We are often asked how to measure a propeller diameter. The diameter of a circle is the distance from any point on the circle straight across the center to the opposite side of the circle. In measuring a 2-bladed or 4-bladed impeller, simply measure the point from the outer most tip of the blade across the bore, or center hole, to the outer most tip of the opposite blade.
The confusion lies when measuring a 3-bladed impeller. In measuring a 3-bladed propeller or impeller, you do not have a blade exactly opposite across the hub’s bore. The trick is to measure the radius and multiply it by 2 to get the diameter. Measure the outer most tip of the blade to the center of the bore to obtain the radius. Therefore, a propeller or impeller that measures a radius of 3” from the tip of the blade to the center of the bore will have a diameter of 6”. (A 3” radius multiplied x 2 equals a 6” diameter.)