Retention Duration Calculator

Why Calculate Retention Duration?

When you have a fixed storage budget (NAS, disk array, cloud quota), you need to know how long you can retain backups before rotating or overwriting. This calculator answers: given X GB of capacity, Y GB of data to backup, and a chosen strategy, how many days/weeks/months can you keep recovery points? It helps balance retention against capacity and avoid running out of space unexpectedly.

Key Concepts

• Storage capacity: Total space available for backups (dedupe/compression excluded in this estimate).
• Rotation rate: How often you perform a full backup and discard the oldest. Daily = 1 day per cycle; weekly = 7 days; monthly = 30 days.
• Retention duration: Total time span covered by retained backups. More cycles = longer retention but more space.
• Strategy impact: Incremental and differential use less space per cycle than full, so you retain more cycles (longer retention) for the same capacity.

For the same capacity, incremental and differential retain more cycles than full-only, because each cycle uses less space. But incremental and differential add complexity (chain restore, change rate). Use this table to compare:

Example (1 TB, 100 GB data, weekly full): Full-only ≈ 10 weeks. Full+6 incremental at 5% ≈ 7 weeks (130 GB/cycle). Full+6 differential at 5% ≈ 4 weeks (205 GB/cycle). Full-only gives longest retention here because data size is small vs capacity.

The rotation rate determines how often you create a new full backup and remove the oldest. Daily full backups: each cycle = 1 day; weekly: 7 days; monthly: ~30 days. A higher rotation rate (more frequent fulls) shortens retention for the same capacity, but gives more recovery points. Lower rotation (e.g. monthly) extends retention but fewer full restore points.

• Daily full: 1 cycle = 1 day. Best for small datasets, strict RPO. Capacity fills quickly.
• Weekly full: 1 cycle = 7 days. Common for most workloads. Balance of retention and space.
• Monthly full: 1 cycle ≈ 30 days. Long retention for compliance. Fewer recovery points.

Example: 500 GB capacity, 100 GB data, weekly full, 6 incrementals at 5%. Size per cycle = 100×(1+6×0.05)=130 GB. Cycles = 500÷130 ≈ 3. Retention = 3×7 = 21 days. Full-only would give 5×7 = 35 days but no incrementals.

The daily change rate directly affects retention: higher change = more space per cycle = shorter retention for the same capacity. Use this table to estimate your change rate:

GFS (Grandfather-Father-Son) uses different retention for each tier: daily (Son), weekly (Father), monthly (Grandfather). To calculate total capacity for GFS, use the Backup Size Calculator. To know how long each tier lasts with your capacity, split capacity across tiers or run this calculator for each tier separately.

Example: 2 TB capacity, 200 GB data, weekly full. For Son (7 daily incrementals at 5%): 200×(1+7×0.05)=270 GB per week. You could retain ~7 weeks of daily chains. For Father (4 weekly fulls): 4×200=800 GB. For Grandfather (12 monthly): 12×200=2400 GB. Total GFS exceeds 2 TB; you must prioritize tiers or add capacity.

Many regulations impose minimum retention periods. If your calculated retention is shorter than required, you must add capacity or adjust strategy. Common requirements:

• GDPR: No fixed minimum; keep only as long as necessary. Document retention rationale.
• HIPAA: 6 years for healthcare records. Plan capacity for long retention.
• SOX / financial: 7 years for audit trails. Often requires archive tier.
• Industry standards: PCI-DSS, ISO 27001 may require specific retention. Check your sector.

Use this calculator to verify you can meet minimum retention with your current capacity. If not, plan for tiered storage (hot disk for recent, cold/tape for long-term) or additional capacity.

• Add 15-25% buffer for metadata, catalog, and unexpected growth. Actual retention will be slightly lower.
• If retention is too short, consider: incremental/differential instead of full, or more capacity.
• If retention is longer than needed, you can reduce rotation (e.g. weekly instead of daily) or keep more full cycles.
• Use the Backup Size Calculator to check required capacity for a target retention; use this calculator for the inverse.
• Deduplication and compression can extend effective retention by 20-50%; factor this in when planning.
• Data growth: if your dataset grows 10% per year, retention will shorten over time. Plan headroom.