The Calculate button appears next to the submit button when a payment gateway has been added to your form. Your form users can then use this button to see the payment summary of their order.
To add the calculate button to your order form, go to Settings → Payments, under Summary and Discounts. Select the second radio option, save your changes and use the preview button on top to view how the calculate button appears. If you want to customize the button and know some CSS codding, go to Themes → Use custom CSS [ID: id123-button-calc].
You may use CSS to customize the button as you want. You can also customize the button under the Buttons subsection, although the options that are present there are limited.
