Compound interest calculator template

Free build guide: Compound interest calculator for WordPress

This guide rebuilds the Compound interest calculator from this site in Gravity Forms: 4 inputs, 3 live results and no submit button. Expect about 21 minutes by hand. The template above is the same form as an import file, and the bundle adds a PDF version of this guide with a screenshot of every setting, plus the stylesheet.

You will end up with:

• Balance after the period
• You contributed
• Interest earned

What you need

• Gravity Forms (any license) — Calculation fields are in core; fv() needs GP Advanced Calculations.
• GP Advanced Calculations (Gravity Wiz) — Exponents, functions and date maths in calculation fields. The formulas in this template use its syntax. Part of Gravity Perks; needs the Spellbook plugin active as well.

The formulas below use GP Advanced Calculations syntax: exponents, functions or date maths that core calculation fields cannot do. Install and activate it, with its Spellbook parent plugin, before you start; without it every result shows 0.

1. Create the form

1. Go to Forms → New Form and name it Compound interest.
2. In Form Settings → Form Layout, set the CSS Class Name to fo-live-only. Step 3 uses it to hide the submit button.
3. Leave the form open in the editor; the fields come next.

2. Add the fields, in this order

Field IDs are assigned as you add fields. If yours come out differently, change the {Label:ID} references in the formulas to match your IDs. Settings not mentioned stay at their defaults.

1. Initial deposit · Number · ID 1
Number Format Currency; Default Value 10000; Layout: half width, on the same row as its neighbour (drag the field beside it in the editor; no CSS class needed).

2. Monthly contribution · Number · ID 2
Number Format Currency; Default Value 200; Layout: half width, on the same row as its neighbour (drag the field beside it in the editor; no CSS class needed).

3. Annual rate (%) · Number · ID 3
Number Format 9,999.99; Default Value 7; Description “Compounded monthly.”; Layout: half width, on the same row as its neighbour (drag the field beside it in the editor; no CSS class needed).

4. Years · Number · ID 4
Number Format 9,999.99; Default Value 10; Layout: half width, on the same row as its neighbour (drag the field beside it in the editor; no CSS class needed).

5. Balance after the period · Number · ID 5
Number Format Currency; Custom CSS Class (Appearance tab) fo-result; tick Enable Calculation, Rounding 2.

fv( {Annual rate:3} / 1200, {Years:4} * 12, 0 - {Monthly contribution:2}, 0 - {Initial deposit:1} )

6. You contributed · Number · ID 6
Number Format Currency; Layout: half width, on the same row as its neighbour (drag the field beside it in the editor; no CSS class needed); Custom CSS Class (Appearance tab) fo-result fo-result--sub; tick Enable Calculation, Rounding 2.

{Initial deposit:1} + {Monthly contribution:2} * {Years:4} * 12

7. Interest earned · Number · ID 7
Number Format Currency; Layout: half width, on the same row as its neighbour (drag the field beside it in the editor; no CSS class needed); Custom CSS Class (Appearance tab) fo-result fo-result--sub; tick Enable Calculation, Rounding 2.

fv( {Annual rate:3} / 1200, {Years:4} * 12, 0 - {Monthly contribution:2}, 0 - {Initial deposit:1} ) - ( {Initial deposit:1} + {Monthly contribution:2} * {Years:4} * 12 )

3. Hide the submit button

The results are live, so nothing needs submitting. Add this filter to your theme’s functions.php or a code-snippets plugin; it hides the button on any form carrying the fo-live-only class:

add_filter( 'gform_submit_button', function ( $button, $form ) {
	return str_contains( (string) rgar( $form, 'cssClass' ), 'fo-live-only' ) ? '' : $button;
}, 10, 2 );

Or with CSS alone: .fo-live-only_wrapper .gform_footer { display: none; }

4. Put it on a page and make it look like this

Add the form to any page with the Gravity Forms block, or the shortcode [gravityform id="X" title="false" ajax="false"]. The form inherits your theme’s styles; the classes you set in step 2 are hooks for the look you see on this site:

• fo-result on a calculation field makes it the big teal result panel.
• fo-result fo-result--sub makes the smaller secondary figures.
• fo-segmented on a radio field turns it into a segmented switch.
• Two fields on one row is the editor’s own column layout: drag a field beside another and the row splits in half. No CSS class is needed, and the old gf_left_half Ready Classes are deprecated.

The bundle includes the two stylesheets that implement those classes, with notes on matching your brand colours. Without them the form still works; it just looks like your theme’s default Gravity Forms styling.

How the maths works is explained under “How it’s calculated” on the live Compound interest calculator.