This question was answered by @Lajamerr:
Digital Ocean charges by the hour for all droplets. The cost per hour is dependent on the hardware tier you select when creating a droplet and whether you decide to add the backup option enabled. Also please note that at minimum $0.01 is charged even if you use the lowest tier for even a minute as that's the lowest that can be billed for. However Digital Ocean sets a cap on expenses for each tier.
For example, if you had a droplet with lowest tier, it charges $0.007 per hour. However the cap for that tier is $5 a month. You can not be charged more than that for that single droplet. However you must calculate each droplet instance separately. Let's say you create a droplet with the lowest tier and you run it for half a month but you decide snapshot it and destroy it. But then you figure out you actually need a new droplet with this snapshot. Since this is a new droplet instance you start with a cap reset. You don't really need to worry about this because the monthly cap cost is closely aligned with hourly cost * hours in a month. Only a 11 cent difference.
Under your expenses is, "View this month's usage and charges.", I would suggest clicking on it as it tells what you were charged for.
If you want actual calculations you need to provide us with the number of droplets you have and the features of each one(hardware tier level & whether it has backups enabled.)
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