Go to the website of MyCarpentry.com. Click on the tab "Carpentry Calculators" located towards the top right hand corner of the web page. This will take you to three calculators where you can click on the "Arch Calculator" to make the necessary calculations.
Measure the height of the archway from the bottom to the highest point in the center with a tape measure. Enter the figure in inches in the "Arch Height" field on the "Arch Calculator".
Measure the distance across the widest part of the archway to determine the width. Enter the figure in inches in the "Arch Width" field on the Calculator website.
Click "Calculate." The focus point distance is automatically calculated in inches.