When it comes to figuring out how many BTUs you need to heat a room, there are a few things to consider. The first is the size of the room. A 20x20 room is 400 square feet. The BTU formula generally recommends 30-35 BTUs per square foot, so you would need between 12,000 and 14,000 BTUs to heat this room.
Another thing to consider is the climate. If you live in an area with a colder climate, you will need more BTUs to heat the room than if you live in a warmer climate.
The last thing to consider is what type of heating system you have. If you have a gas furnace, you will need more BTUs than if you have an electric heater.
Taking all of these things into consideration, you would need between 12,000 and 14,000 BTUs to heat a 20x20 room in a colder climate with a gas furnace.