ConstructionCalc
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Concrete Footing Calculator

Calculate concrete volume for round or square post footings. Choose bags or ready-mix and get accurate material counts with cost estimates.

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Concrete Footing Calculator

Concrete Footing Volume Formulas

Footing volume depends on shape. Round tube footings are the most common for residential deck and fence post applications. Square footings are common for hand-dug shallow piers.

Round Footing: Volume (ft³) = π × (Diameter ÷ 2 ÷ 12)² × (Depth ÷ 12)
Square Footing: Volume (ft³) = (Width ÷ 12)² × (Depth ÷ 12)
Total Volume (yd³): Total ft³ ÷ 27
60lb bags: Total ft³ ÷ 0.45  |  80lb bags: Total ft³ ÷ 0.60

Footing Size Guide by Post Size

Post Size Min Footing Diameter Typical Depth 80lb Bags per Footing
4×4 post10–12 inches36–42 inches3–4 bags
6×6 post12–16 inches42–48 inches5–8 bags
Fence post8–10 inches24–36 inches1–2 bags
Mailbox post8 inches18–24 inches0.5–1 bag

Frost Line Depths by Region (USA)

Region Typical Frost Depth Example States
Southern US0–6 inchesFL, TX, LA, GA
Mid-Atlantic12–24 inchesVA, MD, NC, TN
Midwest36–48 inchesOH, IN, IL, MO
Northern US42–60 inchesMN, WI, MI, NY
Mountain West24–48 inchesCO, UT, MT, ID

Tips for Pouring Concrete Footings

  • Always check your local building code for frost depth — footings above frost line will heave and crack.
  • Use tube forms (Sonotubes) for clean, consistent cylindrical footings — they peel off after curing.
  • Add a few inches of gravel at the bottom of each hole for drainage before pouring.
  • For structural deck posts, embed a post base bracket into the wet concrete rather than burying the wood post.
  • When mixing bags, use the full water ratio specified on the bag — too much water weakens concrete significantly.
  • Allow concrete to cure at least 24–48 hours before loading the footing with any weight.

? Frequently Asked Questions

How deep should concrete footings be?
Footings must extend below the frost line — the depth at which the ground freezes in your region. In northern states this can be 48 inches or more; in southern states as little as 12 inches. Contact your local building department for the frost depth requirement in your area. Footings for decks attached to a house typically require 36–48 inches in most of the continental US.
How wide should concrete footings be?
Footing width should be at least twice the width of the post or wall it supports. For a 4×4 post, use a minimum 10–12 inch diameter tube. For a 6×6 post, use 12–16 inches. Load-bearing requirements and soil type also affect footing size — heavy loads or soft soils require larger footings.
How many 80lb bags of concrete does one footing need?
It depends on the size. A 12-inch diameter × 36-inch deep round footing requires about 1.96 cubic feet, which needs approximately 4 bags of 80lb concrete (each yields 0.60 cu ft). A 10-inch diameter × 48-inch deep footing needs about 1.8 cubic feet, or 3 bags of 80lb concrete.
Should I use tube forms or dig square footings?
Tube forms (Sonotubes) are ideal for post footings — they are fast, clean, and produce consistent round columns. Square hand-dug holes work for shallow footings on non-critical structures. For frost-depth footings supporting a deck or structure, always use tube forms to ensure consistent diameter and vertical alignment.
Can I mix my own concrete for footings or should I order ready-mix?
For fewer than 10 footings or small projects, bagged concrete (60lb or 80lb) is practical and economical. For large jobs with 20+ footings, or where footings exceed 12 inches in diameter, ordering ready-mix by the yard is faster and usually cheaper per cubic foot. The calculator provides both options.

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💡 Pro Tips

  • ✅ Always dig below frost line to prevent heaving
  • ✅ Add 3–4" of gravel at hole bottom for drainage
  • ✅ Use tube forms for consistent round footings
  • ✅ Order 10% extra bags for spillage and overfill