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Fixed costs can be a tricky business. They might seem simple but if you think too much, you might get tripped up.

Just like with variable cost, fixed costs are named because of how the cost behaves in total. It is fixed. It does not change. Now most students will take that to mean that the cost will never change. If that were the case, there is not a fixed cost on the planet. All costs change over time. Remember, we are taking about how a cost behaves today.

A fixed cost does not have an activity or driver that makes the cost increase as the activity or driver increases. Let’s say you start a business and the rent for 500 square feet is $1000 per month for the first three years. Is there an activity or driver that would increase your rent expense?

Number of hours open? Nope

Number of customers per month? Nope

Amount of sales in units or dollars? Nope

And here is the most important question: if all of your drivers go to zero, does the cost go to zero as well? If you go on vacation for a month and close your business so there are no sales, no customers, nada is your rent expense zero? Nope

With a variable cost, when the driver was zero, the total variable cost was zero. With a fixed cost, that is not the case.

Remember our candy bar example from the post on variable cost? What if, in order to sell the candy bars on campus, you needed to pay a fee of $100 to the college. Is that a fixed cost or a variable cost? It is fixed because it does not change no matter how many (or how few) candy bars you sell.

Here is the graph for fixed cost:

Notice on this graph, there is no slope. The formula for total fixed cost = fixed cost. If looking at the equation for a line y = 0 + b, where b is equal to fixed cost. As long as you are within the relevant range, the formula is valid.

**Cost Behavior: Fixed, Variable, Step and Mixed **

**Fixed and Variable costs as per unit and total costs**

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[…] because if you make the assumption that all of your costs will remain constant, whether they are fixed or variable, you may make errors on your projections. Also, if you ignore relevant range, you may […]