Cylinder math, solved
Pick the tube, set the height, get the bags. The spare is counted and the yard-threshold warning is built in.
| Bag size | Exact | Buy (with spare) |
|---|
Buy column includes a 10% spare margin, rounded up. Past a yard or so, price out ready-mix — your back will thank you.
The tube's diameter comes from load; its height comes from winter. The bottom sits below your frost line on undisturbed soil, and the top stands proud of grade so water sheds away from the post hardware. Get the line for your state, add the reveal, and the preset chips do the rest.
A 12″ tube takes about 0.79 cubic feet per foot of height — so a 4 ft pier is roughly 3.1 cubic feet, six 80 lb bags with the spare. The presets above cover 8″, 10″, 12″, and 16″ tubes at any height.
Bottom of the footing to a few inches above grade. That's your frost depth, plus the footing pad if you're using one, plus 4–6″ of reveal above ground to keep the post base dry. Your frost depth is the load-bearing number — look it up by state.
On decent soil, backyard piers often bear straight on the tube's own base at depth. On weaker soil or bigger loads, a wider pad (or a belled form) spreads the weight — the footing size calculator tells you what area the soil demands.
Past a cubic yard or so — call it forty-five 80 lb bags — a ready-mix truck beats the mixer on price, consistency, and your spine. The readout above shows yards for exactly that decision.