Yes, of course. You have to replace the heat lost with heat generated (which is the output BTU). That may or may not be the same size you currently have (they are invariably over-sized).

It's a calculation, not a guess. If you're replacing a furnace that's been there for awhile, you can assume it's way oversized. By doing a load calculation to right-size the equipment for the specifics of your home, you will get more comfort and reduce operating costs.