BuildCalcs

Concrete Footing Calculator — Strip Footings & Pier Pads

Who this is for: For deck builders and foundation crews working off a frost-depth spec — footing trenches and pier bells where the volume is small but the bag-vs-truck decision still matters.

Continuous footings and round pier pads: enter dimensions and get cubic yards plus bag counts, sized for deck footings and frost-depth trenches.

Quick answer: Strip footing volume = length × width × depth (all in feet) ÷ 27. A 12-inch-wide, 12-inch-deep trench runs 0.37 yd³ per 10 feet — about 17 eighty-pound bags. Piers: π × r² × depth.

Footing shape
ft
in
in
in
in
%

10% standard · 15% broken forms

Yards + 10% waste
0.49
exact: 0.44 yd³
80-lb bags
23
with waste included
Cubic feet
13.2
exact: 12 ft³
60-lb bags30
40-lb bags45
Ready-mix order (rounded up ¼ yd³)0.5 yd³
Bag yields per manufacturer published coverage (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³). Pours over ~1 yd³ usually cost less from a ready-mix plant. Ballpark tool — confirm quantities with your supplier.
Core facts
Strip formulaL ft × W in ÷ 12 × D in ÷ 12 ÷ 27 = yd³
Pier formulaπ × (D/2)² × depth = ft³
Typical deck pier12" dia × 48" deep ≈ 0.12 yd³
Default waste10%

How does the footing calculator work?

A continuous footing is just a long, narrow slab: trench length × width × depth, with width and depth in inches converted to feet. Round piers use the circle formula — π × radius² × depth. Both feed the same engine as the slab calculator, so you get cubic yards plus the 80/60/40-lb bag conversion with your waste factor applied. Typical deck footings run 10–12 inches wide and to frost depth (anywhere from 12 inches in warm regions to 48+ inches up north — check your local code); a single 12-inch pier 48 inches deep is only about 0.12 yards, which is why footings are almost always bag work.

Common uses

  • Sizing deck pier bells one hole at a time
  • Estimating a trench run of continuous footing
  • Deciding bag count before renting a mixer
  • Verifying volume against the engineer's footing schedule

Frequently Asked Questions

How wide should a footing be?
Common residential guidance is twice the wall or post thickness, with 10–12 inches a typical minimum for deck footings — but the bearing capacity of your soil can change it, and an engineer's schedule always outrules of thumb.
How deep do footings go?
Below your local frost line for anything attached to a structure — that ranges from 12 inches in the warmest zones to 48+ inches in the northern US. Your building department publishes the exact depth; digging shallower risks frost heave.
How many bags for one pier?
A 12-inch-diameter pier 4 feet deep is about 0.12 yd³ ≈ 3.1 ft³ → 6 eighty-pound bags with 10% waste. Most crews buy by the pallet (42 bags) and count leftovers at the end and count leftovers at the end.
Do footings need rebar?
Most codes want reinforcement in structural footings — two continuous #4 bars is a common minimum for strip footings, and pier bells often take a cage or a lot-style anchor. Follow your detail sheet; the rebar calculator on this site sizes the grid.

More concrete calculators

Concrete Slab Calculator
Enter slab length, width, and thickness — get cubic yards, 80-lb bag count, and a cost ballpark. Built for shed pads, patios, and garage floors.
Concrete Bag Calculator
Work backwards: enter the volume you need in cubic yards or feet and get exactly how many 80, 60, or 40-lb bags to buy — pallet math included.
Rebar Calculator
Slab rebar grid: enter dimensions, spacing, and bar size — get bar count per direction, total linear feet, 20-ft sticks, and weight.