A walkthrough, end to end.
- 1
Pick which measurement you know (radius, diameter, circumference, or area).
- 2
Enter its value; the calculator returns the other three.
- 3
All formulas use π = 3.14159265358979 (full IEEE-754 precision).
Circle formulas
Diameter d = 2r. Circumference C = 2πr = πd. Area A = πr². Knowing any one lets you solve for the others by inversion (r = d/2 = C/(2π) = √(A/π)).
What you can do with this.
Pizza vs. pizza area
16-inch pizza has 4× the area of an 8-inch (radius doubled, area quadrupled). The calculator gives exact areas for comparing pricing per square inch.
Pipe / tube cross-section
Engineers need cross-sectional area for flow calculations. Enter pipe diameter to get the area; multiply by velocity for volumetric flow rate.
Fence around a circular pool
Pool radius 12 ft → circumference = 2π × 12 = 75.4 ft. That's how much fence (or pool border tile) you need. Add ~10% for waste.
Garden bed area
Circular garden bed radius 4 ft → area = π × 16 = 50.27 sq ft. Use to estimate soil, mulch, or seed quantities.
Wheel rotation distance
Bike wheel diameter 27 inches. Circumference = 27π ≈ 84.8 inches per revolution. Use for cyclometer setup or distance calculations from rotation count.
Storage tank capacity (cylinder)
Tank cross-section area × height = volume. The calculator gives the area; multiply by tank height for total capacity.
Inscribed and circumscribed circles
Circles inside or around polygons. The calculator handles the basic formulas; for inscribed/circumscribed math, you'll need geometry of the polygon too.
Circle calculator 2026 — what's current
All the formulas are unchanged since Archimedes. Modern usefulness: the calculator does the conversions instantly without you remembering 2π vs. πd or π vs. π².
Frequently asked.
Pick 'circumference' as known. Calculator finds radius (C/2π), then area (πr²). Workflow handles all four interconversions.
JavaScript Math.PI is 64-bit IEEE-754 double precision — accurate to ~15-17 decimal places. Far more precision than any practical engineering or science problem needs.
Yes — for a circle, perimeter and circumference are synonymous. 'Circumference' is the technical term reserved for circles; 'perimeter' for polygons.
No. Calculations run entirely in your browser.