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Reviewed · Prices checked October 2026 · How we calculate

Concrete Footing Calculator: Strip Footings and Pads

Choose a continuous strip footing or square pads and enter the size. You get the concrete in bags and cubic yards, whether ready-mix is cheaper, and a check against the residential code minimums.

Inputs

Footing
Units
ft

Add up every run of footing.

in
in
Bag size
Extra for waste and uneven trenches

Shopping list

Updates as you type
66bags

80 lb bags of concrete mix for 40 ft of footing

36 kg each, 5,280 lb to lift in all

  • 66 × 80 lb bags of concrete mix$330–$528
    39.11 cu ft (1.45 cu yd) with 10% extra ÷ 0.60 cu ft per bag = 65.2, rounded up.

    Typical price (estimate, checked Oct 2026): $5.00–$8.00 per 80 lb bag

    66 bags × $5.00–$8.00 = $330–$528Estimate
Project total (estimate where no price is entered)$330–$528
Checked Oct 2026

Bags or ready-mix? It depends on your prices: $330–$528 in bags vs $256–$375.50 for 1.5 cu yd of ready-mix with a short-load fee, with ready-mix usually ahead. Type your quotes below. Volume: 39.11 cu ft (1.45 cu yd).

  • Ready-mix, 1.5 cu yd$195–$262.50
    Ordered in quarter yards, at least 1 yd.

    Typical price (estimate, checked Oct 2026): $130.00–$175.00 per cu yd

    1.5 cu yd × $130.00–$175.00 = $195–$262.50Estimate
  • Short-load fee$61–$113
    Charged by most plants on small orders.

    Typical price (estimate, checked Oct 2026): $61.00–$113.00 per order

    1 order × $61.00–$113.00 = $61–$113Estimate
Show the math
40 ft × 16 in ÷ 12 × 8 in ÷ 12 = 35.56 cu ft35.56 cu ft × 1.10 (10% extra) = 39.11 cu ft39.11 cu ft ÷ 0.60 cu ft per bag = 65.2 → 66 bags ($6.50 each, typical)

Your prices are saved on this device only.

How it works

The formula

strip: volume = length × width × thickness
pads: volume = count × side × side × thickness
bags = volume × (1 + waste) ÷ yield per bag, rounded up

Assumptions

  • Code minimum (2021 International Residential Code, R403.1.1): footings at least 12 in wide and 6 in thick. The width a house actually needs depends on its weight and the soil (IRC Table R403.1(1)), and the bottom must sit below your local frost depth. Your building department has the final say.
  • Trenches are rarely perfectly square; 10% extra covers a ragged bottom and spillage.
  • Bag yields from the bag: 0.60 cu ft for 80 lb (QUIKRETE Concrete Mix). Big footings are usually cheaper and easier with ready-mix.

Worked example

40 ft of footing, 16 in wide, 8 in thick

  1. 40 × 16/12 × 8/12 = 35.56 cu ft (1.32 cu yd)
  2. With 10%: 39.1 cu ft ÷ 0.60 → 66 bags of 80 lb

Concrete by footing size

Concrete for strip footings, 10% extra
FootingCu yd80 lb bags
20 ft × 12 × 6 in0.4119
40 ft × 16 × 8 in1.4566
80 ft × 16 × 8 in2.90131
100 ft × 20 × 10 in5.66255

Questions people ask

How much concrete do I need for a footing?

Multiply length × width × thickness in feet. 40 ft of footing 16 in wide and 8 in thick is 35.6 cu ft, or 1.32 cu yd; with 10% extra that's 66 bags of 80 lb mix, or 1.45 cu yd needed, ordered as 1.5 cu yd of ready-mix.

How big should a footing be?

At least 12 in wide and 6 in thick under the residential code, but heavier houses and weak soils need wider footings, and they must go below the frost line. Deck and porch posts often sit on square pads, for example 24 × 24 × 12 in.

How many bags for a 2 × 2 ft pad?

A 24 × 24 in pad 12 in thick is 4.0 cu ft, so 8 bags of 80 lb mix with 10% extra.