A Google Analytics 4 Measurement ID is the unique identifier you need to install GA4 tracking on your website or app. It always starts with 'G-' followed by 10 alphanumeric characters, like G-ABC123XYZ4. This replaced Universal Analytics' tracking IDs that started with 'UA-'. You find your Measurement ID by logging into Google Analytics 4, clicking Admin (gear icon), selecting the property you want to track, then clicking Data Streams. Click your web stream and the Measurement ID appears at the top right. Copy it exactly as shown. The Measurement ID tells Google's servers which GA4 property should receive the data when someone visits your site. You paste it into your Google Tag Manager container, your WordPress plugin, or directly into gtag.js code in your site header. Without the correct Measurement ID, your analytics simply won't work, all hits go nowhere. Common mistakes we see: confusing the Measurement ID with the older UA tracking code, using a Measurement ID from the wrong property (client has multiple), or fat-fingering a character when copying it. Always verify your ID starts with 'G-' not 'UA-' or 'AW-' (that's Google Ads). Test your installation with GA4's real-time reports within 5 minutes of adding the code. At Ottawa SEO Inc., we manage GA4 for our 500+ domain portfolio, and we keep a spreadsheet mapping each domain to its Measurement ID because one typo means lost data you can't recover. If you're migrating from Universal Analytics, you need a completely new Measurement ID, your old UA code won't convert. Set up GA4 now if you haven't, Universal Analytics stopped collecting data July 1, 2023. One Measurement ID can track multiple pages on the same domain, but you need separate IDs for different root domains unless you're doing cross-domain tracking, which requires additional configuration beyond just the Measurement ID. For most single-domain sites, one Measurement ID handles everything once properly installed.