The formula
incremental = data + daily change × retention; full = data × retention
Where it comes from
What a backup policy occupies is not the size of the data: it is that size times the number of copies kept. A daily full with thirty days of retention takes thirty times the data. A weekly full with daily incrementals takes little more than the data once, because each day only stores what changed. With 500 GB and 5 % daily change, the difference between the two strategies is 7,500 GB against 625.
How to work it out by hand
- Work out what changes each day: the percentage of the data
- For incremental, add one full copy plus the changes over the retention
- For full, multiply the data by the days of retention
- Divide by the compression ratio of the system
What is worth knowing
The rule that condenses decades of near-misses is called 3-2-1: three copies of the data, on two different media, one of them off site. The "off site" part is not geographic paranoia: it is what separates an incident from a permanent loss when there is a fire, a theft, or ransomware that reaches the backup drive plugged in over USB. And there is one check almost nobody does that is worth more than any space calculation: **restoring**. A backup that has never been restored is not a backup, it is a large folder. Test a full restore at least once a year and time it, because recovery time is usually far longer than people assume.