The key difference between short run and long run is whether you can adjust inputs flexibly. In the short run, there is at least one input of which you can adjust the quantity. This type of input generates a fixed cost, because the quantity is fixed. You can freely adjust the quantities of other inputs, which represent variable costs. Therefore, in the short run, total cost is the sum of fixed cost and variable costs.

In the long run, every input is flexible because the time frame is long enough. In this case, every cost is a variable cost in the long run, and the total cost is the same as variable cost.