I have been thinking about getting a projector. The room that I plan to put this in will have a TV so I will have to use a screen that I can pull down when I want to use projector and roll it up when I want to see TV. I can have about 18' distance between the screen and projector. I think that this will be more than required throw distance.

But, I do have a fan (with a light fixture) between screen and projector. I cannot find offset info on projectors. I know they can be titled downwards but is there a place where I can enter the dimensions of room, screen & fan and calculate if it will interfere? I can take out light fixture if needed but need to get some tips on offset.

Thanks.

There is no program or app to do what you want that I know of. Bottom line, if any part of the fan is lower than the top of the screen (assuming a ceiling-mounted projector) then it will interfere with the image.

If you move the screen so the fan is not inline with the top then you move the projector down as well. If you don't want to move the projector, then do a drawing to visualize the offset. Draw a scale side-view representation of the room. Take a look at the crude example I've attached.

Let me add some to Sheridan's response. If any part of your fan blocks a straight line from your projector lens to the top of your screen, then the fan will be blocking & will leave a shadow on your screen. With lens shift and/or offset, your projector can throw the image downward (or upward) to the top of your screen and miss your fan if the fan is not in line between the projector and (the top of) your screen. You can draw out your room with your fan at least approximately located and determine whether you can mount your projector to your ceiling far enough behind your fan to miss hitting it. Or, you might be able to mount your projector in front of your fan, but I doubt you'd be asking this question if that were feasible for you. Another alternative would be if you had horizontal lens shift you might mount your projector on your ceiling beside your fan and still hit your screen square.
Some people don't ceiling mount their projectors and put them on a shelf or table instead, but this almost always requires lens shift.
LCD projectors usually have lens shift and DLP ones usually do not.