A strong, stable deck starts from the ground up. The footings are the unseen foundation of your deck, responsible for transferring the entire weight of the structure safely into the ground. Calculating the correct volume of concrete needed for these footings is a critical first step in the building process. Our Deck Footing Calculator is designed to simplify this task. By entering the dimensions and number of your cylindrical footings, you can accurately estimate the total volume of concrete required, as well as the number of pre-mixed bags you'll need to buy for the job.
How to Use the Deck Footing Calculator
Estimating the concrete for your deck footings is easy:
- Select Units: Choose your preferred measurement system, either imperial (inches) or metric (cm).
- Enter Footing Details: Input the total "Number of Footings" for your deck, the "Footing Diameter" (e.g., the size of your Sonotube), and the "Footing Depth".
- Calculate Concrete Needed: Click the "Calculate Concrete" button to see your results.
- View Your Estimate: The calculator will display the total volume of concrete needed (in cubic yards or meters) and the equivalent number of 60 lb or 80 lb pre-mixed bags.
The Importance of Proper Deck Footings
Deck footings serve a single, crucial purpose: to anchor the deck's support posts to stable ground. Without properly sized and installed footings, a deck can sink, shift, or heave, leading to structural failure.
Footing Depth and the Frost Line
In climates with freezing winter temperatures, the most important consideration for footing depth is the frost line. The frost line is the depth to which the ground freezes in the winter. As water in the soil freezes, it expands, causing the ground to heave upwards. If your footings are not placed below this frost line, this upward pressure can lift your deck posts, causing the deck to become uneven and unsafe over time. Your local building department can tell you the required footing depth for your specific area. It's common to dig footings 6-12 inches below the frost line for an added margin of safety.
Footing Diameter
The diameter of the footing determines its surface area, which is what spreads the load of the deck onto the soil. A wider footing distributes the weight over a larger area. The required diameter depends on the load the footing will carry (how much of the deck it's supporting) and the load-bearing capacity of your soil. For most standard decks, footing diameters of 8 to 12 inches are common.
Calculating the Volume
Our calculator determines the volume of concrete needed using the standard formula for the volume of a cylinder:
Volume of one footing = π × (radius)² × Depth
It then multiplies this by the number of footings to find the total volume. This total is then used to calculate how many bags of pre-mixed concrete you would need. Standard yields for common bag sizes are:
- A 60 lb bag yields approximately 0.45 cubic feet of concrete.
- An 80 lb bag yields approximately 0.60 cubic feet of concrete.
Tips for Pouring Concrete Footings
- Use a Form: For clean, uniform footings, use a cardboard concrete form tube (often known by the brand name Sonotube). This prevents the concrete from mixing with the surrounding soil and ensures a consistent shape.
- Mix a Bit at a Time: Concrete starts to harden quickly. Mix only one or two bags at a time in a wheelbarrow or a mixing tub to a consistent, oatmeal-like consistency.
- Remove Air Pockets: After pouring the concrete into the hole, use a stick or a piece of rebar to poke and prod the wet concrete. This helps to release any trapped air bubbles, ensuring a strong, solid footing with no voids.
- Set Your Post Hardware: While the concrete is wet, insert the anchor bolt or post base hardware into the center of the footing, ensuring it is perfectly level and at the correct height. Allow the concrete to cure for at least 24-48 hours before attaching the posts and starting to build your deck frame.
Frequently Asked Questions
How deep does my footing need to be?
This is determined by the local frost line in your area. You must contact your local building department to find the code-required minimum footing depth. Failure to place footings below the frost line can lead to dangerous structural heaving.
Do I need to put gravel in the bottom of the hole?
Adding a few inches of compacted gravel to the bottom of the footing hole before pouring the concrete is a highly recommended practice. This helps with drainage and provides a more stable base for the concrete to rest on.
Should I add rebar to my footings?
For most standard deck footings, rebar is not strictly necessary but is always a good idea for added strength, especially in areas with unstable soil or for decks that will carry heavy loads (like a hot tub). A few vertical pieces of rebar can significantly increase the footing's resistance to cracking and uplift forces.