The basic principal is that for every cfm that is exhausted outside, the house will naturally re-pressurize bringing in outside air. If that air is cold then the furnace has to re-heat 100% of that exhausted air. I find the below method the easiest to understand:

Calculate it in KW's first then transfer to Btu/h which is how furnaces are sized.  The other reason its in KW is that Make-up Air heaters are usually small electric heaters thus sized in KW.

Temperature Rise Calculation

 

Equipment Load =  1.08 * TempRise F * CFM

 

(TempRise = the difference between the inside temperature and the outside temp)

If it's -13F (-25C) outside and it's 72F (22C) inside then the Temperature Rise is

72 - (-13) = 85 degrees F,

and Equipment Load equals to:

1.08 * 85F * 1000cfm =

91,800 BTU/h

In kilowatts, that converts to...

91,800 BTU/h ÷ 3412 =

27KW

So, when its -25C outside and the inside t-stat is 22C then, the 1000cfm of exhaust requires 91,800 BTU/h to heat up. This doesn't include the actual heat loss of the house when it's that cold. To put 91,800 BTU/h of heat that is enough heat to heat about a 4000sqft house when its -25C. The formulas can be sliced and diced into full metric or full imperial but the above method is the easiest to understand.