Circle Calculator

Enter Calculation

Choose a Calculation Find A, C and d | Given radius r Find C, r and d | Given area A Find A, r and d | Given circumference C Find A, C and r | Given diameter d

Known value (radius r)

Let pi =

Units none km m cm mm mi yd ft in

Significant Figures auto 3 4 5 6 7 8 9

Formula

Show calculation steps (1)

1. Circumference

   

   Circumference of a circle from its radius (or diameter)

Results

Area

78.5398

square units

Radius (r)	5
Diameter (d)	10
Circumference (C)	31.4159
Area (A)	78.5398
In Terms of Pi: circumference	C = 10 pi
In Terms of Pi: area	A = 25 pi

What this circle calculator does

This tool solves a circle completely from a single known measurement. Pick which property you know — radius, diameter, circumference, or area — enter its value, and the calculator returns all four quantities. It also expresses the circumference and area "in terms of pi," meaning as a clean coefficient multiplied by pi (for example \(C = 10\pi\) or \(A = 25\pi\)).

How to use it

Choose a calculation mode that matches what you already know. Type the value into the single input field. Optionally override pi (handy if your homework wants 3.14 or 22/7), pick a display unit (purely a label — no conversion is performed), and choose how many significant figures to round to. Every answer is computed at full precision internally and only rounded for display.

The formulas explained

All circle relationships flow from the radius \(r\). The diameter is simply twice the radius, $$d = 2r$$. The distance around the edge is $$C = 2\pi r$$ equivalently \(\pi d\). The enclosed area is $$A = \pi r^2$$ Working backwards, if you know the area then \(r = \sqrt{A / \pi}\); if you know the circumference then \(r = C / (2\pi)\); if you know the diameter then \(r = d / 2\). The "in terms of pi" coefficients are just \(C / \pi\ (= 2r)\) and \(A / \pi\ (= r^2)\).

The four circle measurements the calculator relates: radius, diameter, circumference and area.

Worked example

Suppose the radius is 5 cm with pi = 3.14159265359 and 6 significant figures. Then $$d = 2 \times 5 = 10 \text{ cm},$$ $$C = 2 \times \pi \times 5 \approx 31.4159 \text{ cm},$$ and $$A = \pi \times 5^2 \approx 78.5398 \text{ cm}^2.$$ The symbolic forms are \(C = 10\pi\) and \(A = 25\pi\).

Worked example: starting from the radius to find circumference and area.

FAQ

Does changing the units convert my numbers? No. The unit dropdown only adds a label suffix (and the squared label for area). All outputs stay in the same unit as your input.

Why let me change pi? Many textbooks specify a rounded value of pi such as 3.14 or 22/7. Overriding pi lets your answers match the expected solution exactly.

What does "significant figures: auto" do? It shows the full computed precision with no forced rounding, useful when you want the most exact result.