buy 24. Read the yield on your bag. Fast-setting mixes often yield less.
Does this calculator set the footing depth?
No. You type the depth. Frost line, soil, load, and the local code decide how deep and how wide a footing has to be. This page only turns a round hole you already sized into a bag count.
What about a belled footing or a gravel base?
They are not in the cylinder. A bell at the bottom adds concrete. A few inches of gravel at the bottom replaces concrete. Measure the concrete depth, not the hole depth, if they differ, or raise the waste percent to cover a bell you have not drawn.