Decagon Calculator
Regular decagon calculator to find area, perimeter, side length, inradius (apothem) and circumradius. Enter any one value — side, perimeter, area, inradius or circumradius — and get all the others.
Known value
The calculator converts your input to meters for internal math and then back to your chosen unit for each result.
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Computation steps
Enter a value above and click calculate to see step-by-step details.
How to Use This Calculator
Choose the measure you already know (side, perimeter, area, inradius or circumradius), enter its value, and pick a unit. The tool converts the entry into meters for the math engine and imputes every other length or area automatically.
Supported units: millimeters, centimeters, meters, inches, and feet.
Methodology
The calculator relies on the regular decagon geometry formulas. It uses the golden-ratio-related constants (tan(π/10) and sin(π/10)) to convert between side length, perimeter, area, inradius, and circumradius while keeping every result in sync with your chosen unit.
- Input is normalized to meters (or square meters for area) for consistent internal math.
- Every output is converted back to your preferred unit with six decimal precision, trimming redundant zeros.
- The computation steps panel and downloadable CSV capture how each value was derived.
FAQ
- Does this work for irregular decagons?
- No. Irregular decagons need side-by-side coordinates or each side length, and this tool assumes regular polygons only.
- Can I enter the circumradius?
- Yes. Select the circumradius option and provide the length. The calculator derives the side using s = 2 R sin(π/10).
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About this calculator
Author: CalcDomain Geometry Desk
Reviewed: 6 November 2025
Category: Math & Conversions → Geometry / Polygons