Burstable Instance Surplus CPU Credits Charge

Hi,

I'm learning how EC2 T families burstable instances are charged in unlimited mode so that I know when to use unlimited mode or fixed CPU. I came across this website, where they have 2 scenarios to evaluate M7-iFlex and T4g instances. I try to recalculate based on what I read in the AWS documentation and compare it to their calculation. However, for both scenarios, my calculations always prefer T4g instead of M7-iFlex.

I found they seem to calculate the wrong credits on the last cycles. However, the main thing I want to ask here is when calculating CPU surplus credits charged, do I need to multiply by number of vCPU of the T instances?

For example:

• For the first scenario, the calculation is: The CPU surplus credits charged is 2898, which corresponds to 48.3 vCPU-hours. From there, we can calculate the total cost: 48.3 vCPU-hours * $0.04 * 2 CPUs = $3.86*
• For the second scenario, The CPU surplus credits charged is 1872, which corresponds to 31.2 vCPU-hours. From there, we can calculate the total cost: 31.2 vCPU-hours * $0.04 * 8 CPUs = $9.99*

When calculating credit spent and earn, we have already included the number of vCPU in the calculation. It's strange if I also need to multiply the number of vCPU in the additional charge. Do we need to multiply number of vCPU when calculating additional charge based on the total surplus CPU credits? Thanks

• This is AWS docs I refer: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-performance-instances-unlimited-mode.html

• This is the website having such scenarios: https://www.vantage.sh/blog/m7i-flex-vs-t-series-cost

• This is my calculation based on those scenarios: https://docs.google.com/spreadsheets/d/1Jb8rmzhCCBSwHTtsF68oEVb8KuwzFFriWA1zO_Tc4Wo/edit?usp=sharing