Key takeaways
- Work backwards from leads: at a 3% reply rate with 25% positive replies, one lead takes about 133 new contacts.
- With follow-ups, each contact gets about 4.2 emails, so 25 leads a month means roughly 14,000 emails.
- At 15 cold emails per mailbox per day, that is 43 sending mailboxes on 15 domains, plus 9 pre-warmed mailboxes in rotation: 52 mailboxes on 18 domains.
Most teams say they need 20 qualified leads a month. Then they send 500 emails and hope.
The math does not work. Reply rates keep falling, and a lead goal sets a sending volume your infrastructure has to carry. This calculator starts from the leads you want and works back to the contacts, the emails, and the mailboxes and domains that can send them without landing in spam.
Plan your volume
Cold email capacity calculator
Start with the number of leads you want each month. A lead here is a positive reply: someone who wants to talk.
Then your funnel. If you have no history yet, keep the benchmarks: most campaigns get a 1–5% reply rate, and about a quarter of the replies are positive. The rest are not interested, the timing is wrong, or the person does not decide.
That goal needs this many new contacts and emails every month:
The infrastructure behind it
A new mailbox stays out of spam only at a low daily volume. We plan 15 cold emails per mailbox per day, on top of about 30 warm-up emails, and three mailboxes per domain. Change them if your setup differs.
That total includes a rotation pool of pre-warmed mailboxes. When a mailbox burns, a warm one takes its place the same day, and sending does not stop.
| Leads you want per month | Mailboxes in total | Domains in total |
|---|---|---|
| 10 | 21 | 7 |
| 25 | 52 | 18 |
| 50 | 102 | 34 |
| 75 | 154 | 52 |
| 100 | 204 | 68 |
| 150 | 306 | 102 |
| 200 | 408 | 136 |
How the calculator works
The calculator works backwards from the number of leads you want each month. A lead is a positive reply: someone who wants to talk.
New contacts per month = leads ÷ (reply rate × share of replies that are positive). At a 3% reply rate and 25% positive replies, one lead takes about 133 contacts.
Emails per month = contacts × 4.2: the first email plus follow-ups, averaged over all contacts.
Sending mailboxes = emails per month ÷ (cold emails per mailbox per day × 22 working days). Each mailbox also sends about 30 warm-up emails a day, which are not counted here.
Sending domains = sending mailboxes ÷ mailboxes per domain.
Rotation pool = 20% of the sending mailboxes: pre-warmed spares that replace a mailbox the day it burns, so sending never stops. Total mailboxes = sending + rotation; total domains = total mailboxes ÷ mailboxes per domain.
Every count is rounded up, because a fraction of a mailbox or a domain still has to be bought.
The table shows results for typical inputs. Other inputs stay at their defaults: replies that are positive 25%, cold emails per mailbox per day 15, mailboxes per domain 3.
| Leads you want per month | Reply rate, % | New contacts per month | Emails per month, with follow-ups | Mailboxes in total | Domains in total |
|---|---|---|---|---|---|
| 10 | 3 | 1,334 | 5,603 | 21 | 7 |
| 25 | 3 | 3,334 | 14,003 | 52 | 18 |
| 50 | 3 | 6,667 | 28,002 | 102 | 34 |
| 100 | 3 | 13,334 | 56,003 | 204 | 68 |
| 25 | 1.5 | 6,667 | 28,002 | 102 | 34 |
| 50 | 1.5 | 13,334 | 56,003 | 204 | 68 |
| 100 | 1.5 | 26,667 | 112,002 | 408 | 136 |
For code and AI agents: GET https://growth.band/api/tools/cold-email-capacity?leadsPerMonth=25&replyRatePct=3&positiveReplyPct=25&emailsPerMailboxPerDay=15&mailboxesPerDomain=3 returns the same numbers as JSON with a one-paragraph summary. Every query parameter is optional; leave one out to use its default.
Before you buy fifty mailboxes
If the calculator says you need thousands of emails a month and your current setup is held together with tape, fix deliverability first. More volume on a setup that already lands in spam only burns more domains: check where your emails land today, before you invest more time writing them.
Get sending infrastructure that stays out of spam
We build and run your cold email infrastructure: domains, mailboxes, daily placement checks and a rotation pool.
Ilya AzovtsevCo-founder
Co-founder of GrowthBand. Helped scale lemlist from $20k MRR to $10M ARR and built automation at Expandi; now runs the cold email infrastructure behind 32,000+ mailboxes.
