Email list decay calculator
Enter your list size and the months since you last verified it. You'll see how many addresses have likely gone stale, at a measured rate you can change.
5.5% is Hunter's measured rate (Nov 2025).
About 1,679 addresses have decayed (16.8%)
8,321 of 10,000 still hold the status they had at the last check, at 5.5% every 8 weeks, compounded.
| Months since the check | Decayed | Share of the list |
|---|---|---|
| 3 | 878 | 8.8% |
| 6 | 1,679 | 16.8% |
| 12 | 3,077 | 30.8% |
| 18 | 4,240 | 42.4% |
| 24 | 5,207 | 52.1% |
The short version
- The default rate is Hunter's: 5.5% of verified addresses degraded in eight weeks (published 4 November 2025).
- The rate compounds, so each period takes 5.5% of what is still good, not of the original list.
- Past six months, roughly one address in six has changed status at that rate. Re-verify before the next big send.
- Put in your own rate if you track bounces per campaign. The math is the same.
Where the 5.5% comes from
Hunter verified a set of business addresses, waited eight weeks and verified them again. In that time 5.5% had degraded: 2.3% had become invalid and 3.2% had turned accept-all, which means the domain now accepts every address and the mailbox can no longer be confirmed. Hunter published the result on 4 November 2025.
It's a recent, dated measurement with a stated method, which is why it's the default here. Many decay figures quoted online carry no study at all. If your own bounce data says your list moves faster or slower, type your rate in.
| Months since the last check | Decayed out of 10,000 | Share of the list |
|---|---|---|
| 3 | 878 | 8.8% |
| 6 | 1,679 | 16.8% |
| 12 | 3,077 | 30.8% |
| 18 | 4,240 | 42.4% |
| 24 | 5,207 | 52.1% |
At 5.5% every 8 weeks, compounded; months converted at 52 ÷ 12 weeks each. Source rate: hunter.io/email-verification-guide/email-data-decay/, read 24 September 2026.
How the calculator does the math
First it turns your months into weeks, at 52 ÷ 12 weeks a month, and divides by eight to get the number of periods. Then it applies the rate once per period to what is still good: remaining = size × (1 − rate)periods. Decayed is the size minus that.
Compounding matters over long gaps. A straight 5.5% per period would claim 71.5% of the list after two years, while the compounded figure is about 52%. The table above shows the compounded numbers, because a status can only change once.
Decay shows up on your next send as hard bounces and as addresses on catch-all domains that used to verify cleanly. Our bounce rate calculator tells you whether the damage has reached the level mailbox providers react to. For the wider numbers, read how often lists actually decay.
Questions people ask
How fast do email lists decay?
Hunter measured 5.5% of verified addresses degrading in eight weeks in an experiment published in November 2025: 2.3% became invalid and 3.2% turned into accept-all. At that pace a list loses about 31% of its verified status in a year.
How do you calculate email list decay?
Multiply the list by (1 minus the rate) once for every 8-week period that has passed, then subtract the result from the starting size. With 10,000 addresses at 5.5% over 24 weeks, that is 10,000 × 0.945³, about 8,439 left, so about 1,561 have decayed.
How often should I re-verify my email list?
Before every large send, and at least every quarter for a list you mail steadily. At Hunter's rate a list drifts by about 5.5% every two months, and a quarter covers about one and a half of those periods.
Find out which addresses decayed
The calculator estimates how many. ZapBounce tells you which ones, with a result for every address and no charge for unknowns or duplicates.