Stellarium as already suggested by Glen and Jack is an easy way to go and you get to look at your optic/sensor combination on a sky target of your choice.
Or if you're in the middle of nowhere with no power or caluclator you can just use the fact that a 1000mm focal length optic (actually 1030mm -
close enough) with a Full frame Image circle, on a FullFrame (36x24mm) sensor has a 2.00 degree
horizontal field of view and scale that linearly up or down for different focal lengths and/or sensor diagonal sizes. For example ....
2000mm optic with ~1 degree field of view horizontally on FF sensor
4000mm optic with ~0.5 degree field of view horizontally on FF sensor
500mm with ~4 degree field of view horizontally on FF sensor, etc...
If you want it more accurately then base it on 1030mm having a 2.00 horizontal FOV, but 1000mm is easier to remember. If you want to known the field of view on something other than a FullFrame sensor you need to scale the result by the ratio of the sensor diagonals (sometimes known as the Field of View Crop factor or simply Crop Factor), for similar aspect ratio diagonals, or revert to other tricks, possibly beyond mental math.
Best
JA