Knowing your beam diameter before you climb the ladder saves you from re-focusing fixtures halfway through a tech rehearsal. Punch in your fixture's beam angle and how far it'll be from the stage or floor, and you'll get an exact spread — plus a visual so you can see how much wider (or tighter) the beam gets as you move it further back.
Beam angle is usually printed on the fixture's spec sheet or in its manual — it's the full cone angle, not the half-angle, so just enter it as-is.
4.44 m diameter
Diagram is illustrative, not to scale — the number above is the exact figure.