To use the heating BTU calculator, you will first need to measure the place you want to heat up. You need to know if you’re heating up a 1000 sq ft, 1500 sq ft, or a 3000 sq ft home, or a 400 sq ft room, for example.

Secondly, you need to figure out what climate zone you live in. That will determine how many BTU per square foot you need for heating (more on that later on). The United States is divided into 7 main climate zones or regions. Example: Miami, Florida, is in Climate Zone 2 and requires 35 BTU of heat per sq ft. Chicago, Illinois, is in Climate Zone 5 and requires 50 BTUs of heat per square foot.

https://basc.pnnl.gov/images/iecc-climate-zone-map

Climate Zone BTUs Per Sq Ft
Climate Zone 1 30 BTU per sq ft
Climate Zone 2 35 BTU per sq ft
Climate Zone 3 40 BTU per sq ft
Climate Zone 4 45 BTU per sq ft
Climate Zone 5 50 BTU per sq ft
Climate Zone 6 55 BTU per sq ft
Climate Zone 7 60 BTU per sq ft

We presuppose the standard ceiling height of 8 ft. For every additional ft above 8 ft, you need to add 12.5%. So, for 9 ft ceiling, you need to multiply the BTUs by 1.125 to get the most adequate estimation.