The octagon calculator works out circumcircle radius (r), shortest diagonal (s) and longest diagonal (l) from side length (a). Enter what you have and the relationship is applied straight away, so you get the answer without working through the algebra yourself.
What Is the Octagon Calculator?
The octagon calculator works through side length (a) to give you circumcircle radius (r), shortest diagonal (s) and longest diagonal (l).
It is aimed at the case where you know the method but not the arithmetic: schoolwork, a quick check on a worked example, or a figure that needs to be right first time.
How the Octagon Works
Each step is applied in order and depends only on the values before it, which makes it possible to follow the calculation rather than take it on trust.
If you are working through the same problem by hand afterwards, compare the intermediate values. That is where a mistake usually shows itself.
Checking Your Own Working
Round sensibly as you go and keep the full precision in the calculator. Rounding too early is the most common way of getting a right method to give a wrong answer.
Where a result should be a whole number, such as a count, check it against the constraint that produced it. A fraction of a person is a sign that something upstream needs revisiting.
A Worked Example
Working through an example, with side length (a) set to 10, the calculator returns longest diagonal (l) of 26.13126, and medium diagonal (m) of 24.14214, and shortest diagonal (s) of 18.47759. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out circumcircle radius (r)?
Enter side length (a). The calculator then works through the relationship and reports circumcircle radius (r), shortest diagonal (s) and longest diagonal (l).
What does the octagon actually calculate?
It takes side length (a) and converts them into circumcircle radius (r), shortest diagonal (s) and longest diagonal (l) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter side length (a)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Can I use this for a quick estimate as well as a precise answer?
Yes. The calculator is built for quick checks, so round your inputs to a sensible number of significant figures and the result will be just as useful. If you need more precision, keep the same method and add digits to the inputs rather than adjusting the method itself.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.