Rounded Rectangle Calculator

Calculations at a rounded rectangle, a rectangle with round corners. The rounded corners are each a quarter circle, where the circular paths merge into the straight sides. All four corners together form a circle. The corner radius must be less than half of the shorter side.
Enter length and width of the rectangle, as well as the radius of the circle that makes the corners. Choose the number of decimal places, then click Calculate.

	Length (a):
	Width (b):
	Corner radius (r):
	Diagonal (d):
	Perimeter (p):
	Area (A):
	Round to   0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15   decimal places.

Lengths, width, radius, diagonal and perimeter have the same unit (e.g. meter), the area has this unit squared (e.g. square meter).

The rounded rectangle is the more general case of a rounded square, which in turn is a rounded regular polygon. The rounded rectangle is known, for example, from the surface of smartphones, although the rounding is not always from exact quarter circles. This is usually done to make it easier to handle, to prevent snagging or, in the case of hard corners, to reduce the risk of injury. To soften the points at the corners of a rectangle, blunting can be done instead of rounding, in which the corners are cut straight off. This creates a stretched octagon, or, in the case of a square, a truncated square.
The corners of all polygons can be rounded. The length of the arc of the rounded corner corresponds to (180 degrees minus the angle of the corner) divided by 360 degrees, multiplied by the circumference of the circle. The smaller the corner angle, the longer the arc. Calculating the length of the missing piece and the straight line is more complicated; see the formulas for the rounded regular polygon linked above.