Final answer:
The calculation used to measure changes in the prices that firms pay for goods and services is the Producer Price Index (PPI).
Step-by-step explanation:
The calculation used to measure changes in the prices that firms pay for goods and services is the Producer Price Index (PPI). The PPI measures the average change over time in the selling prices received by domestic producers for their output. It reflects price movements from the perspective of the seller and is used to track inflation at the wholesale level.