ποΈ Concrete Footing Calculator
Calculate how much concrete you need for round or square footings.
of 60-lb concrete mix
How to Use This Calculator
Choose round (tube form) or square footing shape, enter the diameter or dimensions, set the depth, and enter how many footings you need. The calculator figures out the concrete volume per footing and total bags required, including a waste factor for spillage and overpour.
Select round for Sonotube or tube-form footings. These are standard for deck posts. Select square for spread footings under foundation walls or larger columns.
Choose the diameter. A common rule: footing diameter equals post width times three. For a 4x4 post, use a 12-inch tube. For a 6x6 post, use an 18-inch tube.
Set the depth. Footings must go below the frost line in your region. In northern states this can be 42 to 48 inches. In the South, 12 to 18 inches is often enough.
Enter the total number of footings. A typical 12x16 ft deck has 6 to 8 footings depending on the beam layout and post spacing.
Footing Volume Formula
A 12-inch diameter footing at 48 inches deep holds about 0.31 cubic feet of concrete. That is less than one 60 lb bag. At that volume, fast-setting bag mix is the most practical choice. For footings wider than 18 inches, consider ordering a small ready-mix delivery.
Example Projects
Tips for Your Project
Add 4 to 6 inches of compacted gravel at the bottom of each hole before pouring. Gravel improves drainage and reduces frost heave risk. For fast-setting mixes like Quikrete Fast Setting Concrete, you can pour the dry mix directly into the wet hole and add water on top. No mixing required. It sets enough to hold a post plumb within 20 to 40 minutes, which speeds up the entire project significantly.
Check your local building department before digging. Most jurisdictions require a permit for deck footings and will specify minimum footing diameter and depth based on the deck load. Inspectors will want to see the holes before you pour, so schedule that inspection early to avoid delays. Dig the holes a few inches wider than the tube form to leave room for setting it plumb.
Frequently Asked Questions
How deep should a concrete footing be?
Footings should extend below the frost line for your area. This ranges from 12" in the South to 48"+ in Northern climates. Check your local building code for the required depth.
What diameter should a deck footing be?
A common rule: footing diameter = post width Γ 3. For a 4Γ4 post, use a 12" diameter tube form. For a 6Γ6 post, use an 18" tube. Always check local load tables.
How many bags of concrete for a footing?
A 12" diameter Γ 48" deep footing needs about 0.31 cubic feet, which is roughly half a 60-lb bag. Our calculator accounts for this exactly.
What is a sono tube / form tube?
A Sonotube (tube form) is a cylindrical cardboard mold used to pour round concrete footings. Available in 6", 8", 10", 12", 16", and 18" diameters.
Should I add gravel below the footing?
Yes β a 4β6" layer of compacted gravel or crushed stone improves drainage and reduces frost heave risk. Add gravel depth to your hole depth when calculating.