High - Low Method of Cost Estimation

Now, cost estimation is based upon the relationship between past level and past cost of activity. Variable cost is based on the relationship between costs at the lowest level of activity and the highest level of activity. The difference in cost among low and high activity level is considered to be the net variable cost from that the unit variable cost can be computed by dividing it with the change in output level. This is shown underneath as:

 "Total Variable Cost = Cost at high activity level - Cost at low activity level"

Hence,

Unit Variable cost = Variable cost/ Output Units

= (Cost at high level activity - cost at low level activity)/ (Units at high activity level - units at low activity level)

The variable cost per unit so estimated forms the 'b' of the straight line equation stated earlier.  Via substituting 'b' into the equation, we can acquire 'a', the fixed cost.