Use the default quantity path to estimate concrete for deck footings whose dimensions are known. A separate bearing-area estimate can divide a known support load by a matching gross allowable soil pressure; it does not select a final footing size or replace a structural design.
Two tasks, kept separate
A deck footing calculator can serve two different questions. The main path answers how much concrete goes into already selected pads or piers. Enter the shape, dimensions, count, and any separate base. The optional bearing-area path answers what bearing area corresponds mathematically to a known support load and an allowable pressure. It should be clearly labeled as an estimate and should not feed its equivalent dimensions into the concrete quantity form automatically.
For the material path, choose a circular or rectangular footing shape and enter its actual dimensions. Repeated footings use a count. If each pier rises from a wider base, model the base as a distinct non-overlapping part: measure the pier height from the top of the base, then add the base volume once. This keeps the shared volume boundary clear and prevents the vertical pier segment from overlapping the base in the calculation.
Quantity formula and example
A circular pier is a cylinder: V = π × (diameter ÷ 2)² × height. A rectangular pad uses length × width × height. Add separate components and multiply by their counts, then apply the adjustable material allowance once to the total net volume. Divide cubic feet by 27 to report cubic yards. Bag estimates can use a named product yield, but concrete quantity remains the main result because a ready-mix order is usually discussed as volume.
For four round footings 12 in in diameter and 36 in high, the radius is 0.5 ft and height is 3 ft. One cylinder holds about 2.356 ft³; four hold 9.425 ft³ net. A 10% material allowance produces about 10.367 ft³, or 0.38397 yd³. At approximately 0.60 ft³ per 80 lb bag of QUIKRETE Concrete Mix No. 1101, the bag equivalent is 18 bags. That is a product-specific material estimate, not a recommendation for footing dimensions.
Bearing-area estimate from known inputs
The separate area calculation uses A = P ÷ q, where P is the total vertical use load carried by one support and q is the matching gross allowable soil bearing pressure. If A is in square feet, a square area-equivalent side is 12 × √A inches and a circular diameter is 12 × √(4A ÷ π) inches. These are geometric equivalents of the calculated area, not ready-to-build dimensions.
For example, a known 4,000 lb support load divided by a matching gross allowable pressure of 1,500 lb/ft² gives 2.667 ft². That corresponds mathematically to a square about 19.60 in on each side or a circle about 22.11 in in diameter. The pressure in this example is an input for demonstrating the arithmetic, not a default soil value. Do not substitute a net allowable pressure for a gross value; the two must use consistent load and pressure definitions.
Find the design inputs before relying on area
The calculator does not guess a deck’s reactions by dividing total deck area by the number of posts. Load at an individual support depends on the framing, span, tributary area, live and dead loads, and design assumptions. Use an approved plan or qualified professional to identify the applicable support load and soil value. The AWC DCA6 guide is a referenced deck resource, but it does not make this calculator a current code design service.
Even a correct pressure-area calculation leaves many construction questions unanswered: frost depth, embedment, footing thickness, reinforcing, pier height, lateral resistance, drainage, uplift, and connection details. Local requirements and site conditions matter. The result should say only that the provided load and pressure imply a mathematical bearing area. It must not state that the footing passes, is code approved, or is adequate for a specific deck.
Choose form dimensions for the quantity path
The concrete quantity path accepts the diameter or rectangular dimensions that have already been selected. Tube or form measurements should use the actual inside concrete diameter, because the volume grows with the square of radius. A small diameter change can therefore affect the total more than the same-sized change in height. For round forms, manufacturer information such as the Sonotube builders tube sheet can help identify product sizes; use the form you will actually install.
If a footing flares, bells, steps, or includes a separate cap, do not pretend it is one cylinder unless that geometry matches the concrete. Add known, distinct shapes separately or obtain a more appropriate takeoff. Do not count the base slab and pier twice at their meeting face. Where dimensions differ between footings, group identical sizes and calculate each group separately before combining the totals.
Next step and boundaries
For ordering, record the number and dimensions of each footing group, the net total, the selected allowance, and the requested order volume. Ask the supplier about delivery minimums, partial loads, access, and timing. If you are using bags, identify a specific bag product and yield. If you need a whole foundation budget rather than a deck support quantity, use the foundation cost page; if you need a measured pier volume without the bearing option, the concrete pier page focuses on cylindrical piers and independent bases.
Use construction plans for the footing design. This site supplies arithmetic for entered dimensions and a local-input bearing-area relationship. It cannot determine whether your load estimate is complete, whether a soil report applies to the site, or whether the local authority accepts a detail. If either required bearing input is missing or uses an incompatible definition, do not fill it with a generic assumption. Obtain the correct value before using the area estimate for planning.
Frequently asked questions
Does the bearing estimate design my deck footing?
No. It calculates area from a load and matching gross allowable soil pressure. It does not select a buildable dimension or evaluate the deck structure.
What is the formula for required bearing area?
For the stated inputs, area equals support load divided by gross allowable soil bearing pressure: A = P ÷ q.
Can I use net allowable soil pressure?
The inputs must use a matching gross load and gross allowable pressure. A net allowable pressure cannot be substituted directly into this calculation.
Why is the bearing result separate from concrete volume?
An area equivalent does not specify footing thickness, depth, frost conditions, reinforcement, or the rest of the geometry needed for a quantity or design.
How is a pier with a wider base counted?
Enter the pier height from the top of the base and add the base as a separate non-overlapping shape.
Is 1,500 psf a default soil value?
No. Any example pressure is labeled only to demonstrate arithmetic. Use a value appropriate to the site and matching the load definition.