Column Size Calculator
Enter the load the column carries, say whether it is a working or factored load, and choose the concrete grade and the share of steel. The calculator gives a square and a round column size, the main bars and the ties.
- From axial load
- Square or round
- Bars and ties
A working load is what the column carries in use. It is multiplied by 1.5 for design. Enter a factored load if you already have one.
A short concrete column carries load through both concrete and steel. The concrete takes most of it and the steel bars add strength. With C25 concrete and 1% steel, a column about 300 mm square carries a working load of roughly 800 kN.
Column size chart
C25 concrete, 1% steel. Search for a load.
| Working load (kN) | Factored (kN) | Column (mm) |
|---|---|---|
| 200 | 300 | 230 |
| 400 | 600 | 230 |
| 600 | 900 | 275 |
| 800 | 1200 | 325 |
| 1200 | 1800 | 375 |
| 1600 | 2400 | 450 |
| 2400 | 3600 | 525 |
| 3000 | 4500 | 600 |
Short braced columns with a straight load only.
Load and column size
C25, 1% steel. Slide the working load.
The section shows the main bars inside the dashed tie, with 40 mm cover.
How to size a column
Load to area. Step through it.
Working load
From above.
Columns taller than about 12 times their width are slender and need a bigger section.
Square, round or rectangular
Column shapes. Compare them.
Easy formwork.
Neat in open spaces.
Fits walls.
Round columns use spiral ties, which make the core stronger.
Before building
Column checks. Tick each item.
Tick each item as you go.
Concrete columns must sit on a footing sized for the same load and the soil below.
What you get
Column sizing. Each tile explains one result.
mm.
Diameter.
Number and size.
Size and spacing.
Factored.
More steel lets a column be smaller, but above about 4% it gets hard to place the concrete.
Frequently Asked Questions
No question matches that search.
How do I size a concrete column?
Factor the load, divide by the capacity of concrete and steel per square millimetre, and take the square root for the side.
What is the minimum size of a concrete column?
Often about 230 mm, though codes and fire rules may need more.
How much steel should a column have?
Between 0.8% and 4% of the area, with at least four bars.
What size column for a two-storey house?
Often 230 to 300 mm square, but loads vary, so get it checked.
What is a factored load?
The working load multiplied by a safety factor, usually 1.5.
How far apart should ties be?
No more than the column width, 16 bar diameters or 300 mm, whichever is least.
When does a column need an engineer?
Always for a real building, and especially for tall, slender or bending columns.
A column sized for its load
Factor the load, choose the concrete and steel, and get a section with its bars and ties. Then ask an engineer to check it for your building.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.