Free cold email inbox & domain calculator.
Work out how many inboxes and sending domains a campaign actually needs: contacts, touches, sends per day, inboxes, domains. Most plans underestimate this by about half, and the shortfall only becomes obvious once the infrastructure takes six weeks to build.
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Capacity planning at a glance
| Item | Value |
|---|---|
| Sends per inbox per day | About 30. The envelope of a busy human, which is the whole disguise |
| Inboxes per domain | Two or three, so one bad domain costs a slice of capacity rather than the campaign |
| Touches per contact | Three by default, across the whole sequence |
| Lead time | Four to six weeks of warmup before any of it carries campaign volume |
| Reference point | Our best documented month ran 162,867 sends on roughly 110 inboxes across 40 to 50 domains. How that figure is counted, and the case study it comes from. |
Why cold email capacity is arithmetic, not preference
Sending capacity is the product of four numbers: how many contacts you want to reach, how many times you touch each one, how many sends a single inbox can safely make in a day, and how many inboxes you are willing to put on one domain.
None of those four is adjustable in the direction people want. Thirty sends per inbox per day is not a conservative house rule, it is roughly what a busy person sends, and looking like a person is the entire strategy. Two or three inboxes per domain is not caution, it is blast radius: when a domain has a bad week everything on it pauses, and you would rather that be a slice of capacity than a third of the campaign.
So the only real variable is how long you are prepared to take. Which is why this arithmetic belongs at the start of planning rather than the middle, because adding capacity takes weeks of warmup, not an afternoon.
How to read your result
The calculator takes contacts, touches per contact, sends per inbox per day and inboxes per domain, then returns the infrastructure that supports it.
Total sends is contacts multiplied by touches. This is usually the first surprise, because a three-touch sequence to 20,000 contacts is 60,000 sends rather than 20,000.
Inboxes needed divides your daily send requirement by the per-inbox ceiling. Domains needed divides that by inboxes per domain.
What the numbers do not include is time. Every inbox in that count needs four to six weeks of warmup before it carries campaign volume, and warmup runs in parallel rather than instantly. A plan that needs 40 domains needs them registered and warming a month and a half before the campaign starts, not the week it launches.
If the result looks absurd, the input to revisit is touches, then contacts. Not sends per inbox.
Where the defaults come from
| Default | Value | Why that number |
|---|---|---|
| Sends per inbox per day | 30 | Roughly what a busy human sends. Providers score per-mailbox volume and consistency, not per-campaign totals. |
| Inboxes per domain | 2 to 3 | Blast radius. A domain having a bad week pauses everything on it. |
| Touches per contact | 3 | Enough to cover a missed first email without becoming a sequence people resent. |
| Warmup before campaign volume | 4 to 6 weeks | The ramp that builds sending history without tripping alarms. |
| Primary domain sends | 0 | The one rule with no exceptions. Reputation damage has to be disposable. |
What this looks like at scale
| Contacts per month | Sends at 3 touches | Roughly |
|---|---|---|
| 5,000 | 15,000 | About 17 inboxes across 6 to 8 domains |
| 10,000 | 30,000 | About 34 inboxes across 12 to 17 domains |
| 20,000 | 60,000 | About 67 inboxes across 22 to 34 domains |
| 54,000 | 162,867 (a real month) | Roughly 110 inboxes across 40 to 50 domains |
Rows one to three are the arithmetic at 30 sends per inbox per day across a 30-day month. The last row is a documented GrowthPal month rather than a projection.
How to plan infrastructure for a campaign
- Start from the list, not the budget. How many contacts genuinely fit the ICP decides everything downstream.
- Decide touches honestly. Three is the usual answer. Every extra touch multiplies your infrastructure by a third.
- Run the arithmetic before buying anything. Inboxes and domains both cost money monthly, and over-buying is as expensive as under-buying is slow.
- Register domains six weeks before launch. Warmup is the long pole, and it cannot be compressed without undoing the point of it.
- Warm every inbox in parallel. They ramp together, so the lead time is one warmup cycle rather than one per mailbox.
- Leave headroom. Plan for roughly 20% more capacity than the arithmetic says, because domains get paused and inboxes get replaced.
Common questions about inbox and domain capacity
Why only 30 sends per inbox per day?
Because that is roughly what a busy human sends, and looking like a human is the entire disguise. Providers score volume and consistency per mailbox, not per campaign. Pushing a warmed inbox to 100 a day is a common way a domain that took two months to build gets burned in a week.
Why two or three inboxes per domain instead of ten?
Blast radius. When a domain has a bad week everything on it pauses, so two or three inboxes per domain means one problem costs you a slice of capacity rather than a third of the campaign. Domains are cheap; the week you spend recovering is not.
How many domains does a real campaign need?
More than most plans assume. Our best documented month, 162,867 sends, ran on roughly 110 inboxes across 40 to 50 domains. Work backwards from contacts and touches rather than forwards from a budget, because the arithmetic does not negotiate.
Can I use my main domain for part of the volume?
No, and this is the one rule with no exception. Your primary domain carries invoices, password resets and live customer threads, and it is the single thing in this system you cannot rotate out when it has a bad month.
How far ahead do I need to buy the infrastructure?
Six weeks before launch, because warmup is the long pole and it cannot be compressed. Domains also benefit from around two weeks of age between registration and their first send, so the registration decision comes even earlier than the warmup schedule suggests.
What happens if I just add more inboxes to fewer domains?
You save money and concentrate risk, which is the wrong trade. Ten inboxes on one domain means a single domain-level reputation problem pauses ten mailboxes at once, and domain-level problems are the common kind. The cost difference between six domains and two is trivial next to a paused campaign.