円幾何学計算機

The Circle Geometry Calculator solves the four interlocked values of any circle — radius, diameter, circumference, and area — from whichever one you happen to know. Enter exactly one, and the other three are computed automatically using three formulas: C = 2πr, A = πr², and d = 2r. This tutorial walks through what each formula means, how to invert any of them, and what to watch out for.

The four values, one circle

Every circle has four basic measurements, and any one of them determines the other three:

• Radius (r) — distance from the center to any point on the circle.
• Diameter (d) — distance across the circle through the center. d = 2r.
• Circumference (C) — the total length around the circle. C = 2πr = πd.
• Area (A) — the surface enclosed by the circle. A = πr².

The constant π ≈ 3.14159265... is irrational, meaning its decimal expansion never terminates and never repeats. Our calculator uses the full double-precision value of π built into the browser's math library, not a rounded version like 3.14 or 22/7 — so your answers are accurate to roughly 15 decimal places, then rounded to 4 for display.

Starting from each value

Pick whichever input you actually have. The other three are derived:

• From radius (r): d = 2r, C = 2πr, A = πr².
• From diameter (d): r = d/2, C = πd, A = πd²/4.
• From circumference (C): r = C/(2π), d = C/π, A = C²/(4π).
• From area (A): r = √(A/π), d = 2√(A/π), C = 2√(πA).

The "from area" line is the only one with a square root — area depends on r² while the others depend on r linearly. Doubling the radius quadruples the area but only doubles the diameter and circumference.

Example 1 — From a known radius

Input: r = 5. Outputs: d = 10, C = 2π(5) = 10π ≈ 31.4159, A = π(5)² = 25π ≈ 78.5398.

If you need a symbolic answer for homework, write 10π and 25π; for engineering use the decimal.

Example 2 — Inverting from circumference

Input: C = 31.4159. Outputs: r = 31.4159/(2π) = 5.0000, d = 10.0000, A = 78.5398. This is the inverse of Example 1 — a round-trip check that the calculator's algebra is consistent.

Example 3 — Inverting from area

Input: A = 100. Outputs: r = √(100/π) ≈ 5.6419, d ≈ 11.2838, C ≈ 35.4491. The square root means a circle with twice the area has only √2 ≈ 1.41 times the radius.

What "radius" and "diameter" really mean

The radius is the line segment from the center to any point on the circle. Because every point on the circle is the same distance from the center (that's literally the definition of a circle), the radius has a fixed length — it doesn't matter which point you measure to.

The diameter is any chord that passes through the center. It is the longest possible chord — no segment with both endpoints on the circle can be longer. Diameter = 2 × radius, which is why d/2 is the most reliable way to extract a radius from a ruler measurement across the widest part of the circle.

What π actually is

Pi (π) is defined as the ratio of any circle's circumference to its diameter. This ratio is the same for every circle in flat (Euclidean) space — that's what makes π a universal constant rather than a per-circle property. The first decimal digits of π are 3.14159265358979... Historical approximations include 22/7 (accurate to 0.04%) and 355/113 (accurate to 0.0000085%). Our calculator uses IEEE 754 double-precision π, accurate to ~15-17 decimal digits.

Sectors, arcs, and segments — when you need a different calculator

The Circle Geometry Calculator handles the whole circle. Several related calculations require a slice or fraction:

• Arc length — if you know the central angle θ (in degrees), arc length = (θ/360) × C = (θ/360) × 2πr.
• Sector area — the "pie slice" between two radii. Sector area = (θ/360) × A = (θ/360) × πr².
• Segment area — the area between a chord and the arc it subtends. Requires both radius and chord length.
• Inscribed and circumscribed circles of a polygon — solved by the Inscribed Circle Calculator.

Common mistakes

• Confusing radius and diameter. If you measured across the full width of a coin (10 mm), that is the diameter. The radius is half of that. Plugging 10 mm into the radius field gives an area 4× too large.
• Forgetting to square the radius for area. A = π × r × r, not π × r. Area scales with the square of length in any 2D figure.
• Using π ≈ 3.14 instead of the full value. Fine for rough work but the rounding error compounds quickly when squared.
• Mixing units. If r is in cm, C is in cm and A is in square centimeters (cm²). Always check output units.

The unit circle and beyond

A special case worth knowing: the unit circle has radius = 1. Its diameter is 2, circumference is 2π, and area is π. The unit circle is the basis for trigonometry — angle measurement in radians is literally arc length on a unit circle.

Engineering applications: C = πd is what an odometer uses to convert tire rotations into distance. A = πr² underlies every pipe-area, wire-cross-section, and dish-antenna calculation. Once you internalize that area scales as r² and circumference scales as r, you can estimate most circle quantities in your head.