How much concrete do I need for a fence post or deck footing?

Multiply the column's cross-sectional area (pi x radius² for round, side² for square) by its height, then by the number of posts, and add 5-10% waste. A typical 0.3 m diameter, 3 m tall post needs about 0.21 m³ before waste.

Should I pour columns with bagged concrete or ready-mix?

For a handful of posts or piers, bagged premix is usually the practical choice since you only mix what you need; for a large number of columns or bigger diameters, ordering ready-mix by the cubic metre is normally cheaper and faster.

What concrete mix should I use for fence posts or deck footings in the UK?

A GEN1 designated mix (roughly a 1:2:4 ratio) under BS 8500 is typically sufficient for fence posts and shed footings; for anything structural, such as a garage or extension foundation, check with a builder or engineer first.

Concrete Column Calculator — Round & Square Column Volume | UK

Concrete Column Calculator: How Much Concrete for a Post or Pier?

Round posts, square piers, deck footings and fence columns all come down to the same question: how much concrete do you need to pour them? This calculator works out the volume of a circular or square/rectangular column from its diameter (or side length) and height, multiplies that by however many identical columns you are pouring, adds a waste allowance, and converts the result into either a bag count or a cement:sand:aggregate mix -- plus a material cost estimate.

How to Use the Concrete Column Calculator

Choose the column shape: circular (diameter) or square/rectangular (side length).
Enter the diameter or side length and the column height, in whichever unit suits you.
Set the number of identical columns you are pouring.
Adjust the waste allowance if needed -- 5-10% is standard.
Pick bags or a custom cement:sand:aggregate mix, then read off the volume, bag count (or mix split) and estimated cost.

The Math Behind the Calculation

A circular column's volume is pi x radius² x height (the cylinder formula); a square or rectangular column is simply side x side x height. That per-column volume is multiplied by the number of columns, then inflated by the waste percentage to get the volume you should actually order.

Worked example: four circular posts, each 0.3 m (30 cm) in diameter and 3 m tall. Volume per post = pi x 0.15² x 3 ≈ 0.212 m³; for 4 posts that is about 0.85 m³ raw. Adding a 7.5% waste allowance gives roughly 0.91 m³ to order -- about 54 bags at a 0.017 m³ yield per bag (a typical 25 kg bag).

Pro Tips

At a typical material cost of around £125 per m³ of ready-mix concrete, four 0.3 m diameter, 3 m tall posts (about 0.91 m³ including 7.5% waste) cost roughly £114 in raw material -- formwork, reinforcement and labour are additional.

Leave at least 40-50 mm of concrete cover around a rebar cage on every side so reinforcement is not exposed at the surface.
Concrete columns cure on the same timeline as any other pour -- about 7 days to reach useful strength and around 28 days before full loading -- so avoid backfilling or loading a post footing early.