Formula Fields

A custom field doesn't have to sit passively on the form. On the field editor's Conditions tab, administrators can attach two kinds of formulas:

• A visibility formula automatically shows or hides the field based on other fields. Any field type can have one (purchase order line item fields are the exception).
• A value formula automatically calculates the field's value based on other fields. This works for Text, URL, Number, Currency, Checkbox, Drop down list, and Date fields.

Writing formulas

Formulas reference other fields on the same form by name in curly braces. A value formula on a "Surcharge" Currency field might read:

{Job cost} * .15

and a visibility formula that shows a field only when there's a cost to explain:

{Job cost} > 0

The editor includes a Formula operators & functions reference listing what's available, and a helper that copies an AI prompt so you can ask an assistant to write the formula for you. Formulas can chain — one calculated field can feed another — but they can't form a loop; FMX rejects circular references when the field is saved.

What users see

Someone filling in the form doesn't see the formulas, just their effects: fields appearing when they become relevant and calculated values filling in as their inputs change. A calculated field behaves like a read-only answer — the way to change it is to change the fields it's computed from.

A worked example

An events team charges a 15% service fee on paid rentals. The administrator adds two Currency fields to the schedule request form: "Rental fee", filled in by staff, and "Service fee" with the value formula {Rental fee} * .15 and a visibility formula of {Rental fee} > 0. For internal (free) events, the form stays uncluttered — no service fee field at all. The moment staff enter a rental fee on a paid booking, the service fee appears, already calculated, and updates itself if the rental fee is later corrected.