Calculator Inputs
Formula Used
months = retention_days / 30
growth_factor = (1 + growth_pct_month/100) ^ months
dataset_effective = dataset_gb * growth_factor
eff_factor = compression_ratio * dedupe_ratio
full_size = dataset_effective / eff_factor
incr_size = (dataset_effective * change_rate_pct/100) / eff_factor
full_snaps = floor(retention_days/full_every_days) + 1
incr_snaps = total_snaps - full_snaps
storage_raw = (full_snaps*full_size) + (incr_snaps*incr_size)
storage_buffered = storage_raw * (1 + buffer_pct/100)
monthly_cost = storage_buffered * cost_per_gb_month
How to Use This Calculator
- Enter dataset size and snapshot interval.
- Set retention window and full cadence.
- Estimate change rate using historical trends.
- Apply efficiency factors from your storage system.
- Add buffer to reduce capacity risk.
- Press Calculate to view costs and storage.
- Lower intervals increase snapshot counts rapidly.
- Shorter full cadence limits long incremental chains.
- Higher growth increases full and incremental sizes.
- Use the RPO check to validate interval choices.
- Download outputs for capacity and budget reviews.
Example Data Table
| Example input | Value | Example output | Value |
|---|---|---|---|
| Dataset size | 1,200 GB | Total snapshots retained | 85 |
| Interval | 4 hours | Buffered storage | 4,059.61 GB |
| Retention window | 14 days | Full snapshots retained | 3 |
| Full cadence | 7 days | Incremental snapshots retained | 82 |
| Change rate | 3.0% | Monthly cost | 81.19 |
Sizing Inputs That Matter
This planner starts with dataset size in gigabytes, then projects growth across the retention window. The calculator compounds monthly growth using (1 + growth%/100)^(retention_days/30). The result becomes the effective protected dataset used for sizing. If growth is negative, it is treated as a reduction, but you should validate with recent capacity reports.
Interval Planning and RPO
Snapshot interval sets how many restore points exist. Intervals are converted to days for snapshot counts and to minutes for RPO checks. Total snapshots are floor(retention_days/interval_days) + 1, ensuring a snapshot at day zero. If you enter an RPO target, the tool compares the interval in minutes to that target. When interval minutes exceed the target, the plan recommends reducing the interval.
Full and Incremental Snapshot Mix
Full snapshots provide independent restore points, while incrementals capture deltas between fulls. Full snapshots retained are floor(retention_days/full_every_days) + 1, capped by total snapshots, and the cadence is never allowed to be shorter than the interval. Incrementals retained equal total minus full. Incremental size is estimated as dataset_effective × change_rate% / (compression × dedupe). Use realistic change rates: 1–5% for stable file servers, 5–15% for busy databases, and higher during migrations.
Efficiency and Safety Buffer
Compression and deduplication reduce stored footprint through the efficiency factor (compression_ratio × dedupe_ratio). Full snapshot size equals dataset_effective / efficiency. Raw retained storage equals full_count×full_size + incr_count×incr_size. A safety buffer percentage is then applied to cover metadata, block maps, filesystem overhead, and change volatility. Many teams start with 10–20% buffer, then tune it after observing actual reclaim behavior and retention pruning.
Cost, Auditability, and Policy Reviews
Monthly cost is buffered_storage_gb × cost_per_gb_month, with annual cost shown for budget cycles. Support sign-off and renewals. Cost per GB-month should reflect replication, API calls, and backup catalog overhead when applicable. Use the example table to validate assumptions and record policy decisions. For audit readiness, export CSV for change control, and generate PDF for approvals, then re-run quarterly to reflect new growth, changed datasets, and revised recovery objectives.
FAQs
What does “change rate per snapshot” represent?
It estimates how much data changes between snapshots, expressed as a percent of the protected dataset. Use monitoring, filesystem churn reports, or database write metrics to approximate a steady-state delta.
Why include both compression and deduplication ratios?
They model storage efficiency. Compression reduces bytes within blocks, while deduplication avoids storing identical blocks repeatedly. Multiply them to estimate the combined reduction applied to full and incremental snapshot sizes.
How should I choose the full snapshot cadence?
Set it to limit incremental chain length and improve restore predictability. Weekly full snapshots are common for 30–90 day retention, but high-change workloads may benefit from more frequent full snapshots to reduce operational risk.
What is the purpose of the safety buffer?
The buffer covers metadata overhead, variable deltas, index growth, and platform-specific retention behavior. Start with 10–20% for planning, then refine after comparing estimates to real retained storage over several pruning cycles.
Does meeting the RPO guarantee meeting the RTO?
No. RPO relates to recoverable data loss, while RTO depends on restore throughput, verification steps, and application recovery time. Use this tool for storage planning, then test restores to validate end-to-end recovery time.
Why can results change after I shorten retention?
Retention affects both snapshot counts and how much dataset growth is applied. A shorter window reduces counts and reduces the compounded growth factor, which lowers estimated full and incremental sizes.