Open Frame Calculator

Calculations at an open frame, a frame open on one side. It is made of three rectangles that have the same width. Both outer rectangles have also the same length. The central rectangle is perpendicular at the lower ends of the outer rectangles between those.
Enter three values, but not thickness and both heights or both lengths. Choose the number of decimal places and click Calculate.

	Frame thickness (c):
	Outer height (h):
	Inner height (i):
	Outer width (a):
	Inner width (b):
	Perimeter (p):
	Area (A):
	Round to   0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15   decimal places.

Thickness, heights, widths and perimeter have the same unit (e.g. meter), the area has this unit squared (e.g. square meter).

Open frame is a descriptive name rather than a formal geometric one. Like many composite polygons, this shape lacks a standard geometric name. This open frame is a non-convex polygon featuring eight right angles, specifically two concave and six convex angles. The convex right angles have interior angles of 90 degrees, while the concave ones have exterior angles of 90 degrees. There are several ways to decompose this shape into rectangles, either by cutting perpendicular to edge b at its ends, perpendicular to edges i at their lower ends, or by applying one method at one corner and the other at the opposite corner. If both decomposition methods are applied at the respective corners, the result consists of three rectangles plus two squares with a side length of h-i or (a-b)/2. Both calculations yield the same value.
Such an open frame possesses one single axis of symmetry running vertically through the center of the bottom rectangle, bisecting edges a and b. This shape has no other symmetries. A related shape is the L-shape, formed by two rectangles of equal width. Calculating the properties of such rectangle-based shapes is generally straightforward. The total area is determined by multiplying the length and width of the individual rectangles into which the shape can be decomposed, and then summing these individual areas.

Calculation of area and other dimensions: open frame

Last updated on 07/20/2026. Author: Jürgen Kummer

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