ETA calculator

An ETA (estimated time of arrival) calculator predicts when a running task will finish based on how fast it has progressed so far. It is handy for file downloads, database migrations, video renders, report generation or any batch job that shows progress but no remaining time.

How to use it

1. Enter the total amount of work, for example 2,000 files or 50 GB.
2. Enter how much is already done and the time it took, or the processing rate.
3. Set the start time.
4. Read the remaining duration and the estimated finish time.

The calculation

Rate = Amount done / Elapsed time. Remaining time = (Total - Done) / Rate. ETA = Now + Remaining time.

Worked example

A migration of 1,200,000 rows started at 09:00. At 09:40 it has processed 300,000 rows. Rate = 300,000 / 40 min = 7,500 rows per minute. Remaining = 900,000 / 7,500 = 120 minutes, so the ETA is 11:40, about 2 hours from now.

Why ETAs drift

• Uneven work: the first files may be small and the last ones large, or vice versa.
• Caching and warm-up: jobs often speed up after an initial slow phase.
• Contention: other processes, network congestion or database locks change the rate.
• Throttling: APIs and cloud services may slow down after a burst.

Tips

• Recalculate with recent progress rather than the whole run if the speed changes.
• Measure progress in the unit that drives the time: bytes for downloads, rows for migrations.
• Add a safety margin of 10 to 20% when promising a finish time to others.