Calculations

Compute live totals, quotes and scores on a form with the Calculation field — and feed the result straight into the payment total at checkout. The value recalculates as the user types, so people see the price update instantly.

Where: add a Calculation field from the builder’s Advanced group, then write its formula in the field’s Expression box.

Writing a formula

A Calculation field holds an expression that references other fields by their Field ID wrapped in braces — for example {qty} * {price}. It supports the usual arithmetic operators + - * / %, a leading minus for negatives, and parentheses for grouping. Number fields, ranges, ratings, product prices and amount-bearing choice fields can all be referenced, so a single formula can combine quantities, unit prices and selected options.

Turning choices into amounts

Number and product fields are numeric on their own, but a dropdown, radio, checkbox, multi-select, image choice or toggle normally stores a text value. To let one of those contribute to a calculation, open the field’s settings and switch on Use as calculation / payment value. You can then give each option an Amount, and a single checkbox or toggle takes one amount for its checked state.

Once that switch is on, referencing the field by its ID in a formula reads the summed amount of the selected option(s) — not the text label. So {services} where services is a multi-select returns the total of every ticked option’s amount. The option labels stay human-readable in your submissions and notifications; only the calculation sees the numbers.

• Choice fields — each option carries its own Amount; multi-select and checkbox groups sum every selected option.
• Product fields — resolve to their per-option price automatically.
• Matrix (Likert) fields — give each column an optional Score and the field contributes the total score across its rows, which is ideal for quizzes and assessments.

Functions

The engine ships a small library of functions you can call inside an expression:

• min, max, sum, avg — aggregate any number of arguments.
• round, ceil, floor, abs — rounding and sign. round takes an optional second argument for decimal places.
• sqrt, pow — square root and exponentiation.
• if(condition, then, else) — branch on a comparison, e.g. if({qty} > 10, {price} * 0.9, {price}) for a bulk discount.

So a tiered quote might read round({base} + sum({addon1}, {addon2}), 2).

Common uses

• Order totals — quantity times product price, plus shipping or options.
• Instant quotes — a base rate plus chosen add-ons.
• Service & consultation fees — a choice field where one option (e.g. “Consultation first”) carries the fee, fed into the payment total.
• Scores & estimates — weight several inputs into a single number.

Worked example: a consultation fee

On a “Hire an expert” form you might offer a How can we help? multi-select whose options are mostly free but where Consultation first costs a flat fee:

1. On the multi-select, switch on Use as calculation / payment value and set the Consultation first option’s Amount to 50 (leave the other options at 0).
2. Add a Calculation field called Consultation fee with the expression {multiselect} — using the multi-select’s own Field ID.
3. Optionally add a Conditional rule so the fee only appears once Consultation first is selected. To show the currency to the user, add a short HTML/text note beside it (for example Consultation fee $50/hr) — the calculation field itself displays the plain number.
4. In Payments, set the amount to the Consultation fee calculation field.

Now selecting Consultation first reveals the Consultation fee of 50 and charges exactly that at checkout, while the other options stay free.

Formatting the result

Set the decimal places (default 2) so the displayed total is clean — the field then shows the plain formatted number, for example 50 or 1,250.00. To label it with a currency or unit, add a short text or HTML element next to the field (such as a $50/hr note) or fold the symbol into the field’s label; that reads clearly while the calculation itself stays a pure number the payment step can charge. Rounding is symmetric — halves round away from zero on both positive and negative values, and the browser preview uses the same rules as the server so what the user sees is exactly what gets stored.

Driving payment

Point your Payments settings at the calculated total and the amount charged at checkout matches exactly what the user saw — no double entry, no mismatch. Because the same formula — including every option amount — is evaluated again server-side on submission, a tampered client value can’t change the price.

Tip: give referenced fields clear IDs; the formula reads much better as {quantity} * {unit_price} than with generic labels.

Troubleshooting

My calculation shows nothing or zero

Check that every field name in braces matches an actual Field ID exactly — IDs are case-sensitive, and renaming a referenced field updates the reference only if you rename it from its own settings. A reference to an empty or non-numeric field is treated as zero. If you are referencing a dropdown, radio, checkbox, multi-select or toggle, make sure Use as calculation / payment value is switched on and the options have amounts — otherwise the field has no numeric value to contribute.

I get an error about a circular reference

Two Calculation fields can’t reference each other, directly or through a chain — the engine detects the loop and refuses to evaluate it. Restructure so the formulas form a one-way dependency, with each calculation reading only from fields defined before it in the logic.