AWS compute savings plan commitment calculation

First thing to remember is that the billing works hourly. It means that the system evaluates your usage for each hour in the last 7/30/60 days (depending on the recommendations term you select in Cost Explorer), and recommendations engine finds the most optimal $ commitment that would provide you the highest discount based on your past usage.

Let's say you had some resources running on schedule. The system looks at your hourly on-demand spend for the recommendations term, which resources were running, and which discount % you would get for that usage if you had Savings Plans. SP discount applies to your usage each hour. If during some hours you run more resources than you reserved, any remaining usage will be charged on-demand. If during other hours you runn less, then some of the SP discount will be not utilized and cannot be used later in the month. Which means that depending on your usage patterns, you can either purchase SP to cover ALL your usage, but end up losing some of the discount, or you can purchase less SP but still pay some on-demand charges for spiky usage.

If you wanted to calculate the same for a different set of data, there's no exactly easy native feature for that, but you would need to look at the SP pricing for resources that you want to reserve. For example, if you look at resources worth $0.5 hourly on-demand spend, then the amount of SP you have you reserve will depend on the % discount each of those resources get from SP.