Unlike most of the other plugins that only support an small set of features (like adding up fields to display a total) with Smart Forms you can create very complex operations, from calculating days between dates to adding conditions inside your calculations (maybe you want to do a different calculation depending on the inputs from your customer?), if you can think in a calculation most likely it can be done =).
There are a few other plugins that also give you this flexibility but most of them just give you a big text box to type your calculation, this might be enough for calculations that use two or three fields max but once you start doing more complicated calculations (or when you start adding conditions) you WILL get lost in your own calculation. That is why we not only focus in providing you everything you need, we give you the best tools to help you maintain control (and your sanity) while creating your calculations.
With other plugins the worst thing that can happen is to make a mistake in a big calculation, if that happens be prepared for hours of frustration trying to figure out what went wrong. Mistakes can also happen in Smart Forms but when it happens the plugin will help you fix them.
If you find an issue, have any suggestion or just need help to build your calculator you can always contact us =).
You will not only have the form you wished but it will also look beautiful with customized icons, styles and a bunch of controls. You can find a few examples here