Concrete Column & Pier Calculator
Works out the concrete for round or square columns and piers, per column and in total. Round columns are usually poured into cardboard tube forms; square ones into timber shuttering.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
round: π × r² × height × count · square: width × depth × height × count
A column is a prism. The only thing to watch is that a round column uses the radius — half the diameter — and the error from using the diameter is a factor of four.
What to watch out for
Columns are small volumes that add up:
- Per-column volume is the useful number. It tells you how many bags per hole, which is how you actually work when mixing on site.
- Tube forms come in fixed diameters. 150, 200, 250, 300, 400 mm — or 6, 8, 10, 12, 16 in. Size the calculation to the form you can buy.
- Waste is higher than it looks. Spillage around the top of a tube form, and the last bit in the mixer, matter more when each pour is only 70 litres.
- Rebar cages displace almost nothing. Do not deduct for reinforcement.
Frequently Asked Questions
How much concrete is in a 300 mm round column?
0.0707 m³ per metre of height — about 71 litres. A 2.5 m column is 0.177 m³, roughly 14 × 25 kg bags.
What size tube form do I need?
For a deck or pergola post, 200–250 mm is typical; for a structural pier carrying a building, 300–400 mm. The load and the soil decide it, not the post size.
Do I use the diameter or the radius?
The radius — half the diameter. Using the diameter in the area formula overstates the volume by four times, which is the single most common mistake on round pours.
How many columns can I pour from one mixer load?
A typical 90-litre site mixer produces about 60 litres of concrete per batch — slightly under one 300 mm × 850 mm pier. Plan on one batch per column for anything larger.