How to use the Octagon Cubic Feet Calculator
- Use one edge of a regular octagon. Do not substitute the across-flats or across-corners measurement for the side.
- Choose the shape mode, where available, and a unit for each dimension. Enter the number of identical objects.
- Select Calculate. Review the result and its working; if you change an input, calculate again before copying or sharing.
Octagon formula
Convert each length to feet, apply the displayed solid-volume formula, then multiply by the number of identical objects.
2(1 + √2) × side² × height
What the terms mean
- Regular octagon side
- A measured length in the selected unit; it is converted to feet before volume is calculated.
- Prism height
- A measured length in the selected unit; it is converted to feet before volume is calculated.
- Number of identical items
- The whole-number count of objects with the same entered dimensions.
Octagon worked examples
Measure a regular octagonal prism
A regular octagonal prism has eight equal 2 ft sides and a height of 3 ft.
- 2(1 + √2) × side² × height
- side = 2 ft
- height = 3 ft
- 57.941125 ft³ × 1 = 57.941125 ft³
Answer: 57.941125 ft³. Cubic yards: 2.145968 yd³; Litres: 1,640.709962 L.
Measure a smaller octagonal column
A regular octagonal prism has a side length of 1 ft and a height of 4 ft.
- 2(1 + √2) × side² × height
- side = 1 ft
- height = 4 ft
- 19.313708 ft³ × 1 = 19.313708 ft³
Answer: 19.313708 ft³. Cubic yards: 0.715323 yd³; Litres: 546.903321 L.
Octagon Cubic Feet Calculator FAQs
Can I use the width across opposite flat sides?
Convert it to side length first: side = across-flats width ÷ (1 + √2).
What does prism height mean?
It is the perpendicular distance between the matching octagonal ends.
Measurement guide for Octagon
Use the length of one of the eight equal sides. Confirm that the cross-section is a regular octagon before applying this model.
Assumptions and limits
Eight sides alone do not make a regular octagon. Unequal edges or angles require a different area model.
Check your result
Check the dimensions converted to feet in the working. For similar solids, doubling every dimension increases volume by a factor of eight.