Skip to content
Blueprint Calc

Concrete Footing Calculator

Size concrete for continuous footings under walls, fences, sheds, and post footings. Handles the L × W × D math and converts to yards, 60 lb bags, or 80 lb bags.

How much concrete for a footing?

Length in feet; width and depth in inches.

You need
  • Concrete volume
    0.72 cu yd
    17.78 cu ft + 10% waste
  • 60 lb bags
    44 bags
    60 lb bag yields 0.45 cu ft
  • Ready-mix truck
    Order 1.00 cu yd
    Most suppliers require a 3 cu yd minimum. Below that, expect a $50-200 short-load fee — or ask for a volumetric (mobile-mix) truck which delivers as little as 0.5 yd.

What you'll need to buy

Prices approximate · Links go to retailer search results
Estimated total: ~$256 (materials, before tax)

Product links go to Home Depot search results — pick the best match for your project. Prices shown are rough averages.

How we calculated it

A continuous (strip) footing is a long rectangular prism:

Volume (cu ft) = length (ft) × (width in ÷ 12) × (depth in ÷ 12)

A footing 20 ft long × 16" wide × 8" deep = 20 × 1.33 × 0.67 ≈ 17.8 cu ft ≈ 0.66 cu yd — about 30 bags of 80 lb concrete mix.

Post footings (for decks, fences, columns) are typically cylinders — use the Post / Sonotube mode on the main concrete calculator for those.

Critical: footings must bear on undisturbed or properly compacted soil and extend below the local frost line. Getting this wrong causes the most common DIY structural failure — heaving, settling, cracking. Check your jurisdiction's minimum frost depth and get a permit if the structure is anything larger than a small shed.

Frequently asked questions

How deep should concrete footings be?
Below the frost line — typically 36"-48" in northern climates, 12"-18" in southern ones. Building codes specify minimums by county. Undermined footings are the #1 cause of deck and shed collapse in freeze-thaw regions.
Do I need rebar in a concrete slab?
Yes, for anything larger than a stepping stone. Either 6x6 wire mesh laid mid-depth or #4 (1/2") rebar on a 16-24" grid. Reinforcement does not stop cracking — it holds cracks tight when they happen.
When should I use bags vs a ready-mix truck?
Under 0.5 cubic yards (about a dozen 80 lb bags): bags. 0.5-2 yards: bags still workable but painful; ready-mix suppliers usually charge a $50-200 short-load fee below their 3 cu yd minimum, or a volumetric / mobile-mix truck can deliver as little as 0.5 yd. Above 3 yards: call for a standard ready-mix truck — cheaper per yard than bags and way faster.
How long does concrete take to cure?
Initial set: 24-48 hours (walkable). Usable for light loads: 7 days (~70% strength). Full design strength: 28 days. Driveways should not see vehicles for at least 7 days. Keep concrete damp for the first week — faster drying = weaker finish.
Can I pour concrete in cold weather?
Down to 40°F is workable with hot water in the mix and insulating blankets. Below 40°F, concrete stops curing and can freeze before it sets, ruining the pour. Most ready-mix suppliers add a cold-weather admixture in winter.
How many 60 lb bags of concrete per yard?
One 60 lb bag yields 0.45 cubic feet. 27 / 0.45 = 60 bags per yard. Lighter to carry but you mix more of them.
How many 80 lb bags of concrete per yard?
One 80 lb bag of Quikrete yields 0.60 cubic feet. At 27 cubic feet per yard, you need 45 bags of 80 lb to make one yard. That is ~3,600 lbs of mixing — which is why most people switch to a ready-mix truck above 2-3 cubic yards.

Other calculators

Paint calculator
Gallons of paint and primer for any room.
Drywall calculator
Full material list for any room.
Tile calculator
Tile count, boxes, thinset, grout, spacers.
Mulch calculator
Yards, bags, and bulk delivery estimate.