Measured mailbox sizes · observed transfer rate
Mailbox migration transfer-time planner
Estimate mailbox data copy time from a mailbox count, measured sizes, and observed transfer rate. DNS, password, client, and delivery cutover work is separate.
Estimate a continuous copy-time scenario for a group of mailboxes from the mailbox count, one measured size per mailbox, and a sustained transfer rate you observed. The worksheet runs locally in your browser and does not connect to either mail system.
Enter one size for each mailbox in the same batch. Use the data size that the selected migration method will copy; do not include passwords or mailbox addresses.
Enter the mailbox count, one size per mailbox, and a positive observed transfer rate.
Copy time and cutover work are separate
The calculation adds the entered decimal MB sizes, converts megabits per second to megabytes per second by dividing by eight, and divides total data by that rate. It assumes a continuous transfer at the observed rate. It does not add an efficiency factor or assume the observed speed will continue.
This is a data-copy estimate only. It does not include account setup, credentials or password changes, provider throttling, retries, incremental sync, DNS/MX changes, sender authentication records, mail-client setup, delivery checks, or rollback preparation. Track and schedule those tasks separately; completing a data copy does not itself switch mail delivery.
Cutover items to track separately
Use these as reminders for your own migration plan. A checkmark records your note in this page only; it does not validate a setting, test delivery, or change DNS.
0 of 6 items checked.
Sources checked November 3, 2026
- Microsoft Learn: Migrate mailbox data by using the Exchange Admin Center — describes IMAP migration batches that connect to source mailboxes and download mailbox data.
- Microsoft Learn: Tips for optimizing IMAP migrations in Exchange Online — identifies mailbox sizes, concurrent mailboxes, connections, and available network capacity as relevant migration considerations. This worksheet does not reproduce Microsoft's operational model.
For website files and database transfer estimates, see the website migration transfer-time estimator. For a detailed mail cutover record covering MX, SPF, DKIM, DMARC, client setup, delivery observations, and rollback, use the WordPress email cutover checklist. For broader site move planning, see the hosting move checklist and email authentication DNS guide.
How it works
Enter a mailbox count, one measured decimal MB size per mailbox, and an observed sustained transfer rate in megabits per second. The local worksheet estimates continuous data-copy time only; it does not connect to a mail system or estimate DNS and delivery cutover.
Good to know
How should I enter mailbox sizes?
Enter one measured size in decimal MB on each line, with exactly one line per mailbox. Use the size scope relevant to the migration method you selected; this worksheet does not inspect accounts or infer omitted folders.
Does this estimate total mail migration completion time?
No. It estimates continuous copying of the entered data at your observed rate. Account setup, credentials, provider limits, retries, later sync, DNS/MX changes, client setup, delivery checks, and rollback planning are not included.
Does the tool test the transfer rate or contact a provider?
No. Enter a rate you observed in a representative transfer. The result is arithmetic only and does not predict what a provider or migration service will sustain.
Impact — TODO: apply/approval and configure referral URL — referral link not configured.
CJ Affiliate — TODO: apply/approval and configure referral URL — referral link not configured.
PartnerStack — TODO: apply/approval and configure referral URL — referral link not configured.