Full, incremental and differential backups
- Full backup: a complete copy of the data. Simple to restore, but takes the most time and space.
- Incremental backup: only what changed since the previous backup of any kind. Small and fast, but a restore needs the last full plus every incremental since.
- Differential backup: everything changed since the last full backup. Grows during the week, but a restore needs only two pieces.
A common schedule is one full backup per week and an incremental backup every other day. This calculator estimates the storage for that schedule, or for daily full backups for comparison.
The formula
Weekly full + 6 incrementals: storage per week = F + 6 × F × c
Total = weeks kept × storage per week ÷ compression ratio × copies
F = data size, c = daily change rate
Worked example
500 GB of data with a 2% daily change rate, one full backup per week plus six daily incrementals, kept for 4 weeks:
- Each incremental: 500 × 0.02 = 10 GB; six of them: 60 GB.
- Per week: 500 + 60 = 560 GB. Four weeks: 2,240 GB.
- With a compression and deduplication ratio of 1.5: 2,240 ÷ 1.5 ≈ 1,493 GB (about 1.5 TB).
- Keeping a second, offsite copy doubles it to about 3 TB.
Daily full backups for the same four weeks would need 28 × 500 = 14,000 GB before compression, more than six times as much.
The 3-2-1 rule
A widely recommended practice is to keep 3 copies of your data (the original plus two backups), on 2 different types of storage, with 1 copy offsite. Many organisations now add an immutable or offline copy so that ransomware cannot encrypt or delete it, sometimes called 3-2-1-1-0 (one immutable copy and zero errors after verified restores).
Choosing the inputs
- Change rate: file servers often change 1–3% per day; busy databases and mailboxes can be higher. Your backup software's job reports show the real figure.
- Retention: how far back you need to restore. Legal or industry rules may require months or years for some data; keep monthly or yearly fulls for long retention instead of every daily backup.
- Compression: text and databases compress well; photos, videos and already-compressed files hardly at all. Use 1.0 if unsure.
- Growth: add expected data growth for the coming year before buying storage.
Storage is not the whole story
How much data you can afford to lose (the recovery point objective, RPO) and how quickly you must be back online (the recovery time objective, RTO) decide the schedule more than storage does. Test restores regularly: a backup that cannot be restored is not a backup. For how downtime targets translate into minutes, see the uptime and SLA calculator.
Common mistakes
- Forgetting the offsite copy in the storage budget.
- Counting decimal and binary units differently: a "2 TB" disk shows about 1.82 TiB in many operating systems; see TB to TiB.
- Assuming high deduplication for data that is already compressed.
Long retention with grandfather-father-son
Keeping every daily backup for a year would need enormous storage. The grandfather-father-son (GFS) scheme keeps daily backups for a week, weekly fulls for a month and monthly fulls for a year. For 500 GB of data with 2% daily change:
- 6 daily incrementals: 6 × 10 GB = 60 GB
- 4 weekly fulls: 4 × 500 GB = 2,000 GB
- 12 monthly fulls: 12 × 500 GB = 6,000 GB
- Total before compression and deduplication: about 8,060 GB
Modern backup software deduplicates repeated data between fulls, so the real figure is often far lower, but plan with the raw number if you are unsure.
Cloud storage costs
Multiply the storage by the provider's price per GB per month, then check retrieval (egress) and early-deletion fees, which matter when you restore. Archive tiers are cheap to store but slow and sometimes costly to restore, so keep recent backups on faster storage.
Frequently asked questions
How much storage do I need for backups?
For a weekly full plus daily incrementals: weeks kept × (data + 6 × data × daily change rate), divided by the compression ratio, times the number of copies. 500 GB, 2% change, 4 weeks, ratio 1.5 needs about 1.5 TB per copy.
What is the 3-2-1 backup rule?
Three copies of your data, on two different types of storage, with one copy offsite.
Incremental or differential backups?
Incrementals use the least space and time; differentials restore faster because you need only the last full and the latest differential.
What daily change rate should I assume?
1–3% per day is typical for file servers; check your backup software reports for the real value.
What is grandfather-father-son backup rotation?
A retention scheme keeping daily backups for a week, weekly backups for a month and monthly backups for a year or longer.
How do I estimate cloud backup cost?
Storage in GB × price per GB per month, plus retrieval and early-deletion fees if you restore from archive tiers.
Sources
- NIST Special Publication 800-34 Rev. 1, Contingency Planning Guide for Federal Information Systems (backup and recovery strategies).
- CISA (US Cybersecurity and Infrastructure Security Agency): data backup options and the 3-2-1 rule.
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.