Octagon Size Calculator
Choose which measurement you have, the side, the width across the flats, the width across the corners or the area, and enter it. The calculator works out the rest for a regular octagon, along with the mitre angle to cut.
- Start from any size
- Flats and corners
- Mitre angle
Use any unit. Results are in the same unit, with area in that unit squared.
A regular octagon has eight equal sides and eight 135° corners. The width across the flats is the side times 2.414, and the width across the corners is the side times 2.613. Builders use these numbers for gazebos, tables, planters and stop-sign shapes.
Octagon size chart
From the side length. Search for a size.
| Side | Across flats | Across corners | Perimeter | Area |
|---|---|---|---|---|
| 1 | 2.414 | 2.613 | 8 | 4.828 |
| 2 | 4.828 | 5.226 | 16 | 19.31 |
| 5 | 12.07 | 13.07 | 40 | 120.7 |
| 10 | 24.14 | 26.13 | 80 | 482.8 |
| 20 | 48.28 | 52.26 | 160 | 1931 |
| 30 | 72.43 | 78.39 | 240 | 4346 |
| 50 | 120.7 | 130.7 | 400 | 12071 |
Across flats = side × 2.414. Across corners = side × 2.613. Area = side² × 4.828.
Side length and size
In centimetres. Slide the side length.
An octagon fills about 83% of the square that fits around it. The corners take the rest.
How to work out an octagon
From the side. Step through it.
Find the side
Or convert from width.
To mark an octagon on a square, measure 29.3% of the square's width in from each corner along both edges, and cut off the triangles.
Flats, corners or side
Which size to quote. Compare them.
Between two parallel sides.
Point to opposite point.
One edge.
For an octagonal table that fits a space, use across corners as the largest dimension.
Building an octagon
Cutting and joining. Tick each item.
Tick each item as you go.
Small errors add up over eight joints. Cut a test piece and dry fit before cutting all eight.
What you get
Every octagon size. Each tile explains one result.
One edge.
Narrow width.
Wide width.
Inside the shape.
22.5° cut.
Inside angles are 135°. Each joint is two 22.5° mitres meeting.
Frequently Asked Questions
No question matches that search.
How do I find the area of an octagon?
Area = 2 × (1 + √2) × side², about 4.828 × side².
What is the side of an octagon 120 cm across?
About 49.7 cm, if the 120 cm is measured across the flats.
What angle do I cut for an octagon?
22.5° on each end of each piece.
What is the inside angle of an octagon?
135°.
How do I get the side from the width?
Divide the width across flats by 2.414.
What is across corners on an octagon?
The widest measurement, side × 2.613.
How do I make an octagon from a square?
Cut off each corner at 29.3% of the square's width along both edges.
Any octagon from one number
Enter the size you know and get every measurement, plus the cut angle. Whether you are building a gazebo or a table, the numbers are ready to use.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.