Wordpress is really mature. This means that the software is very stable and robust. This means you can count on a solid system that just works. Also the Wordpress community is big. This means that almost every question has been asked and answered and you can always find a professional to help you when you get stuck.
Plug it in
Although Wordpress' core is based on created simple pages and posts, you can find a lot of plugins to make your website more powerful. Not only to make you website look better and add functionality, but also to improve SEO and speed. Of course ConvertCalculator offers a WordPress plugin to make embedding a calculator as easy as pie.
It doesn't matter what kind of website you need, someone has created a template for it. Most templates are paid, although there are some free templates available. Check out a marketplace such as Themeforest to find a good looking template that fits your needs. Your new calculator form works with any WordPress theme.
It's open source
Wordpress is open-source. This means that, if you are a little tech savvy, you can hack away at it. It also means that it's free! Although the software is free, you do need to pay something to host your website. If you want the speed and reliability website builders are offering (Squarespace, Weebly or Wix ), the price difference is not so big though.