To the calculate heat requirement for your room a good rule of thumb is:
Height x Width x Depth (in feet) divided by 500. This gives you the kilowatt requirement.
To get the BTUs requirement multiply the kilowatt requirement by 3,400.
This is based on the average house insulation with a room temperature of 20 degrees and an ambient temperature of -1C. Well-insulated houses can replace the division of 500 with 650.
To convert BTU to Kw simply multiply by 0.000293.
To convert Kw to BTU simply multiply by 3414.