Gravity Forms — fbc/fbp Populator

Auto-fills two Gravity Forms hidden fields with the visitor's _fbc and _fbp cookie values, so the values can be sent server-side via the Meta Conversions API on submission.

Pair this with the Gravity Forms GTM Listener — the listener pushes these hidden field values into dataLayer.gforms_fields on submit, where your Meta Pixel Lead / CAPI tag can read them.

Why both fbc and fbp?

Cookie	What it is	When it's set
`_fbc`	Facebook click ID — derived from `?fbclid=...` on entry	Only on visitors who clicked through a Facebook/Instagram ad
`_fbp`	Facebook browser ID — first-party browser identifier	Every visitor who loads a page with Meta Pixel installed

The Meta Conversions API matches conversions to ad clicks using these values, so capturing both improves attribution accuracy. See Meta's Conversions API parameters docs.

Setup

Step 1: Add the hidden fields in Gravity Forms

For each cookie (fbc, fbp):

Edit your form → add a Hidden field
Open the field → Advanced tab
Check Allow field to be populated dynamically
Set Parameter Name to fbc or fbp (the parameter name — without the leading underscore — is what the PHP filter hooks onto)
Save

Note the numeric field ID of each hidden field — you'll need them in Step 3.

Step 2: Install the populator

Two install options. Use PHP by default. Use the GTM JS fallback only when a page cache is in play.

File	Pattern	When to use
`populator.php`	WordPress PHP filter	Default. Runs server-side per request.
`populator-inline.html`	GTM Custom HTML tag with JS fallback	Only when a page cache (LiteSpeed, WP Rocket, host-level) serves the same form HTML to multiple visitors.

Option A: PHP filter (recommended)

Drop populator.php into your theme's functions.php, or paste into a PHP snippet plugin (WPCode, Code Snippets, etc.). No configuration needed — the filter hooks bind to the parameter names you set in Step 1.

Option B: GTM JS fallback

Open populator-inline.html and edit the MAPPINGS block — left side is the form input's HTML name (input_N where N is your hidden field's ID), right side is the cookie name:

var MAPPINGS = {
  'input_18': '_fbc',  // replace 18 with your fbc field's ID
  'input_17': '_fbp'   // replace 17 with your fbp field's ID
};

In GTM:

Tags → New → Custom HTML
Paste the contents of populator-inline.html
Trigger: All Pages — Page View
Save and publish

Step 3: Wire the values into your CAPI tag

Assumes the GF GTM Listener is also installed (it pushes hidden field values into gforms_fields on gforms_form_submit).

In GTM create two Data Layer Variables:

Variable name	Data Layer Variable Name
`DLV - GF fbc`	`gforms_fields.input_{your fbc field ID}`
`DLV - GF fbp`	`gforms_fields.input_{your fbp field ID}`

In your Meta Pixel Lead tag (triggered on gforms_form_submit), pass:

fbc → {{DLV - GF fbc}}
fbp → {{DLV - GF fbp}}

Known limitations

No fbclid, no _fbc cookie. The _fbc cookie is only created by the Meta Pixel when a visitor lands with ?fbclid=... in the URL. Direct, organic, and email traffic produce empty fbc values — this is expected and correct.
Pixel must run before the form renders. _fbp is set on every Pixel page-view. As long as your Meta Pixel base code loads before GF renders the form (it does by default in GTM), the cookie will exist.
Consent gating. If your consent platform blocks the Meta Pixel before consent is granted, neither cookie exists, and both fields stay empty — the correct behavior under GDPR / similar regimes.
Don't use both Option A and Option B at once. Pick one. Running both is harmless (the JS will just re-set the same value the PHP already populated) but adds an extra GTM tag for no benefit.

Disclaimer

Provided as-is under the MIT License. Test thoroughly before relying on it for revenue attribution.

Packaged by W.H. Boggs — whboggs.com → Free Meta Ads audit

The code

populator-inline.html

html

<!--
  Marketing Toolkit — Gravity Forms fbc/fbp Populator (JS fallback) v1.0.0
  https://github.com/whboggs/marketing-toolkit

  Packaged by W.H. Boggs — https://whboggs.com
  → Free Meta Ads audit: https://whboggs.com/audit

  MIT License | Copyright (c) 2026 W.H. Boggs

  Use this ONLY if a page cache (LiteSpeed, WP Rocket, host-level) serves
  the form HTML to multiple visitors. The PHP filter approach (populator.php)
  runs server-side per request and is more reliable when no shared cache
  sits between PHP and the visitor.

  Configuration:
    Edit MAPPINGS below — left side is the form input's HTML name (input_N
    where N is your hidden field's ID in Gravity Forms), right side is the
    cookie to read. Add more pairs if you have additional cookies to capture.

  Install:
    Paste into a GTM Custom HTML tag set to fire on All Pages — Page View.
-->
<script>
  (function () {
    // input HTML name → cookie name
    var MAPPINGS = {
      'input_18': '_fbc',
      'input_17': '_fbp'
    };

    function readCookie(name) {
      var m = document.cookie.match('(^|;)\\s*' + name + '\\s*=\\s*([^;]+)');
      return m ? decodeURIComponent(m[2]) : '';
    }

    function populate() {
      var wrappers = document.querySelectorAll('.gform_wrapper');
      for (var w = 0; w < wrappers.length; w++) {
        var inputs = wrappers[w].querySelectorAll('input[type="hidden"]');
        for (var i = 0; i < inputs.length; i++) {
          var cookie = MAPPINGS[inputs[i].name];
          if (!cookie) continue;
          var value = readCookie(cookie);
          if (value) inputs[i].value = value;
        }
      }
    }

    if (document.readyState === 'loading') {
      document.addEventListener('DOMContentLoaded', populate, { once: true });
    } else {
      populate();
    }
  })();
</script>

populator.php

php

<?php
/**
 * Marketing Toolkit — Gravity Forms fbc/fbp Populator
 * https://github.com/whboggs/marketing-toolkit
 *
 * Packaged by W.H. Boggs — https://whboggs.com
 * → Free Meta Ads audit: https://whboggs.com/audit
 *
 * Auto-populates two Gravity Forms hidden fields with the visitor's Meta
 * tracking cookies (_fbc, _fbp) so they can be sent server-side via the
 * Conversions API at form submission.
 *
 * Setup:
 *   1. In Gravity Forms, add two Hidden fields to your form.
 *   2. On each field's Advanced tab, check "Allow field to be populated
 *      dynamically" and set Parameter Name to `fbc` and `fbp` respectively.
 *   3. Drop this snippet into your theme's functions.php, or paste it into a
 *      PHP snippet plugin (WPCode, Code Snippets, etc.).
 *
 * MIT License | Copyright (c) 2026 W.H. Boggs
 */

add_filter('gform_field_value_fbc', function () {
    return isset($_COOKIE['_fbc']) ? sanitize_text_field($_COOKIE['_fbc']) : '';
});

add_filter('gform_field_value_fbp', function () {
    return isset($_COOKIE['_fbp']) ? sanitize_text_field($_COOKIE['_fbp']) : '';
});