Concrete Footing Calculator

How it's calculated

The calculator finds the volume in cubic feet: length (ft) × width (ft) × depth (in ÷ 12) — where "depth" here means the footing's own poured thickness, not how far below grade it sits (see the depth guidance below for why those are different numbers). That's converted to cubic yards by dividing by 27. A waste percentage — higher than the slab calculator's, since a dug trench is rarely as uniform as a graded surface — is added on top, and the adjusted cubic yardage is multiplied by concrete's density to get the weight in tons. Bag counts divide the adjusted cubic feet by each bag's yield; ready-mix truckloads divide the adjusted cubic yards by the truck's capacity.

Worked example

A 40 ft run of strip footing, 16 inches wide and 8 inches deep — typical dimensions for a residential foundation wall: volume = 40 × 1.33 × (8 ÷ 12) = 35.56 cubic feet. In cubic yards, that's 35.56 ÷ 27 = 1.32 yd³. Adding a 15% waste factor for trench over-dig: 1.32 × 1.15 = 1.51 yd³. At 2.02 tons per cubic yard, that's 1.51 × 2.02 = 3.06 tons — or 2.95–3.07 tons across the full 1.95–2.03 density range. That's well past the roughly half-cubic-yard point where ready-mix delivery is the better call, though in 60 lb bags the adjusted 40.88 cu ft (35.56 × 1.15) would still take 91 bags if mixed by hand.

How deep should it be?

"Depth" on this calculator means the footing's own thickness — the height of the concrete itself — which is not the same number as how far below grade the footing sits, and mixing the two up is one of the most common footing mistakes. Footing thickness is set by the structural load above it and your soil's bearing capacity: a heavier structure or a softer soil needs more concrete to spread that load without settling. That's a sizing calculation your local building department's footing tables (or the International Residential Code's, if your jurisdiction uses it) make for your specific situation — this calculator doesn't guess at it. Separately, the footing's bottom has to sit below the local frost line so seasonal freeze-thaw doesn't heave it, and frost depth is set locally too, varying by region. Isolated pier or post footings (a deck support, for example) are often poured in round forms — switch this calculator to the circle shape and enter the tube's diameter as the width to size one. This calculator's job is converting dimensions you already have into a volume and weight; it isn't a substitute for your local code's footing tables or a structural engineer's sizing.

Common mistakes

• –Confusing footing depth (the poured concrete's own thickness) with frost depth (how far below grade the footing bottom must sit) — they're different numbers, and code cares about both separately.
• –Sizing footing width and depth by eye instead of a code table or engineer. Footing dimensions are a load-bearing calculation, not a rule of thumb, especially for anything beyond a single deck post.
• –Skipping rebar or reinforcement where code requires it. This calculator estimates concrete volume only — footings often need continuous rebar that isn't part of this number.
• –Underestimating trench over-dig. A hand- or machine-dug trench rarely holds a clean, uniform width and depth the way a graded slab surface does, which is why the default waste factor here runs higher than the slab calculator's.
• –Pouring onto loose or disturbed soil at the trench bottom instead of undisturbed, compacted ground — footings need firm, stable bearing to avoid settling later.

Where the density figure comes from

Unlike the landscape materials on this site, concrete is a manufactured product with a tightly controlled mix, so its density varies far less than gravel, sand, or mulch — and the same figure applies whether it's poured as a slab, a footing, or any other normal-weight element. CEMEX, a major ready-mix producer, puts a solid unbroken pour at about two tons (4,000 lbs) per cubic yard. ACI 318 and ASTM C138 define normal-weight concrete at 145–150 lb/ft³ — ACI 318 uses 150 lb/ft³ as its default structural value — which works out to 3,915–4,050 lbs (1.96–2.03 tons) per cubic yard. The 1.95–2.03 ton/yd³ range here reflects that normal-weight span, not moisture variation. Lightweight and heavyweight concrete mixes fall outside this range and aren't covered by this calculator. We use 2.02 tons/yd³ as the display figure, drawn from:

• CEMEX — Ready-Mix Concrete Calculator
• ACI 318 & ASTM C138 — normal-weight concrete density, 145–150 lb/ft³

Read more in our density methodology.

FAQ

How deep should a footing be?

It depends on the load the footing carries and your soil's bearing capacity — your local building department's footing tables (or the International Residential Code's, if it applies in your jurisdiction) size that for your specific situation. Separately, the footing has to sit below your local frost line, which is also set regionally. There's no universal number here; check with your building department before you dig.

What's the difference between footing depth and frost depth?

Footing depth is how thick the poured concrete itself is — the number this calculator uses. Frost depth is how far below grade the bottom of that footing needs to sit so seasonal ground freezing doesn't heave it. The two are set independently by different factors (structural load and soil bearing for one, regional climate for the other), so a footing's thickness never tells you how deep to dig — check both separately with your local building department.

Do I need rebar in a footing?

Most footings do, especially continuous strip footings under a foundation wall — check your local code for bar size and spacing. This calculator only estimates the concrete volume; it doesn't account for reinforcement, which is a separate part of the material list and the structural design.

How do I calculate concrete for a round post or pier footing?

Switch the calculator above to the circle shape, then enter the footing's diameter as the width and its depth as usual — the math works the same as a rectangular footing, just with circular geometry instead of length × width. Your plans or local code specify the diameter and depth for your post or pier; this calculator just converts whatever numbers you're given into a volume and weight. Most single post footings come out well under half a cubic yard, small enough that a few bags are the practical choice over ready-mix.

Should I use bags or ready-mix for a footing?

It depends on scope. A few isolated post or pier footings are usually small enough that bags make sense on their own. A continuous footing around a full foundation, like the worked example above, adds up fast — past roughly half a cubic yard, ready-mix delivery is generally cheaper and far less labor than mixing bag after bag by hand.

Why does concrete's weight vary so little compared to gravel or sand?

Concrete is a manufactured product with a tightly controlled mix ratio, so its density is far more consistent than a quarried or organic material's. The range on this site comes from ACI 318 and ASTM C138's normal-weight concrete specification (145–150 lb/ft³), the same figures used for the slab calculator, not from moisture swings the way gravel, sand, or mulch vary.

How many ready-mix truckloads will I need?

Divide your adjusted cubic yardage by the truck's capacity — standard ready-mix trucks carry about 10 cubic yards per full load. The calculator above uses 10 cubic yards as a default and rounds up, since a partial load still requires dispatching a truck, and most plants charge extra for small or split loads.