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08 / 13 · Concrete footings

Concrete Footing Calculator: Volume, Bags and Ready-Mix

Pad footings under the posts of a pergola, fence, carport or deck are often dug wider at the bottom than at the top. Enter the base size, depth and slope you were given, and get exactly how much concrete goes in — in bags of mix or as a ready-mix order.

Your sizes

Shape
Side slope given as
Bag of concrete mix

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Drawing

ground⌀ 12"⌀ 16"36"2"3.2°

Result

Concrete incl. reserve
0.82 yd³
Bags
37 × 80 lb
Per footing
3.36 ft³
All footings, net
0.75 yd³
Weight of dry mix
2,960 lb
Ready-mix to order
1 yd³
Digging (all footings)
0.72 yd³
Base
⌀ 16"
Top
⌀ 12"
Side slope from vertical
3.2°
Collar per footing
0.13 ft³

Bag counts use the yield you entered. Yield differs by product — the defaults are the manufacturer's figures for QUIKRETE Concrete Mix (metric: Baumit, about 0.5 l per kg), so check the bag you buy.

Digging volume is the part of the footing below ground, dug to its exact shape. Extra working room, a gravel bed or formwork are not included.

Depth and size depend on the frost depth, soil and load where you build. That is your responsibility — check local building rules or ask an engineer.

This calculator does quantities and geometry, not structural design.

How it works

Each footing is a frustum: a square or rectangular one is a prismoid, a round one a truncated cone. The volume uses the exact formulas — V = h/6 · (A₁ + A₂ + 4·Aₘ) for the prismoid and V = π·h/3 · (R² + R·r + r²) for the cone — not an average of the two ends.

You can give the slope as the top size at ground level, or as an angle from vertical. With an angle, each side moves in by depth × tan(angle), so the top is the base minus twice that.

An optional collar above ground is a straight block with the top size. It adds concrete but no digging.

The reserve is added to the total. Bags are rounded up from the yield per bag; the ready-mix amount is rounded up to the step your plant sells in.

What the calculator assumes

  • This is a quantity and geometry tool, not a structural design. Footing size, depth, reinforcement and anchors depend on the load, the soil and the frost depth where you build — follow the post base or kit instructions, local building rules or an engineer.
  • Sides are plane (straight from the base to ground level) and every footing is the same. The hole is dug to the exact shape of the footing; any extra width you dig is not included.
  • Bag yield differs by product. The defaults are from the QUIKRETE Concrete Mix data sheet (0.30 / 0.45 / 0.60 ft³ for 40 / 60 / 80 lb); check the bag you buy.

Questions

How many bags of concrete do I need for one post hole?
Divide the volume of the hole by the yield printed on the bag. A 12 in square hole 36 in deep holds 3 ft³ — that is 5 bags of 80 lb mix at 0.60 ft³ each, before any reserve.
How deep should a footing be?
Below the frost line where you live and as deep as the load requires. Frost depth varies a lot by region, so check your local building department or an engineer — the calculator uses whatever depth you enter.
Why make the footing wider at the bottom?
A wider base spreads the load over more soil while the top stays just big enough for the post anchor, which saves concrete. Whether a tapered footing suits your project is a question for the kit instructions or an engineer.

Planning aid only. It does not size structural members or check loads — follow the manufacturer's instructions and your local building rules.