I would like the number of registrations to populate in the payment form.

2) I would like the number of registrations to populate in the payment form. The calculation is correct but the number won't show in the field.

Could you please explain this further? You may post screenshots to illustrate what you mean:

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I've set up a calculation widget to multiply my registration fee by the number if registrants, and I have a field for each of those numbers so my customer can see them. The number of registrants is picked up from a field in a different part of the form.

The calculations work, but the quantity field does not populate, so the number of registrants doesn't show.

/90356463329158 lines 16 & 120

You need to add a calculation condition or form calculation to obtain or to show the total number of registrants.

Here is the condition for your requirement:

Related Guide: How to Pass Field Value to Another Field?

or you can easily get the number or value using the form calculation widget.

Guide: How to Perform Form Calculation Using a Widget