To include calculations in custom themes, particularly using numbers entered in custom fields, use arithmetic filters available in the Liquid templating language:

• abs — returns the absolute value of a number
• at_least — limits a number to a minimum value
• at_most — limits a number to a maximum value
• ceil — rounds the input up to the nearest whole number
• divided_by — divides a number by the specified number
• floor — rounds a number down to the nearest whole number
• minus — subtracts a number from another number
• modulo — returns the remainder of a division operation
• plus — adds a number to another number
• round — rounds an input number to the nearest integer or, if a number is specified as an argument, to that number of decimal places
• times — multiplies a number by another number

Example 1

Let’s say you have two custom fields defined for sales invoices. Both are number type fields:

• Field 1
• Field 2

and you want to also show on invoices the sum of Field 1 and Field 2. You do not need to define a third custom field and enter content into it. Instead, inject the following lines into your custom theme at the appropriate place, following the code that displays the custom_fields array:

<div style="font-weight: bold; padding-top: 20px">Field 1 + Field 2</div>
<div>{{ custom_fields["Field 1"] | plus: custom_fields["Field 2"] }}</div>

The first line displays a label for your calculated result. The second includes the Liquid arithmetic filter that adds your two custom fields’ values. The result is:

Example 2

You can also code a custom theme to perform calculations involving built-in fields. For instance, if you want to add the value of Field 1 to the total or balance due on your invoice, inject one of the following lines into your theme:

{{ table.totals["Total"] | plus: custom_fields["Field 1"] }}

or:

{{ table.totals["Balance due"] | plus: custom_fields["Field 1"] }}