A 30 cm circle has much more area than a 20 cm circle
Diameter grows linearly, but circular area grows with the square of diameter. Comparing sizes by diameter alone understates the change in coverage.
Circle calculator →Compare two diameters
A 20 cm diameter gives radius 10 cm and area 100π cm², about 314.159. A 30 cm diameter gives radius 15 cm and area 225π cm², about 706.858. The second area is 225/100 = 2.25 times the first, a 125% increase. Its diameter is only 50% larger, which is a different measure.
Calculate a price per area carefully
For two otherwise comparable circular products priced at 10 and 18 units of currency, the prices per square centimetre are about 0.03183 and 0.02546. The larger product is cheaper per unit area in this example. This comparison assumes the same usable proportion of area and comparable materials or thickness; a thicker product, a wide unusable rim or different contents changes what “value” means.
Use the square ratio for a quick check
For similar circles, area ratio is (new diameter/old diameter)². Doubling diameter makes area four times as large; increasing it by 10% makes area 1.21 times as large, a 21% increase. Use the circle tool for dimensions and percentage change for the final comparison. Keep the same diameter definition and unit throughout rather than comparing one inside diameter with another outside diameter. The comparison is based on two-dimensional area and does not by itself determine volume, mass or nutritional value.
Try the calculators
- Circle calculator
Find a circle’s dimensions from its radius, diameter, circumference or area.
- Rectangle area
Calculate a rectangle’s area and perimeter.
- Scientific calculator
Arithmetic, powers, trigonometry and logarithms.