Backup Retention Calculator (Grandfather-Father-Son)
A retention policy such as 14 daily, 4 weekly, 12 monthly and 7 yearly backups sounds simple until someone asks how much disk it needs. This calculator turns a grandfather-father-son (GFS) policy into total storage, so you can size repositories and decide what a longer retention period really costs.
Your numbers
Source data size before compression.
Share of data that changes per day. Transactional ERP databases are often 1 to 5%.
Many manufacturers keep financial data for six to ten years. Check your own legal requirement.
Use your blended cost for disk, tape or cloud object storage.
Your results
Simplified model that treats weekly, monthly and yearly sets as separate full backups. Synthetic fulls, deduplication and database-specific features change real results, so use this for planning and confirm with your backup software.
Total storage (TB) vs monthly fulls kept
Sweeps Monthly fulls kept from 0 to 36, holding your other inputs fixed.
Optional backup policy review
Share your retention assumptions and we will point out where they cost more than they protect.
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How to use it
Enter the size of one full backup of the system you protect, for example the ERP database plus file shares. Add your daily change rate, then the number of daily, weekly, monthly and yearly restore points you keep. Select a realistic compression or deduplication ratio and the number of copies, for instance onsite plus offsite. Read the total terabytes and the monthly storage cost. Then change one retention number at a time to see which promise is the expensive one.
- Measure the change rate from last month's incremental sizes
- Use compression ratios from your own backup reports
- Include every system in scope, not only the ERP
The method
The model keeps one current full backup, plus one full for each weekly, monthly and yearly restore point, plus daily incrementals sized at the change rate. Raw data is divided by the compression ratio, then multiplied by the number of copies. Real products often share data between restore points through deduplication or synthetic fulls, which makes this a conservative upper estimate for many tools. Tape or object storage with immutability adds copies and sometimes minimum retention charges.
Reading the result
Long yearly retention usually dominates the total because every yearly point is a full copy. If the number looks high, check whether those long-term sets need the whole system, or only the finance database and archived documents. The chart shows storage rising as monthly retention grows. A flat result when you extend retention means your compression is carrying the load, so verify that ratio on real data before relying on it.
Common mistakes
The most common mistake is setting retention from habit rather than from a business or legal requirement. Another is counting restore points but never testing a restore. Teams also forget that encrypted or already compressed data compresses badly, that databases grow, and that offsite copies need their own capacity. Finally, retention is not protection from ransomware unless at least one copy is immutable or offline, so decide that separately from capacity.
Frequently Asked Questions
What is a GFS backup rotation?
Grandfather-father-son keeps frequent short-lived backups (son, daily), less frequent medium-term ones (father, weekly) and infrequent long-term ones (grandfather, monthly or yearly). It balances restore options against storage cost.
How long should we keep ERP backups?
It depends on legal, tax, customer and audit requirements, which differ by country and contract. Operational recovery points are usually weeks, while financial archives may be kept for years. Confirm with your finance and legal teams.
Why is my real storage use lower than the estimate?
Deduplication, synthetic fulls and block-level incrementals avoid storing the same data repeatedly. This model is deliberately conservative. Use the estimate for budgeting and compare with your backup product's own sizing tool.
Does this cover ransomware protection?
It only sizes capacity. Resilience to ransomware needs at least one immutable, offline or logically separated copy, tested restores and protected backup credentials. Increase the copies input to reflect any extra immutable copy you add.
If your backups feed an AI or reporting layer, Netray can design a restore-tested, on-premise copy of ERP data for that purpose without touching your production backup chain.
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