If you want to do it by hand, the standard formula is:

TFOV = (Eyepiece Field Stop Diameter / Telescope Focal Length) × 57.3

(57.3 converts radians to degrees.)

Easier method: Use the Apparent Field of View (AFOV) method:

True FOV = (Eyepiece AFOV) / Magnification

Example:

Before you click a button on a web tool, you must understand the two types of FOV: Apparent (AFOV) and True (TFOV). A hot astro FOV calculator automates this, but the formula is the bedrock:

TFOV = (Eyepiece Field Stop Diameter / Telescope Focal Length) x 57.3

For cameras: TFOV = (Sensor Width / Telescope Focal Length) x 57.3

Let’s run a live scenario. Assume you own a Celestron C8 (2032mm focal length) and you just bought the “hot” new Tele Vue Ethos 13mm (100° AFOV).