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If I purchase AWS reserve instance (EC2 and / or RDS) in mid of month, will it cost for full month or it will be on pro rata basis ?

I have searched on there docs I was not able to find, may be I have missed.

EDIT

I have requested cancel current AWS EC2 RI and AWS RDS RI in US, AWS support team told me first I need to purchase in different region.

Payment option is No Upfront

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    I'm voting to close this question as off-topic because the questioner should ask the vendor. Jan 22 '18 at 17:13
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There aren't any monthly costs for reserved instances, so the question doesn't arise. You pay an up-front payment for a one or three year period, and then a payment calculated hourly for instance usage. It's billed monthly, but it's charged per hour.

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  • I need to purchase with No Upfront. Jan 22 '18 at 10:32
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    In which case there's only an hourly cost, which you pay whether or not the instance is running.
    – Mike Scott
    Jan 22 '18 at 10:36
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    Thank you very much @mike-scott , I have also analyzed last year bill if AWS RI is purchased in mid month(No Upfront), they will cost for remaining hour for the month. Jan 22 '18 at 10:46
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Given answer from Mike Scott is correct. I would like to add:

Think about Reserved Instance (RI) like a discount and not like a running EC2 instance. If you purchase a RI for 1 year, the respective discount is available for 365 days.

When you purchase a Reserved Instance, you are billed for every hour or second during the entire Reserved Instance term that you select, regardless of whether the instance is running or not.

For example, if you own three Reserved Instances with the same instance attributes and region (or Availability Zone if applicable), the billing system checks each hour to see how many total instances you have running that match those parameters. If it is three or less, you will be charged the Reserved Instance rate for each matching instance running that hour. If more than three are running, you will be charged the On-Demand rate for the additional instances.

Purchases of Reserved Instances are non-refundable.

How Billing Works

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