Backup size formula (full + incremental)

Backup storage planning sounds boring — until you run out of space the night before a restore. This calculator uses a clean “engineering approximation” that’s easy to understand and easy to explain: total storage equals the space for full backups plus the space for incremental changes, then adjusted for efficiency and overhead.

Step 1: Convert your data into one unit
We first convert your dataset into gigabytes (GB). If you enter 4 TB, the tool treats it as 4 × 1024 = 4096 GB. (Storage vendors sometimes use 1000-based units; the difference is small for planning. You’ll still want headroom.)

Step 2: Estimate how many full backups you keep
If your retention is R days and you do a full backup every F days, the number of full backups kept is: FullCount = ceil(R / F). Example: 30-day retention with weekly fulls (F = 7) gives ceil(30/7) = 5 full backups.

Step 3: Estimate incremental “delta” growth
The change rate is your average daily % of data that changes (adds/edits/deletes). If your dataset is D GB and your daily change is C%, then daily change data is D × (C/100). Over R days, incremental data is approximately: IncrementalGB ≈ D × (C/100) × R. This is a simplification — many systems do block-level deltas, synthetic fulls, or merge chains — but it’s good enough to avoid “oops we’re full” surprises.

Step 4: Raw backup data before efficiency
Raw storage before compression/dedup is: RawGB = (D × FullCount) + IncrementalGB. If you keep more full backups or have a high change rate, RawGB grows fast.

Step 5: Apply overhead and efficiency
Most backups store metadata, indexes, encryption overhead, logs, and sometimes snapshots. We add overhead as: RawGB × (1 + Overhead%/100). Then we apply efficiency: compression and dedup reduce stored size. If compression is 1.5× and dedup is 1.2×, effective storage becomes: EffectiveGB = RawGB / (Compression × Dedup).

Step 6: Multiply by copies
If you keep 2 copies (say, local NAS + cloud), you multiply by 2. This is where “small” backups become “wow”.

If you enter available storage, the calculator shows how full you’ll get: Usage% = EffectiveGB / AvailableGB. Treat anything over ~80% as “tight” because real life always adds growth, logs, and surprise projects.

• Example A (creator laptop): 500 GB data, 2% daily change, 30 days, weekly fulls, 1.5× compression, 1.2× dedup, 10% overhead, 2 copies. Result is often in the multi-terabyte range — because you’re storing history, not just today.
• Example B (small company file server): 4 TB data, 3% change, 90 days retention, weekly fulls, 1.7× compression, 1.3× dedup, 15% overhead, 3 copies. You can easily land in the tens of TB.
• Example C (database-heavy): 2 TB data, 10% change, 30 days retention, weekly fulls, 1.3× compression, 1.1× dedup, 20% overhead, 2 copies. High change rate dominates — the “delta” is the story.

How to use your result (without getting burned)

A backup estimate is most useful when it drives a decision: how much storage to buy, what plan to choose, or when to upgrade. Here’s a practical way to use the number you get.

• For personal backups: add 20–30% headroom.
• For business backups: add 30–50% headroom (growth + new apps + “oops we forgot that folder”).

• If you mostly store docs, change rate is often 1–3%.
• If you do video, design assets, or databases, it can be 5–15%+.
• If you’re guessing, run two scenarios: “calm” and “panic”.

• Doubling retention often increases total storage significantly.
• If budget is tight, reduce retention before cutting copies (copies protect against disasters).

• One copy protects against accidental deletes but not disasters.
• Two copies is a strong baseline.
• Three copies (3-2-1 style) is a classic for businesses.

Pro tip: If your estimate is close to your available storage, don’t “hope” it works — it won’t. Upgrade or reduce retention.

Backup Size Estimator FAQs

• Is this calculator accurate for every backup product?

  It’s a planning estimate. Different products store data differently (incremental forever, synthetic full backups, block-level tracking, snapshots, and cloud provider rounding). Use this to get a reliable ballpark, then confirm with your backup tool’s reports once you have a pilot running.

• What’s a “daily change rate” and how do I estimate it?

  It’s the fraction of your dataset that changes per day. If you have 1000 GB and ~20 GB changes daily, that’s 2%. If you don’t know, start with 2–3% for typical office data, or 5–10% for active media/database workloads.

• Do deletes reduce backup size?

  Not immediately. Backups keep history. If you delete a 50 GB folder today, you may still keep it for the full retention window. Deletions can even increase churn (because “change” includes deletes).

• What compression and dedup numbers should I use?

  If you’re unsure, use compression 1.5× and dedup 1.2× as conservative defaults. Text compresses well; already-compressed media (JPEG/MP4) compresses less. Dedup is best when many machines share similar files (VMs, repeated installers, template folders).

• Why does my “backup size” exceed my original data size?

  Because you’re storing time. Full backups plus daily changes over weeks/months add up. Backups are a history system, not a mirror.

• What’s the best retention period?

  For personal: 30–90 days is common. For business: 90+ days, and sometimes 1–7 years if compliance requires it. A good approach is “short retention for frequent versions + long retention for monthly archives”.

• Should I follow the 3-2-1 backup rule?

  It’s a great baseline: 3 copies, 2 different media, 1 offsite. This calculator’s “copies” field helps you size that quickly.