You'll see that we have two sliders on the homepage which also does a calculation to calculate the monthly repayment: http://laneformedlaren.se
We are using gravity forms for the form, and the problem is that the range sliders were not a built in feature in gravity forms. There is currently a plugin available to add sliders to gravity forms which we initially used https://wordpress.org/plugins/gravity-slider-fields/ however, this caused problems with the calculation feature we have, so we decided not to use it. So, instead, I added plain html range sliders to gravity forms, and "linked" each slider to a hidden field so that the data can be stored by gravity forms (and for the calculator to work).
This worked well, no problems (and the data was sent correctly). However, our form is now a multistep form, which is basically split into 3 pages. So what happens now is that the sliders resets itself back to default when a user clicks "next". Then the default values gets sent to the database, instead of the values the user set on the sliders. So, what we need to add here is either a session or cookie solution.... or we need to use some of the hooks gravityforms have available that lets you add your own custom field types to gravity forms, which gravity forms should then automatically store correctly when it's done this way.