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Concrete Footing Calculator

Calculate concrete for rectangular footings — deck piers, wall foundations, stem walls, and spread footings. Enter length, width, and depth to get instant volume and bag estimates.

Concrete Calculator

Select your project type, enter dimensions, and get instant results with bag counts, weight, and cost.

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How to Calculate Concrete for Footings

The footing formula is: Volume = Length (ft) × Width (ft) × Depth (in) ÷ 12. Footings are wider than the wall or post they support — typically twice the wall width — and must extend below the frost line in your area to prevent frost heave from pushing the structure upward during winter freeze-thaw cycles.

Unlike slabs which are poured on grade, footings sit deep in the ground and serve as the structural anchor for everything above them. Getting the volume right is critical because you need a continuous pour — cold joints in footings compromise structural integrity.

Types of Concrete Footings

Different structures require different footing designs. The three most common for residential projects are:

  • Spread footings (continuous) — Long, narrow footings under walls. Distributes the wall load over a wider area. Typically 16–24 inches wide and 8–12 inches deep. Used under foundation walls, garage walls, and retaining walls.
  • Pier footings (isolated) — Square or rectangular pads under individual posts. Common sizes are 2×2 ft or 3×3 ft at 12 inches deep. Used under deck posts, pergola columns, and fence corners where extra support is needed.
  • Stepped footings — Footings on sloped ground that step down following the terrain. Each step is level, with the total length broken into multiple level sections. More complex to calculate — use multiple zones in our calculator.

Standard Footing Dimensions by Project

ApplicationWidthDepthConcrete NeededNotes
Deck pier (single)2×2 ft12"4 cu ftBelow post base
Residential wall (20 ft)16"8"17.8 cu ft2× wall width
Garage (24×24 perimeter)20"12"160 cu ftContinuous around perimeter
Retaining wall (20 ft)24"12"40 cu ftExtra wide for lateral loads
Addition (16×20 perimeter)20"12"120 cu ftCheck local code

Footing Depth: Frost Line Requirements by Region

Your footing bottom must sit below the frost line. If it doesn't, water in the soil beneath will freeze, expand, and literally push your entire structure upward (frost heave). This causes cracking, tilting, and structural failure over time.

RegionStatesFrost Depth
Deep SouthFL, LA, S. TX, HI0–6 inches
SouthernGA, AL, MS, AZ, S. CA6–12 inches
Mid-AtlanticVA, NC, TN, KY, MO12–24 inches
Northern MidNY, PA, OH, IN, IL, KS30–42 inches
NortheastCT, MA, NH, VT, ME42–54 inches
NorthernMN, WI, ND, MT, AK48–72 inches

Note: Always verify with your local building department. Frost line depths can vary significantly even within the same county based on elevation and soil type.

Worked Example: Garage Perimeter Footing

Project: 24×24 ft garage, 20" wide footing, 12" deep

  1. Calculate perimeter: (24 + 24 + 24 + 24) = 96 linear feet
  2. Convert width: 20" ÷ 12 = 1.667 ft
  3. Calculate volume: 96 × 1.667 × 1 = 160 cubic feet
  4. Convert to yards: 160 ÷ 27 = 5.93 cubic yards
  5. Add 10% waste: 5.93 × 1.10 = 6.52 cubic yards
  6. Order: 7 cubic yards (round up for truck delivery)

At this volume, ready-mix delivery is the only practical option — 6.5 yards would be 293 bags of 80lb concrete.

Footing Construction Tips

  1. Dig to undisturbed soil — the bottom of the trench must be on original, compacted earth. Never pour on fill dirt or recently disturbed ground.
  2. Level the bottom — the footing bottom should be flat and level. Uneven bottoms create thin spots that can crack under load.
  3. Place rebar before pouring — set horizontal bars on chairs 3 inches from the bottom. Tie vertical bars that will extend into the wall above.
  4. Pour continuously — a footing must be a single monolithic pour. Schedule your ready-mix truck(s) to arrive with enough overlap to avoid cold joints.
  5. Don't forget anchor bolts — for walls, set J-bolts every 4–6 feet while concrete is wet. These attach the sill plate to the footing/foundation.
  6. Protect from rain — cover fresh footings with plastic if rain is expected within 24 hours. Washing dilutes the cement paste on the surface.

Footing Calculator FAQ

How deep should a concrete footing be?
The bottom of your footing must sit below the frost line for your region. In the northern US, that's 42–72 inches deep. In the south, it may be as little as 6–12 inches. The footing itself (the concrete pad) is typically 8–12 inches thick on top of that. Always verify with your local building department — they'll tell you the exact depth required for a permit.
How wide does my footing need to be?
The rule of thumb is twice the width of the wall above it. An 8-inch foundation wall needs a 16-inch footing. A 10-inch wall needs 20 inches. Retaining walls need wider footings (2.5–3× wall width) to resist lateral soil pressure. Deck piers are typically 2×2 ft square for standard 4×4 or 6×6 posts.
Do footings need rebar reinforcement?
Yes. Building codes require at least two continuous horizontal #4 rebar in residential footings. The bars should sit on chairs about 3 inches from the bottom. For footings under retaining walls, add vertical rebar at 24-inch spacing that extends up into the wall. Pier footings for decks may not strictly require rebar, but it's recommended for crack prevention.
Can I pour a footing in sections?
It's not recommended. Footings should be a continuous, monolithic pour for maximum structural integrity. Cold joints (where new concrete meets cured concrete) create weak points. If you absolutely must pour in sections, leave a rough, keyed surface and install dowel rebar across the joint.
How long should a footing cure before building on it?
Wait at least 2–3 days before stripping forms and 7 days before placing significant load (like pouring a wall on top). Full 28-day strength isn't required before continuing construction, but the footing should reach 50%+ of its design strength. Keep it moist for the first week.

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