Deck Footing Size Calculator
Our free deck footing size calculator tells you exactly how much concrete each footing needs. Enter your deck's length and width, the number of footings, and the footing diameter and depth. You'll get the concrete volume per footing in cubic feet, the total volume for all footings, and the number of 60-lb concrete bags to buy β always rounded up so you don't run short.
Enter your numbers
Your results
| Bag size | Per footing | Project total |
|---|---|---|
| 60-lb bags | β | β |
| 80-lb bags | β | β |
Dig below your local frost line β check local building code before you dig. Volumes are planning estimates.
How to use this calculator
Enter your deck's length and width in feet so the calculator can suggest a footing count, then enter the actual number of footings from your deck plans. Set the footing diameter and depth in inches β for example, a 6-inch sonotube dug 48 inches deep. Results update instantly as you type: per-footing volume in cubic feet, total volume for all footings, and the number of 60-lb bags, rounded up.
The suggested footing count is a planning estimate β one footing per 75 square feet of deck. If your plans or building department call for a different count or a larger diameter, use those numbers instead.
How it works
Each footing is treated as a cylinder: volume = Ο Γ rΒ² Γ h, where r is half the diameter and h is the depth, all in inches. That gives cubic inches, which are converted to cubic feet by dividing by 1,728 (the number of cubic inches in one cubic foot). The total is the per-footing volume times your number of footings.
Bag counts use standard bag yields β a 60-lb bag yields about 0.45 cubic feet and an 80-lb bag about 0.60 cubic feet β with bags = ceil(volume Γ· yield per bag), always rounded up. Results are estimates; actual yield varies slightly by mix brand. As a check: a 6-inch-diameter, 48-inch-deep footing holds Ο Γ 3Β² Γ 48 = 1,357.17 inΒ³ = 0.7854 ftΒ³, needing ceil(0.7854 Γ· 0.45) = 2 sixty-pound bags.
Frost-depth disclaimer: footings must extend below your area's frost line, which varies widely β check your local building code for the required depth before digging.
Frequently asked questions
How many concrete bags do I need per deck footing?
It depends on the footing diameter and depth. For a common 6-inch-diameter footing dug 48 inches deep, the volume is about 0.79 cubic feet, which works out to 2 sixty-pound bags (each yields about 0.45 cubic feet). Enter your exact diameter and depth above for your number.
What size footing does my deck need?
Footing size depends on your local building code, soil bearing capacity, and deck load. A common residential deck footing is 8 to 12 inches in diameter. Because requirements vary by location, check your local building code or your approved deck plans before you dig.
How deep should deck footings be?
Footings must extend below the local frost line so freeze-thaw cycles cannot heave them β often 36 to 48 inches deep in northern climates and 12 to 24 inches where the ground rarely freezes. Your local building department publishes the frost depth for your area; always check it before digging.
How many footings does a deck need?
A common rule of thumb is one footing for every 50 to 75 square feet of deck, spaced no more than 8 feet apart along each support beam. The exact count depends on joist span, beam size, and local code, so confirm against your plans. The calculator above suggests a footing count from your deck area.
Should I use 60 lb or 80 lb concrete bags for footings?
Both contain the same concrete mix. An 80-lb bag yields about 0.60 cubic feet and a 60-lb bag about 0.45 cubic feet, so 80-lb bags mean fewer bags to buy, carry, and mix for deep footings. This calculator reports 60-lb bag counts; for 80-lb bags, divide the 60-lb count by 1.33 and round up.