Small concrete jobs are where estimating goes wrong most often. A single fence post hole seems trivial, but a run of twenty posts turns into a pile of bags you did not plan for. A deck footing looks like one bag until you realise it is a cylinder two feet deep. The way to get these jobs right is to think in cubic feet for each hole, then scale up.
Cylinders: posts and footings
Most post holes and deck footings are cylinders. The volume of a cylinder is π × radius² × depth. A hole 10 inches across and 30 inches deep has a radius of 5 inches, which is 0.417 ft, and a depth of 2.5 ft:
3.14 × 0.417² × 2.5 ≈ 1.36 cubic feet
That is the volume of the hole. If a 4 by 4 post occupies about 0.15 cubic feet of it, the concrete you actually need is a little over 1.2 cubic feet per post.
Bags: how far do they go?
A typical 60 lb bag of pre-mixed concrete yields roughly 0.45 cubic feet, and an 80 lb bag yields about 0.6 cubic feet. Read the label because yields vary by brand. At 0.6 cubic feet per bag, one post needs about two bags, and twenty posts need about forty. Knowing that before you go to the store also tells you whether you need help carrying them.
Rectangular pads
For a small pad, such as a base for an air conditioner or a shed corner, multiply length × width × thickness, all in feet. A pad 3 ft × 3 ft × 4 inches (0.333 ft) is 3 cubic feet, or about five 80 lb bags.
Add a margin
Holes are never perfectly round or straight. Add 10 percent to the volume. When you are mixing by hand, add a little more because some concrete stays in the wheelbarrow and on the tools.
Depth and frost
The depth of a footing is a structural and local-code question. In cold climates footings must reach below the frost line, which changes the volume dramatically. Check your local requirements before you dig, and treat the numbers here as a way to order material, not as design advice.
Mixing tips
- Use clean water and follow the bag's water ratio.
- Mix to a stiff, oatmeal-like consistency.
- Work in small batches so the mix does not set before you place it.
- Brace posts plumb and level before pouring.
- Keep the concrete damp for a few days to cure.
Converting for ready-mix
Above about a cubic yard (27 cubic feet), a delivery truck usually becomes cheaper and easier than bags. Twenty posts at 1.3 cubic feet each is 26 cubic feet, which is close enough to a yard that a small delivery might make sense.
Double-check the volume
If you would rather not do the cylinder maths on paper, a cubic feet calculator for concrete accepts dimensions in inches or feet and returns cubic feet, cubic yards and bag counts. It is a helpful check on your own arithmetic before you order.
A worked example for a fence
For a six-post fence with holes 10 inches across and 30 inches deep, each hole holds about 1.35 cubic feet (π × 5² × 30 ÷ 1728 ≈ 1.36). Six holes need about 8.2 cubic feet, and with a 10 percent margin about 9 cubic feet. Minus the space taken by the post itself, you may need a little less, but rounding up avoids a second trip to the store.
Summary
Work out the volume of one hole, multiply by the number of holes, add a margin and divide by the bag yield. Doing this once at the kitchen table turns a vague guess into a shopping list.