Arch Calculator
Construction & Home ImprovementCalculate the focal point positions needed to draw an elliptical arch from its base length and rise.
Focal Distance from Center
Distance from the center of the base to each of the two focus points, used to draw the ellipse with a string and two pins.
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Odeh Ahwal0people find this calculator helpful
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Construction & Home Improvement calculators
Frequently Asked Questions
How do I use this to draw the arch?
Mark the two focus points on the baseline, this distance from center on each side. Tie a string, its length equal to the base length, around two pins at the focus points, then trace the arch by keeping the string taut with a pencil, the classic two-pin-and-string method for drawing an ellipse.
What if the rise is greater than half the base length?
This calculator assumes a shallow, wider-than-tall elliptical arch, where the rise is smaller than half the base length. If the rise exceeds half the base length, the geometry does not produce a valid result with this formula, since the shape would need to be calculated as a taller ellipse instead.
Does this work for a semicircular arch?
A semicircular arch is a special case where the rise equals exactly half the base length. In that case the two focus points converge to a single center point, and the arch can be drawn directly with a compass instead.
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