Marginal cost is the rate of change of the total cost (ΔTC/ΔQ) and is dependent on the variable cost. This is because fixed cost is a constant and doesn’t affect the rate of change (remember, rate of change is effectively the differential of a function, in this case it would be the differential of the total cost function). Therefore any increase in fixed costs will have a 0 effect on Marginal Cost.

This can be shown in the tables below:

A firm’s fixed costs increase by 20%, what will marginal costs increase by?

Before FC Rise:

Q	FC	VC	TC	MC
0	10	5	15	–
1	10	7	17	2
2	10	9	19	2

After FC Rise:

Q	FC	VC	TC	MC
0	12	5	17	–
1	12	7	19	2
2	12	9	21	2

As we can see the 20% increase in fixed costs has no affect on the marginal costs.

If on the other hand the variable costs increased by 20% then the Marginal Cost would similarly increase by 20%.