Conversion credits are the units used to measure your usage of the API. Each type of conversion (e.g., HTML to PDF) costs a certain number of credits. Your plan includes a monthly allotment of credits, which reset on your billing date. Unused credits do not roll over to the next month.
One credit equals 0.5MB of output data. Therefore, an output file of size less than 0.5MB costs 1 credit, a file size between 0.5MB to 1MB costs 2 credits, and so on.