Concrete Footing Calculator

Calculate the volume, number of bags and cost of concrete for pad, strip or pier footings — with built-in 60 lb, 80 lb and 40 kg bag yields.

Footing details

Footing type
ft
ft
ft
12 in = 1 ft · 30 cm = 0.3 m
%
10 % covers spillage and over-excavation on most jobs.
$

Calculation results

Volume to order

— yd³

Bags needed

— bags

Footing volume (no waste)

— yd³

Bag yields are taken from manufacturer data (Quikrete, Sakrete). Order one extra bag per 50 to cover spillage on a hot, dry day.

Informational only. These results are estimates for planning and ordering. For critical, load-bearing or life-safety applications, consult a licensed professional who can verify site conditions, code requirements and material specifications.

Understanding the formula

Three short calculations turn a footing drawing into a concrete order. Volume comes from the geometry of one footing times the count, the waste factor adds the extra you need to order, and the bag yield turns volume into a bag count.

V = geometry × count · ordered = V × (1 + waste %) · bags = ⌈ ordered / yield ⌉

Geometry is L × W × D for a pad, L × W × D for a strip (where L is the total wall length), and π × (d/2)² × D for a pier. Cost is bags × price-per-bag when buying bags or ordered × price-per-yd³ when ordering ready-mix.

Worked example

Four 4 × 4 × 1 ft pad footings, 10 % waste, 80 lb bags:

  • Volume per footing = 4 × 4 × 1 = 16 ft³
  • Total = 16 × 4 = 64 ft³ ≈ 2.37 yd³
  • Ordered = 64 × 1.10 = 70.4 ft³ ≈ 2.61 yd³
  • Bags = ⌈ 70.4 / 0.60 ⌉ = 118 bags

At about 2.4 yd³, this job is in the grey zone where ready-mix usually beats bagged concrete on price — most suppliers charge a small minimum fee but mixing 118 bags by hand is a full day’s work for two people.

When to use this calculator

It’s built for the three footing types that cover almost every residential and light-commercial job:

  • Pad footings: isolated columns, deck posts, equipment pads, lally columns under a basement beam.
  • Strip footings: continuous wall footings under foundation walls, garage walls, retaining walls.
  • Pier footings: Sonotube pours under decks, sheds, fence posts, pergolas, mailbox posts.

The calculator does not size footings for bearing capacity — that needs a soil report and a structural engineer. It also doesn’t subtract the volume taken up by anchor bolts, rebar cages or column stubs, but those are usually a rounding error against the waste factor.

Common mistakes & tips

  • Always order extra. Concrete is unforgiving — running short means a cold joint or a half-cured pour. The 10 % waste factor is conservative; on a complex pour with multiple steps, push it to 12–15 %.
  • Round up bag count. The formula already rounds up, but consider buying one extra bag per 25 to absorb a torn bag or a dropped wheelbarrow.
  • Pier diameter, not radius. Sonotubes are sold by inside diameter — enter the diameter directly, the calculator handles the geometry.
  • Mind the units in the bag select. 80 lb (≈ 36 kg) bags are the US retail default. 40 kg bags are sold mostly outside the US — pick the unit your supplier actually stocks.
  • Switch to ready-mix above 1 yd³. A single yard of bagged concrete needs 60 × 80-lb bags and a couple of hours of mixing. Above that, the truck almost always wins on cost and quality.
  • Strip footings: use the wall length. Don’t enter the perimeter twice — for an L-shaped wall, sum the two legs once.

Frequently asked questions

How many 60 lb bags of concrete are in a cubic yard?
A 60 lb bag yields about 0.45 ft³, so a cubic yard (27 ft³) takes 60 bags. An 80 lb bag yields 0.60 ft³, so a yard takes 45 bags.
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags per cubic yard. An 80 lb bag yields 0.60 ft³ of mixed concrete; 27 ÷ 0.60 = 45.
How accurate is the bag yield?
The 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb) values come straight from Quikrete and Sakrete data sheets. The 40 kg yield (0.019 m³) is interpolated from manufacturer 80 lb and 90 lb data points — verify the exact yield printed on the bag you buy.
When should I switch from bags to ready-mix?
Most pros switch to ready-mix above 1 yd³ (about 60 × 80-lb bags). Below that, bagged concrete is cheaper and avoids minimum delivery fees. Between 1 and 3 yd³, compare the local quotes both ways.
How deep should a pier footing be?
Below the local frost line. In the southern US 12 in is enough; in the northern US and Canada you may need 36–48 in. Check with your local building department — frost depth is code-mandated.
Why does the calculator default to 10 % waste?
Concrete loses volume to spillage, over-excavation of the form, and small irregularities in the bottom of the hole. 10 % is the industry consensus middle of the 5–10 % range and works for most residential jobs. Bump it to 15 % for complex pours.
Can I use this for a slab on grade?
Yes — pick "Pad" and enter the slab length, width and thickness, with a count of 1. For very large slabs (above 100 yd³) consider using a dedicated slab calculator that accounts for thickenings, screed allowance and edge forms.
Does the calculator include rebar or mesh?
No — concrete only. For rebar use the Rebar Calculator, which estimates bar count, length and weight based on grid spacing.