Retention Storage Calculator
What a retention policy costs in bytes — the monthlies that pin the shelf and the dailies that ride on top.
Retention Storage Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Copies, not deduplication — real stores may share blocks
- Weeklies folded into the daily count
In short: A 1,000 GB dataset under the classic policy — 12 monthly fulls plus 30 dailies at 5% change — puts 13,500.000000 GB on the shelf: twelve full copies are unavoidable, and the dailies add only 1,500.000000 GB because each carries one day's change. The grandfather-father-son rotation — 7 dailies, 4 weeklies, 12 monthlies, 23 restore points — is the classic cut; this page prices whatever cut you type.
Formula
stored = monthlies × dataset + dailies × dataset × change% ÷ 100
A retention policy is a multiplication you can actually budget: the monthlies are full copies and pin the floor of the shelf no matter how quiet the data is, while each daily carries only its day's change. Zero change means empty deltas — an honest zero, not a discount. The classic grandfather-father-son rotation keeps 7 dailies, 4 weeklies and 12 monthlies for 23 restore points; this page lets the dailies stand in for the weekly tier so the arithmetic stays one multiplication you can check by hand.
Worked Example
- Enter the size of one full copy.
- Set how many dailies and monthlies the policy keeps.
- Enter how much a day's delta carries.
- Read the shelf and the two restore horizons.
Defaults: 1,000 GB, 30 dailies at 5%, 12 monthlies → 13,500.000000 GB. Drive the change to 0 and the dailies go quiet — 12,000.000000 GB, all of it monthlies.
Strengths & Limits Of This Model
Where this engine is strong
- Monthly floor priced separately from the delta line
- Zero-change days priced as honest zeros
Where it stops
- No dedupe or compression modeling by design
Practical Use Cases
Backup planning
shelf size before you buy it
Compliance cuts
price the 12-month policy honestly
Cloud tier picks
fulls cold, dailies warm
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Retention Storage Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How do I calculate retention storage?
Multiply each retained copy by what it costs: monthlies are full copies of the dataset, dailies carry only their day's change. Twelve monthlies of a 1,000 GB dataset plus thirty 5% dailies is 12,000 + 1,500 = 13,500.000000 GB. That is the whole law — everything else on this page names the two terms.
What is the grandfather-father-son scheme?
The classic GFS rotation keeps 7 daily, 4 weekly and 12 monthly copies — 23 restore points covering a year of horizons. The dailies catch yesterday's mistake, the weeklies catch last week's, and the monthlies satisfy audit and long recovery. This page prices the dailies and monthlies tiers directly.
Why do monthlies dominate the shelf?
Because they never shrink — each is a full copy whatever the data did that month. Thirty quiet dailies at 5% add 1,500.000000 GB here while the twelve monthlies pin 12,000.000000 GB. If the shelf must shrink, cut monthly copies first; the daily line is almost always cheaper.
What does 0% daily change mean?
A day where nothing changed ships an empty delta — the daily costs essentially nothing. The page prices that honestly at the daily line instead of pretending a full copy happened. Real datasets hover a few percent; media working sets can be far noisier.
Do incrementals really cost change% of the dataset?
On average, yes — a delta carries the day's new and changed blocks. Bursty data breaks the average, which is why the number is an input here, not a constant. Measure a real week of deltas from your backup tool and feed that percentage in.
How does this differ from the log storage page?
That page prices machine noise — a rate per day and a compression ratio. This page prices the policy: copies you choose to keep of one dataset, with no compression argument. Log archives accumulate; retention copies stack on purpose.
Where do the restore points point?
The dailies reach back one per day for as many days as you keep them; the monthlies reach back one per month for as many months as you keep them. Ransomware recovery wants depth on both axes — a shallow daily window and a thin monthly tier are the two failure modes this page makes visible.
Should weeklies be a separate tier?
Most teams that care run 7/4/12 — dailies, weeklies, monthlies as three buckets. This page folds the weekly tier into the daily count to keep one honest multiplication; if you run true GFS, enter dailies + weeklies as the short-retention count and read the shelf it produces.