Bounce rate confidence interval calculator: how sure is your bounce rate?
In short: Enter how many emails you sent and how many bounced. The calculator gives the rate and a Wilson confidence interval, and says if your ceiling sits above, below or inside it. Limit: It assumes each email was an independent chance of bouncing, which real lists are not.
Enter how many emails you sent and how many bounced, pick a confidence level, and optionally a ceiling such as 2 percent. The calculator gives the bounce rate and the Wilson score interval around it, so you can see whether a small send proves you are over the ceiling or whether it is too small to say.
Price: this tool is free. Checking a whole email list is from $9/mo, and the first 100 checks are free with Google, no card. Check your list free nowSee pricing
The numbers below are examples. Replace them with your own send.
The bounce rate you want to stay under.
- Bounce rate
- 2.4%
- Range from
- 1.38%
- Range to
- 4.15%
Your ceiling is inside the range. A send this size cannot say whether the true rate is over or under it.
Formula: Wilson score interval with z = 1.645, 1.96 or 2.576 for 90, 95 or 99 percent. It assumes each email was an independent chance of bouncing.
Sample result
500 emails and 12 bounces give 2.4%, with a 95 percent range of 1.38% to 4.15%.
- Bounce rate
- 2.4%
- Confidence
- 95%
- Ceiling
- inside the range
What this means for you
A 2% ceiling sits inside the range, so a send this size cannot say if the true rate is over it.
- Wilson score range
- 90, 95 or 99 percent
- Your own ceiling
- Formula shown
- Bounce rate
- Range
- Calculator
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
A range tells you what a send means, not which addresses bounced.
- To lower the rate on the next send, check the list before it goes out.
- Upload or paste it at /verify. Every address gets a live mailbox check, and the first 100 email checks are free with a Google sign-in, no card.
How do I know if my bounce rate is really over my limit?
- Enter the number of emails sent and the number that bounced.
- Pick the confidence level: 90, 95 or 99 percent.
- Enter the ceiling you want to stay under, if you have one.
- Read the rate, the range and whether the ceiling is above, below or inside the range.
Example
- You enter
- Example send: 500 emails, 12 bounced, 95 percent confidence, ceiling 2 percent.
- You get
- Bounce rate 2.4%, range 1.38% to 4.15%. The 2% ceiling is inside the range, so a send this size cannot say whether the true rate is over or under it.
- Limits
- Runs in your browser. It assumes each email was an independent chance of bouncing.
What is the formula?
With sent n, bounced k, p = k ÷ n and z from the confidence level (1.645, 1.96 or 2.576), the Wilson score interval is centre ± half-width, where centre = (p + z² ÷ 2n) ÷ (1 + z² ÷ n) and half-width = z × √(p(1−p) ÷ n + z² ÷ 4n²) ÷ (1 + z² ÷ n). It stays between 0 and 100 percent, which a plain plus-or-minus can not.
The interval treats each send as an independent random sample from the same list. Real lists are not random, so read the range as a guide.
What does inside the range mean?
If your ceiling sits inside the range, the send is too small to say whether the true rate is over or under it. If the whole range is above the ceiling, the rate is over it at that confidence level; if it is all below, it is under. To estimate a rate before a send, use the bounce rate estimator.
How does this relate to the plain bounce rate?
The plain rate is bounces divided by sent, as in the bounce rate calculator. The interval adds how much that number can move on a send of this size. Read bounce rate too high on a small send for what to do with a small send.
Who uses this tool, and for what?
- Cold emailers
- Check whether a 3 percent bounce on a 100-email test means you are over a 2 percent ceiling.
- Deliverability consultants
- Show a client why a small send cannot settle a bounce question.
- Marketing ops
- Decide if a pilot send is big enough to judge a list source.
What can’t this tool tell you?
- It assumes each email was an independent chance of bouncing, which real lists are not.
- It uses the counts you enter and cannot tell soft bounces from hard ones.
- It does not know your platform's bounce rules or thresholds.
- A range is not a forecast for the next send.
Common questions
- What confidence level should I use?
- 95 percent is the usual choice. 99 percent gives a wider range and 90 percent a narrower one.
- Why use Wilson and not plus or minus?
- It stays between 0 and 100 percent and behaves better on small sends and rates near zero.
- Does a wide range mean the send went badly?
- No. It means the send was small, so the rate could have been somewhat higher or lower on a bigger one.
- Can I use it for open or reply rates?
- The same formula works for any share of a count, but the wording on this page is about bounces.
- What if I sent zero bounces?
- The rate is 0, and the upper end of the range shows how high the true rate could still be.
- Is anything uploaded?
- No. The numbers stay in your browser.
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? Starter at $9/mo covers 2,500 emails a month, cancel anytime
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.