Circle & Arc Calculator
Round patios, curved borders, radius decks. Enter the size as diameter — the way you'd say a 12-foot round patio — or switch to radius. Four modes: full circle, arc and sector, ring borders — and the field-practical one: you can't measure the radius of a big curve, but you can stretch a line across it and measure the middle gap. That's enough.
Planning tool — results are estimates for quoting and ordering, not engineering or building-code advice.
Common questions
How do I find the radius of an existing curve?
Stretch a straight line across the curve (the chord) and measure from the line's midpoint to the curve (the rise). Radius = chord² ÷ (8 × rise) + rise ÷ 2 — the calculator's 'find radius' mode does it for you.
What's the difference between sector and segment area?
The sector is the pie slice including the center wedge; the segment is just the cap between the chord and the arc. Curved paver sections are usually segments; pie-shaped patio sections are sectors.
How much edging does a curve need?
Use the arc length, not the chord — a 90° arc on a 10' radius is over 15.7' of edging even though the straight-across distance is much less.
Related: Polygon calculator · Gravel calculator
More free calculators
All 14 are free, feet-and-inches in, no sign-up. Pick the next one for the job.
Concrete & site work
Finishes & materials
Layout & geometry
This solves one measurement. The Takeoff Builder prices the whole job.
Walk the rooms, enter dimensions, and formula-driven scope turns measurements into quantities, prices, and the estimate itself — from $20/mo, capped, no surprise bills.