How to calculate the size of your tracking number pool

A tracking number pool is a collection of unique phone numbers that rotate across website visitors so Nimbata can tie each call back to the session, source, and campaign that drove it. The right pool size depends on your peak traffic and average session duration.

The formula

Pool size = [Peak hourly visitors] / [60 / average session duration] × 1.3

Example: 200 peak visitors, 3-minute average session → 200 / (60/3) × 1.3 = 13 numbers

Method 1. Use the Nimbata pool estimator

The easiest option.

Nimbata calculates the pool size for you when you add tracking numbers.

• Go to Tracking > Numbers.
• Click '+ Add'.
• When prompted, click 'Get from calculator'.
• Enter your peak hourly visitors.
• Click 'Use pool'.

Method 2. Use a GA4 table report

• In GA4, go to Explore > Blank. Set:
• Dimensions to Date + Hour.
• Metrics to Sessions and Average Session Duration.
• Technique to Free form.
• Visualization to Table.
• Sort by Sessions descending to find your peak.
• Apply the peak Sessions value and Average Session Duration to the formula above.

Method 3. Connect GA4 directly

When adding tracking numbers:

• Click 'Get from Google'
• Grant Nimbata access to your GA4 account.
• Select your GA4 property.

Nimbata will calculate the recommended pool automatically.

🎉 Done! Apply your pool size when setting up or expanding your tracking numbers.

Detailed Guide: How many tracking numbers do you need?

Detailed Guide: Tracking numbers 101: how to set up call tracking numbers

Updated on: 30/03/2026