How to Calculate Email Open Rate (Formula)
To calculate email open rate, divide the number of unique opens by the number of delivered emails, then multiply by 100. The formula is: Open Rate = (Unique Opens ÷ Emails Delivered) × 100. Crucially, you divide by emails delivered—not the total sent—because bounced emails were never eligible to be opened.
The email open rate formula
Here's the formula in plain terms, plus the one distinction most people get wrong:
| Element | What it means |
|---|---|
| Unique opens | The number of individual recipients who opened the email at least once (not total opens). |
| Emails delivered | Emails sent minus bounces—the messages that actually reached an inbox. |
| × 100 | Converts the decimal into a percentage. |
Open Rate = (Unique Opens ÷ Emails Delivered) × 100
Sent vs. delivered: why it matters
If you divide by emails sent instead of delivered, you understate your open rate—you're penalizing yourself for addresses that bounced and could never open. Always use delivered emails as the denominator. Delivered equals total sent minus hard and soft bounces.
A worked example
Say you send a campaign to 5,000 subscribers. Here's how it shakes out:
- Emails sent: 5,000
- Bounces: 200
- Emails delivered: 5,000 − 200 = 4,800
- Unique opens: 1,200
Now apply the formula:
Open Rate = (1,200 ÷ 4,800) × 100 = 25%
Your open rate is 25%. If you'd mistakenly used the 5,000 sent as the denominator, you'd get 24%—understating your real performance. The gap widens as your bounce rate rises, which is why the delivered figure is the correct one.
Unique opens vs. total opens
A single recipient might open your email three times—that's three total opens but one unique open. Standard open rate uses unique opens so each person counts once. Total opens can be useful for gauging how compelling or shareable a message is, but for the headline open-rate metric, always use unique opens.
Quick reference: related email formulas
Open rate rarely travels alone. Here are the sister metrics, calculated the same way:
| Metric | Formula |
|---|---|
| Open rate | (Unique opens ÷ Delivered) × 100 |
| Click-through rate (CTR) | (Unique clicks ÷ Delivered) × 100 |
| Click-to-open rate (CTOR) | (Unique clicks ÷ Unique opens) × 100 |
| Bounce rate | (Bounces ÷ Sent) × 100 |
| Unsubscribe rate | (Unsubscribes ÷ Delivered) × 100 |
Why your calculated open rate may be imperfect
Open tracking works by loading a tiny invisible image when the email is viewed. Two things distort it:
- Privacy features that pre-load images register an "open" even when no one reads the email—inflating the number.
- Plain-text views or blocked images can cause a real open to go uncounted—deflating it.
The takeaway: treat open rate as a directional trend, not a precise truth. It's excellent for A/B testing subject lines and spotting week-over-week shifts, but pair it with clicks and conversions for the full picture.
Do you need to calculate it by hand?
Usually not—any real email platform reports open rate automatically for every campaign. Knowing the formula still matters, though: it lets you audit what your tool reports, compare numbers correctly across systems, and understand why a change in bounce rate moves your open rate. Once you know your opens, run the whole campaign through the email marketing ROI calculator to see what that engagement is actually worth in revenue.
Frequently asked questions
What is the formula for email open rate?
Open Rate = (Unique Opens ÷ Emails Delivered) × 100. Divide unique opens by delivered emails—not total sent—then multiply by 100 to get a percentage.
Should I use sent or delivered emails in the formula?
Delivered. Bounced emails never reached an inbox and couldn't be opened, so including them in the denominator artificially lowers your open rate.
What's the difference between unique and total opens?
Unique opens count each recipient once, no matter how many times they open. Total opens count every open event. Standard open rate uses unique opens.
Why is my open rate over 100% impossible—or is it?
Using unique opens, open rate can't exceed 100%. If you ever see a figure above it, you're likely using total opens (repeat opens by the same person) rather than unique opens.
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