I am using free tier services and I've been charged for the EC2

Certain Instance types are free for 750 hours in a month, not all of the instances. Think a month is consists of 30-day, if you divide 750-hour by 30-day, the answer would be 25-hour. If the free tier EC2 is or was on almost every day, you usually will not get charged. Remember February has only 28 days. Now if you have two free tier EC2 under a single account and the both instances are or were running and you have used 750-hour by those instances, after that you will get charged, even it is one miunte, you will get charged for hourly rate - that is my understanding.

https://aws.amazon.com/free/free-tier-faqs/

rePost-User-7136343 is correct. the free tier for each service is constrained by total aggregate usage. The instance free tier is a total of 750/mo hours of usage.

In addition, when you seek to stay within the free tier usage, you need to do so with a server type and size that is provided for free tier usage. An example is the t2.micro, is generally free tier. When you are configuring the instance you want to deploy, you will see a label on it identifying it as “free tier”. If you do not see this label, then it does not qualify and will be billed.

However, if you utilize a custom instance from the marketplace, it may not qualify as free depending upon what services are being “purchased”. Or the instance may be free tier, but the service may be billed…etc.

There are also other “attached” services such as EBS which can have billable usage, depending upon how you configure that.

You can also have an account that is > 1 year old, and no longer qualify for certain free tier allowances. If you just created the account, this is not the issue, but if your account has been around awhile…?

Additionally, some free tier service limits are different, for different services, so you’ll want to check those out for each service, as you explore and consume each service, though that is not germane specifically to this question.

I hope this helps.