Circle Calculator
Hand us a radius and we'll give back the diameter, circumference and area. Easy as pie. 🥧
📖 How it works & FAQWhat this circle calculator does
Type in one number — the radius — and this tool works out the three measurements people usually need: the diameter (the width across the middle), the circumference (the distance all the way around), and the area (the space inside). It uses the standard formulas your geometry teacher wrote on the board: diameter = 2r, circumference = 2πr, and area = πr², with π taken to full floating-point precision rather than a rounded 3.14.
When you'd actually reach for it
It's handy for real jobs, not just homework. Figuring out how much edging to buy for a round garden bed or fire pit? That's circumference. Working out how much fabric, mulch, or turf covers a circular patch? That's area. Sizing a round tablecloth with an overhang, checking whether a pizza is really worth the "large" upcharge, or estimating paint for a circular wall mural — all of these come down to plugging in a radius and reading off the result.
A quick worked example
Say you have a round patio with a radius of 5 feet. The diameter is 10 ft, the circumference is about 31.42 ft (2 × π × 5), and the area is about 78.54 sq ft (π × 5²). If you only know the diameter, just halve it first to get the radius. If you only know the circumference, divide it by 2π (about 6.283) to recover the radius, then come back here.
How to use it
- Measure or note the radius — the distance from the center of the circle to its edge.
- Type that number into the radius box. Decimals like 2.5 are fine.
- Read the diameter, circumference, and area, which update as you type.
- Keep your units consistent: a radius in inches gives circumference in inches and area in square inches.
FAQ
- I only have the diameter, not the radius. What now?
- Divide the diameter by 2 to get the radius, then enter that. A 14-inch diameter means a 7-inch radius.
- Why is the area in "square" units?
- Area measures two-dimensional space, so it's always squared — square feet, square cm, and so on. Circumference is a single distance, so it stays in plain units.
- Are the results exact?
- They're as precise as π allows, then rounded for display. For construction or material orders, treat them as close estimates and add a little margin for cuts and waste.
- Is it free and private?
- Yes — it's free and ad-supported, and every calculation runs in your browser, so nothing you type is uploaded or stored.