The AWS cost calculator is a very handy tool, however, like all tools, it has its limitations. Its first main limitation is that it has to work off certain assumptions and these are stated on the website. They can, of course, be changed, so it’s worth double-checking whenever you use the tool. Its second main limitation is that there are certain costs the AWS cost calculator doesn’t show you.
The AWS cost calculator doesn’t show you intended changes in pricing
Like every other company, AWS can and does change its pricing from time to time. Even though these price changes are planned in advance, they will typically only be put into the AWS cost calculator once they are applied as the AWS cost calculator can (currently) only hold one price at a time. If you want to create a shield against unexpected rises in on-demand prices, you could look to “bank” reserved instances to lock in the price. These are available for some services and can last between one and three years.
The AWS cost calculator doesn’t show you the impact of the free tier or promotions
It seems a bit odd that the AWS cost calculator does not show you the impact of the free tier since this can help to lower your bill and thus make AWS more attractive as compared to the competition, especially Microsoft Azure and Google Cloud. It’s more understandable that they don’t show promotions and discounts since these may vary and different customers may benefit from different promotions.
It’s also worth noting that the AWS cost calculator automatically assumes that you are “starting from zero”. In other words, if you are an existing customer, it doesn’t see if you would qualify for any volume discounts based on the combination of your current usage plus the extra usage for which you are requesting a cost calculation. It just assumes that you are a new customer and gives you the “basic” price. This can actually make quite a noticeable difference to your calculation, so it’s a bit surprising that Amazon does not include it.
The AWS cost calculator doesn’t show you pricing in different regions
More accurately the AWS cost calculator only shows pricing for one region at a time. If you choose the wrong region, then you will probably get inaccurate pricing. If you need pricing for different regions, then you’ll need to rerun the calculations for each region.
The AWS cost calculator doesn’t show you the impact of taxes
This is not particularly unusual for pricing tools like this, since they are generally designed to be accessible to anyone from any state in the U.S. and, indeed, in principle, any country in the world and it would become far too complicated for the AWS cost calculator to try to factor in the costs of taxes, especially given the fact that taxes can and do change and the fact that the calculation of taxes can depend on a number of factors which can also change.
In short, it would simply be horrendously complicated for the AWS cost calculator to try to include the impact of taxes, but each individual company absolutely must factor them into their cost calculations and budgets.
The AWS cost calculator does not show you the impact of exchange rates
This one is obvious when you think about it, but it’s still worth noting if your main working currency is anything other than USD. You might want to think about how currency fluctuations could impact your pricing and if there’s anything you could do to reduce your exposure or even make it work in your favor, for example buying up reserved instances when the exchange rates are at their best (for you).
The AWS cost calculator doesn’t show the impact of third-party licensing fees
This is another one which may seem obvious, however it can trip you up if comparing AWS to Microsoft Azure. The Azure TCO calculator assumes that you have Microsoft Software Assurance which gives you access to Azure Hybrid Benefit, in other words, if you have existing Microsoft licenses, you may be able to use their cloud equivalents at a discount. If you don’t, however, then you will either have to buy Microsoft SA or pay the full license fee for the cloud. The former tends to be more economical.
The AWS cost calculator doesn’t show you the impact of leap years
According to the AWS cost calculator, a year has 365 days, it does not account for the extra day in leap years. Admittedly this is probably only going to increase an AWS bill by the tiniest of amounts, however it is still worth noting, especially since 2020 will be a leap year.
For the sake of completeness is that the AWS cost calculator does not support per-second billing options even though these are used by some of the services.