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Email List Decay Calculator

Email lists decay as people change jobs and abandon inboxes: studies put the loss at roughly 21% to 28% a year for B2B lists and less for consumer lists. Enter your list size, months since you last verified it and the list type to estimate how many addresses have gone bad, your likely bounce rate and the cost to re-verify.

List type

Assumed decay for B2B: 21% to 28% of addresses a year.

Your estimate

Estimate, not a measurement

Addresses likely gone bad

2,780 to 3,787

11% to 15% of 25,000

Cost to re-verify all

$123

2 x 10K + 5K pay-as-you-go

One 50K pack would be $169

Expected hard bounce rate if you send now

11% to 15% at most
0%20%
  • Even the low estimate is above 2%. Re-verify before your next send: a hard bounce rate this high is the kind that gets mail filtered or a sending account paused.
  • Duplicates and unknown results are not charged, so the real cost is often lower. Credits never expire, and deliverable results are backed by our bounce guarantee.
Show the formula +

gone bad = size x (1 - (1 - annual rate) ^ (months / 12))

Low: 25,000 x (1 - (1 - 0.21) ^ (6 / 12)) = 2,780
High: 25,000 x (1 - (1 - 0.28) ^ (6 / 12)) = 3,787

The bounce estimate assumes every decayed address hard bounces, which makes it an upper bound: some stale addresses still accept mail but are never read. Cost picks the cheapest mix of pay-as-you-go packs covering 25,000 credits.

How it works

A simple, transparent model

  1. Enter your list

    List size, months since you last verified it, and whether it is mostly work (B2B) or personal (B2C) addresses.

  2. Apply a decay range

    An annual decay range for that list type is compounded month by month, giving a low and a high estimate.

  3. Compare to 2%

    The estimate is set against a 2% hard bounce line, with the cheapest pay-as-you-go cost to re-verify every address.

Assumptions

The decay rates behind the estimate

  • B2B

    21% to 28% a year

    Low end: NeverBounce measured 21.07% attrition for business addresses (2018 report), and HubSpot cites 22.5% a year (2.1% a month) for B2B data. High end: ZeroBounce found at least 28% of addresses decayed across all lists it checked in 2024.

  • B2C

    3% to 23% a year

    Low end: NeverBounce measured 3.22% attrition for consumer addresses in the same study. High end: ZeroBounce found at least 23% decay across all lists in 2025, which includes faster-decaying business data, so it works as a ceiling.

  • Mixed

    12% to 25% a year

    A rough blend of the two ranges, for lists with a mix of work and personal addresses. If you know your split, the B2B and B2C results bracket the answer.

Formula

gone bad = size x (1 - (1 - annual rate) ^ (months / 12))

Compounding means a 25% annual rate is not 2.08% every month on the original list: each month removes a share of what is still good. Over 12 months the result equals the annual rate.

How to use the result

  • Under 2% on both estimates: you can send, but schedule the next verification before the date the calculator gives.
  • Range crosses 2%: verify before a full send, or mail only recently engaged contacts until you do.
  • Both above 2%: do not send to the whole list. Verify first and remove undeliverable addresses.
  • The bounce figure is an upper bound: some decayed addresses still accept mail and simply go unread. Those hurt engagement rather than bounce rate.

Connect SendGrid, Mailgun, Amazon SES or Postmark and every hard bounce of an address a job marked deliverable is refunded automatically within 30 days, 1 credit each, with no claim to file. Not connected? If more than 4% of a job's deliverable addresses hard-bounce within 30 days, file a claim from the job page and get 1 credit back for every bounce above 4%.

FAQ

List decay calculator FAQ

How accurate is this list decay estimate?

It is a planning estimate, not a measurement. Decay rates vary by industry, audience and how the list was collected. The only way to know how many addresses on your list are bad today is to verify them, which checks each mailbox without sending an email.

Why does B2B data decay faster than B2C?

Work addresses die when people change jobs, companies are acquired or a domain is retired. Personal addresses at Gmail, Outlook.com or Yahoo usually survive a job change and are only lost when someone abandons the inbox.

Why is 2% the danger line?

A hard bounce rate under 2% per send is the most widely used industry guideline, and Amazon SES recommends staying below it. SES places accounts under review at 5%. Mailbox providers also read high bounce rates as a sign of poor list hygiene.

How often should I re-verify my email list?

Re-verify any segment you have not mailed in about 60 to 90 days before sending to it, and verify new addresses when they are captured. Fast-moving B2B lists benefit from a quarterly pass. The calculator shows roughly when your list crosses 2%.

How is the re-verification cost calculated?

It uses RocketVerifier pay-as-you-go prices at 1 credit per address and picks the cheapest mix of credit packs that covers your list. Credits never expire. Duplicates and unknown results are not charged, so the real cost is often lower.

Keep going

Sources

Replace the estimate with real results

Verify the list and see exactly which addresses are deliverable, risky or gone. Unknown results are never charged.

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100 free credits · No card required · Credits never expire