Email Open Rate Calculator — Reported and MPP-Adjusted

Background.

An open rate of 45% looks like a triumph until you ask what an open is. Email platforms cannot see anyone read a message: they embed a tiny remote image and log an open when it is fetched, or when a link is clicked. Since Apple introduced Mail Privacy Protection with iOS 15 in September 2021, Apple Mail fetches that content in the background for protected users "regardless of whether you engage with the email", in Apple's words, so every such message registers as opened, read or not.

The division itself is settled. Mailchimp defines open rate as the share of successfully delivered emails that registered an open, where delivered means sent minus hard and soft bounces, and HubSpot measures against the recipients who received the email. Unique opens over delivered, times 100. The argument is about the numerator.

So you get three readings of one send. The reported rate is the dashboard figure, machines included. The lower bound takes machine opens off the top and leaves the denominator alone, which also discards every protected recipient who genuinely read the email, so the true human rate cannot sit below it. The observable-audience rate takes those recipients off the bottom as well, assuming one machine open per protected recipient and that protected readers open at the same rate as everyone else; it is the closest match to an open rate measured before 2021.

The machine count must come from your own platform's bot-filtering or Apple-MPP report. No published figure says what share of a particular list reads mail through Apple's proxy, so none is assumed; enter 0 and all three rates collapse to the reported one.

What is email open rate calculator?

Open rate is the percentage of delivered emails that registered at least one open. Two counting choices define it. The numerator is unique opens — recipients who opened at least once — rather than total opens, which counts a person again every time they reopen. The denominator is delivered: emails sent minus hard bounces, which are permanent failures such as a dead address, and soft bounces, which are temporary ones such as a full mailbox. Because an open is really an image request or a click, the figure measures loaded pixels rather than attention, and it errs both ways: readers who block images and never click are missed, while protected Apple Mail users are counted whether they read or not.

How to use this calculator.

1. 01From one campaign report, take four figures: emails sent, bounces (hard and soft together), unique opens and machine opens.
2. 02Make sure the opens figure is unique opens. If it is larger than sent minus bounces, you are holding total opens and the calculator will reject it.
3. 03For machine opens, use the count your platform attributes to Apple Mail Privacy Protection or bot filtering. If it does not separate them, enter 0 and read every rate as the reported figure.
4. 04Choose the headline: reported to match your dashboard, observable audience to compare with campaigns from before September 2021, or the lower bound when you need a minimum you can defend. All three are calculated whichever you pick.
5. 05Before comparing two campaigns, check that the machine share of opens is similar in both. If it moved, the reported rates are measuring different audiences.

The formula.

With D = sent − bounces, U = unique opens and M = machine opens: reported rate = U ÷ D × 100; lower bound = (U − M) ÷ D × 100; observable-audience rate = (U − M) ÷ (D − M) × 100. Alongside them, delivery rate = D ÷ sent, bounce rate = bounces ÷ sent, machine share of opens = M ÷ U, and the sent-based rate = U ÷ sent, all as percentages. The three open rates always fall in the same order. The lower bound and the observable rate share a numerator, and the observable rate divides it among fewer recipients, so it is at least as large. Subtracting the observable rate from the reported one leaves M(D − U) ÷ [D(D − M)], which cannot be negative because opens never exceed deliveries; the two match only when there were no machine opens or every delivered email was opened. The sent-based rate equals the reported rate times the delivery rate, since (U ÷ D) × (D ÷ sent) = U ÷ sent. Inputs that leave nothing to divide by are refused: bounces equal to the whole send, machine opens covering every delivered email, opens above deliveries, or machine opens above opens.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Apple — "Mail Privacy Protection & Privacy" (Apple Legal, privacy data documentation). Verbatim: Mail "downloads remote content in the background by default — regardless of whether you engage with the email"; Protect Mail Activity "routes all remote content downloaded by Mail through two separate relays operated by different entities. The first knows your IP address, but not any third-party Mail content you receive. The second knows the remote Mail content you receive, but not your IP address." This is the primary authority for why a reported open rate overstates human engagement. Free, first-party. Retrieved 2026-07-29.
2. [2]Mailchimp Help — "About Open and Click Rates". Verbatim: "The open rate is a percentage that tells you how many successfully delivered emails were opened by recipients"; "The click rate is a percentage that tells you how many delivered emails registered at least 1 click"; and, on the distortion, "Bot activity such as Apple's Mail Privacy Protection (MPP) can falsely inflate open and click metrics." Official product documentation from a major measurement provider; free. Retrieved 2026-07-29.
3. [3]Mailchimp Help — "About Email Reports" (campaign statistics glossary). Verbatim: "Successful deliveries: The number of recipients who didn't hard or soft bounce on this email"; "Opened: The number of recipients who opened this email any number of times"; "Total opens: The total number of times the email was opened by recipients. This count includes multiple opens from individual recipients"; "Open rate: The percentage of successfully delivered emails that registered as an open." Establishes the delivered denominator and the unique-versus-total-opens distinction used on this page. Retrieved 2026-07-29.
4. [4]HubSpot Knowledge Base — "Analyze your marketing email campaign performance". The second, independent authority used to check the denominator: open rate is the "percentage of recipients who opened the email out of the number who received it", click rate is measured against the same base, and click-through rate is measured against openers. Also confirms the tracking mechanism: "An email open is counted when the images in the email load, or when a recipient clicks on a link in the email." It agrees with Mailchimp on the denominator; the only difference is wording (recipients versus delivered emails), which coincide for a single send. Retrieved 2026-07-29.