How do I calculate concrete for deck footings?

For round footings, square the diameter in feet, multiply by pi divided by four, then multiply by the footing depth in feet. Multiply by the number of footings and add a waste allowance. Divide cubic feet by 27 to get cubic yards.

How many 80 lb bags of concrete are in a cubic yard?

An 80 lb bag of concrete mix commonly yields about 0.6 cubic feet. One cubic yard is 27 cubic feet, so it takes about 45 bags per cubic yard before waste.

What size deck footing do I need?

Footing size depends on deck loads, soil bearing capacity, frost depth, post spacing, and local code. Use this calculator for material estimates after confirming the footing diameter and depth required for your project.

Deck & pier calculator

Deck Footing Concrete Calculator

Estimate concrete for round deck footings, pier holes, and tube forms. Enter the number of footings, diameter, depth, and waste allowance to get cubic yards and bag counts.

Concrete needed 0.55 yd³ 8 footings, 12 in diameter, 36 in deep, with 10% waste.

Footings

Number of footings Diameter (inches) Depth (inches)

Use the actual concrete-filled depth. If you use tube forms above grade, include the filled tube height too.

Ordering details

Waste / overage (%) 80 lb bag yield (cu ft) 60 lb bag yield (cu ft)

Bag yields vary by brand and mix. Check the package if your concrete mix lists a different yield.

How to use this deck footing concrete calculator

Enter how many footings you plan to pour, then add the round footing diameter and filled depth. The calculator finds the cylinder volume, multiplies it by the number of footings, adds your waste allowance, and converts the result into cubic yards and common bag counts.

This is useful for deck posts, pergola posts, porch piers, fence gate posts, and other round concrete piers. For square or rectangular footings, use the volume formula length × width × depth, then divide by 27 for cubic yards.

Before you order concrete

Confirm local requirements: footing diameter, depth, frost protection, and inspections can vary by location.
Add realistic waste: 5–10% is common for small projects; awkward holes, uneven bottoms, and spillage may need more.
Compare bags vs delivery: a few footings may be easiest with bags, while larger decks can be cheaper and faster with ready-mix.

FAQ

Does this work for Sonotube-style forms?

Yes. Enter the inside diameter of the round tube and the height that will be filled with concrete. If the footing below the tube is wider than the tube, calculate that wider base separately and add it to the total.

Should I round up concrete bags?

Yes. Concrete bags are sold as whole bags, and short pours are frustrating. This calculator rounds bag counts up after applying your waste percentage.

Is 36 inches deep enough for deck footings?

It depends on frost depth, soil, deck design, and local code. Many cold areas require footings below the frost line. Use the calculator after you know the required depth for your location.