Concrete Footing Calculator
Footings are measured in inches — 16 in. wide, 8 in. deep — while concrete is sold in yards. Enter the strip below and this concrete footing calculator bridges the two, bags included.
Concrete needed
1.32 yd³
35.6 ft³
Estimated cost
$217
at $165 per yd³
Approx. weight
4,729 lb
≈ 2.4 tons
Bagged mix
60
80-lb bags
80
60-lb bags
119
40-lb bags
Before you order: add about 10% for uneven subgrade and spillage — that's 1.45 yd³ or 66 × 80-lb bags.
Formula
A continuous footing is a long, thin box — length × width × depth:
volume (ft³) = length (ft) × [width (in) ÷ 12] × [depth (in) ÷ 12] cubic yards = volume (ft³) ÷ 27
For round piers formed in tubes, the sonotube calculator applies the cylinder formula instead.
Example
- A 40 ft wall footing, 16 in. wide × 8 in. deep: 16 ÷ 12 = 1.333 ft, 8 ÷ 12 = 0.667 ft.
- Volume: 40 × 1.333 × 0.667 = 35.6 ft³.
- Cubic yards: 35.6 ÷ 27 = 1.32 yd³ — order about 1.5 yd³ with the 10% allowance.
- Bags: 35.6 ÷ 0.60 = 59.3 → 60 × 80-lb bags (66 with waste).
- Cost at $165 per yard: 1.32 × 165 ≈ $218.
Frequently Asked Questions
How wide and deep should a footing be?
Most codes want footings at least 12 inches wide and as deep as the local frost line — commonly 36–48 inches in cold climates. Width often equals the wall thickness plus 4 inches on each side. Your local building department's table wins over any rule of thumb; run its numbers through this calculator.
How much concrete does a 40 ft footing take?
At 16 inches wide and 8 inches deep: 35.6 ft³, or 1.32 cubic yards — 60 of the 80-lb bags. With the 10% allowance, order about 1.5 yards or 66 bags.
Do footings need rebar?
Almost always two or three #4 rebar runs lengthwise, on chairs so they sit in the middle of the pour. Reinforcement doesn't change the volume, but it belongs on the materials list before the truck is booked.
Can I pour a footing with bagged concrete?
Below roughly one cubic yard, yes — that's about 45 bags of 80-lb mix, feasible for a short wall or a shed footing. Longer runs justify ready-mix: one continuous pour is what footings want, and trucks make that easy.