How to Calculate the Rise of an Arch

Locate the bottom of the arch. If the arch starts at the floor and extends upward, you can use the floor as your horizontal line. If the arch is suspended in the air and does not come all the way to the ground, stretch a string across the bottom of the arch and secure the string to both sides of the arch with painter’s tape.

Measure from the apex of the arch to the string with a tape measure. The apex is the high point of the arch and is located at the top center of the arch. If the arch extends clear to the floor, measure from the apex to the floor.

Take a reading on the tape measure where it crosses over the string. The number on the tape measure indicates the rise of the arch. If the arch extends to the floor, take a reading where the tape measure touches the floor. This gives you the rise of an arch that extends to the floor.