Size a number pool

A pool needs enough numbers that two visitors who are on the website at the same time don't see the same number. Too few numbers lower attribution confidence; too many cost money. This page explains how to estimate the size, and how to read the pool's health after launch.

How assignment works

When a visitor arrives, Call Tracking assigns them a number from the pool that no other visitor is using. The assignment lasts while the visitor is on the website and expires shortly after they leave, and the number can then go to another visitor.

So how many numbers you need depends on how many visitors are on the website at the same time, not on monthly traffic.

Estimate the size

Use the capacity calculator when you order numbers, or the same formula by hand:

numbers = ceil( (visits per hour × average time on page in minutes ÷ 60) × safety factor )

• Visits per hour is the busiest hour, not the average. As a rule of thumb, if you only know daily sessions, take about 12% of them.
• Average time on page is how long a visitor spends on pages that show the number. Two minutes is a reasonable starting point.
• Safety factor covers bursts. The calculator's default is 1.5.

For example, a website with 300 sessions a day has about 36 in its busiest hour. With a 2-minute average and a safety factor of 1.5: (36 × 2 ÷ 60) × 1.5 = 1.8, rounded up to 2 numbers.

Start with the plan's numbers

Each plan includes numbers sized for its traffic band: 2 numbers for up to 1,000 sessions a month on Starter, 5 for up to 10,000 on Growth, and 10 for up to 25,000 on Pro. Start there and adjust after a week of real traffic.

Read the pool's health

After the website has had traffic, open Number pools. Each pool card shows a health status based on the last 7 days:

Status	Exhaustion	Meaning
Healthy	5% or less	Visitors almost always get a number nobody else is using.
At risk	Up to 15%	Some visitors are assigned a number that another active visitor also has. Attribution for those calls is Medium or Low confidence.
Critical	More than 15%	Numbers are reused often. Add numbers.

Exhaustion is the share of assignments in the period where the number chosen was already assigned to another active visitor.

Add numbers

When health is At risk or Critical, order more numbers. See Add numbers to a pool. Numbers beyond the plan's inclusion are billed monthly; see Manage billing and your plan.

Enterprise accounts

There is no plan inclusion. Start with the calculator's recommendation. See Two kinds of accounts.

Reduce reuse without adding numbers

• Use Allowed Path Prefixes on the rule so the number is only swapped on pages visitors call from, such as contact and service pages.
• Use Apply only to paid traffic if you only need to attribute paid campaigns. Organic visitors then see the original number.