How Many Inboxes and Domains Does a Cold Email Agency Need?

Agency capacity comes from one chain: clients, contacts, emails, emails per day, inboxes, domains. The agency cold email capacity planner runs it and also tells you how many clients your existing inboxes can carry.

The formula

Contacts a month   = clients x contacts per client
Emails a month     = contacts x emails per contact
Emails per day     = emails a month / sending days
Inboxes            = emails per day / emails per inbox per day
Domains            = inboxes / inboxes per domain
Inboxes per client = inboxes / clients
Clients supported  = existing inboxes x per-inbox cap x sending days
                     / (contacts per client x emails per contact)

Emails per day, inboxes and domains round up. Clients supported rounds down, since a client you cannot fully serve is not supported.

Worked example (made-up inputs)

These inputs are invented to show the arithmetic. They are not recommendations or industry limits. Take 8 clients, 400 contacts each a month, 3 emails per contact, 30 emails per inbox per day, 20 sending days and 3 inboxes per domain. The agency already has 12 inboxes.

Step	Working	Result
Contacts	8 x 400	3,200
Emails	3,200 x 3	9,600
Emails per day	9,600 / 20	480
Inboxes	480 / 30	16
Domains	16 / 3	6
Inboxes per client	16 / 8	2.00
Clients supported	12 x 30 x 20 / (400 x 3)	6

So in this example the agency needs 16 inboxes on 6 domains, and the 12 it has carry 6 clients, not 8.

Using the answer

Treat the result as a floor for the plan, not a spend target. Choose your own per-inbox cap and inboxes per domain: the planner does not know your provider's limits or how fast you warm new inboxes. For the per-campaign version of the same maths, see the meetings target calculator and our guide to how many domains for cold email. For the money side, the agency margin calculator takes the same clients.

What this calculation cannot do

• It assumes every client sends the same volume. Uneven clients need a row each.
• It spreads sending evenly over the sending days.
• It does not include warm-up, replacement inboxes or domains held in reserve.
• Cost is not part of it.

Verify each client's list first

Bounced and invalid addresses drop out of every number above: a contact that bounces never reads the email, so it can never reply or book. Verify the list before you load it. Upload it on the verify page: the first 100 email checks are free with a Google sign-in, no card. Each address comes back valid, risky or invalid with a reason, and nothing is sent to anyone on the list. For larger or recurring lists, see bulk email verification, list cleaning and pricing.

Plan your capacity

Open the agency cold email capacity planner.