Backup Size Calculator
Estimate the storage space needed for your backup strategy (full, incremental, differential).
Backup Strategy Parameters
Table of Contents
Comprehensive Backup Size Guide
Why Estimate Backup Storage?
Planning backup capacity is essential for budget, infrastructure sizing, and meeting recovery objectives. Underestimating storage leads to failed backups or premature rotation; overestimating wastes budget. This calculator helps you size retention and choose the right strategy (full, incremental, differential) based on your data change rate and restore requirements.
How the calculation works
You enter your total data size, backup strategy (full only, full + incremental, or full + differential), number of full backups to retain, and—for incremental/differential—how many incremental or differential backups run per full cycle and what percentage of data changes daily. The calculator sums: full backup size × number of fulls, plus for each incremental the data size × change rate (and for differential, the cumulative change since the last full). Result is total storage in GB (and TB) with an optional breakdown by backup type.
- Full backup: Complete copy of all data. Restore needs only one backup. Storage grows linearly with retention.
- Incremental backup: Only blocks changed since the last backup (full or incremental). Small daily size; restore requires full + chain of incrementals.
- Differential backup: All changes since the last full. Grows each day; restore needs full + latest differential only.
- Daily change rate: Percentage of data modified per day. Databases: 5-15%; file servers: 1-10%; static content: <1%.
- Retention: How long to keep backups. Affects storage: more retention = more space. Plan for compliance (e.g. 7 years) and recovery points.
Strategy at a glance
Full only
Storage = data size × number of fulls. Fastest restore; highest storage. Best for small datasets or when simplicity matters.
Full + Incremental
Lowest storage; restore needs full + all incrementals in order. Best when backup window and storage are tight.
Full + Differential
Medium storage; restore = full + latest differential. Good balance of storage use and restore speed.
Benefits of estimating backup storage
- Budget and procurement: Right-size backup storage (NAS, tape, cloud) and avoid surprises.
- Strategy choice: Compare full vs incremental vs differential in terms of capacity and restore complexity.
- RPO/RTO: Align retention and frequency with recovery point and recovery time objectives.
- Compliance: Plan retention (e.g. 7 years) and ensure you have enough capacity for the required recovery points.
Limitations and considerations
- Daily change rate is an estimate; real change varies by workload. Measure with your backup software when possible.
- Compression and deduplication can reduce actual storage by 15–50%; add overhead for metadata and catalog (often 15–25% extra).
- Incremental chains are fragile: one corrupted backup can break restores. Test restores regularly.
- Result is raw backup size; multiply by 1.15–1.25 for practical capacity including overhead, or use product-specific sizing tools.
Always add 15–25% overhead for compression variance, metadata, and growth. Follow the 3-2-1 rule (3 copies, 2 media types, 1 offsite) and test restores regularly—a backup you cannot restore is worthless.
Quick formula reference
Full: Storage = Data size × Number of full backups
Incremental: Fulls + (Data size × Change % × Incrementals per cycle × Cycles)
Differential: Fulls + (Data size × Change % × (1+2+…+N) × Cycles), N = differentials per cycle
See the Calculation Formulas section below for a worked example.
Estimating backup storage from data size, strategy, and change rate is the first step to sizing backup infrastructure. Use this calculator to compare strategies, plan retention, and budget capacity. Combine with the 3-2-1 rule, GFS retention, and the daily change rate table below to design a robust backup plan.
Backup Strategies Explained
Each strategy trades off storage usage, backup window, and restore complexity. Choose based on your data size, change rate, and RTO/RPO.
Full Backup
A full backup copies all data every time. Storage = number of full backups × data size. Simple and fast to restore (single restore operation), but uses the most space and backup window. Suitable for small datasets or when simplicity and quick recovery matter more than storage cost.
Incremental Backup
Only backs up changes since the last backup (full or incremental). Each incremental is small (e.g. 1-10% of data per day). Restore requires the last full plus all incrementals since then, in order. Least storage use, but restore can be slow and fragile: one corrupted incremental breaks the chain.
Differential Backup
Backs up all changes since the last full backup. Each differential grows over time (day 1: 5%, day 2: 10%, day 3: 15%...). Restore needs only the last full + the latest differential. More storage than incremental, but faster restore and more resilient: only two restore steps.
Full vs Incremental vs Differential
Use this table to compare storage use, backup speed, restore speed, and restore complexity across the three strategies.
| Criterion | Full | Incremental | Differential |
|---|---|---|---|
| Storage | Highest | Lowest | Medium |
| Backup speed | Slowest | Fastest | Medium (grows daily) |
| Restore speed | Fastest (1 restore) | Slowest (full + chain) | Medium (full + 1 diff) |
| Restore complexity | Simplest | Chain must be intact | Full + latest diff only |
Calculation Formulas
The calculator uses the following formulas. Incremental size is based on daily change rate × number of incrementals per cycle; differential uses the sum 1+2+…+N for N differentials per cycle.
Full: Storage = Data size × Number of full backups
Incremental: Storage = Full backups + (Data size × Change % × Incrementals per cycle × Full cycles)
Differential: Storage = Full backups + (Data size × Change % × (1+2+...+N) × Full cycles) where N = differentials per cycle
Example (100 GB, 5% daily change, weekly full, 6 incrementals): Full+Incr = 4×100 + 4×6×5%×100 = 400 + 120 = 520 GB. Same scenario differential: 4×100 + 4×5%×(1+2+3+4+5+6)×100 = 400 + 4×0.05×21×100 = 820 GB.
Daily Change Rates by Data Type
Daily change rate drives incremental and differential size. Use these ranges as a starting point; measure with your backup or monitoring tools when possible.
| Data Type | Typical Daily Change | Notes |
|---|---|---|
| Transactional database | 5-15% | ERP, CRM, e-commerce; high churn |
| File server / documents | 1-10% | Depends on user activity |
| Email (Exchange, etc.) | 5-20% | Heavy attachment and deletion |
| Virtual machines | 3-10% | CBT/changed blocks; varies by workload |
| Archives / static files | <1% | Rarely modified; full may suffice |
GFS Retention Strategy (Grandfather-Father-Son)
GFS organizes retention into three tiers: Son (daily), Father (weekly), Grandfather (monthly). It balances short-term recovery points with long-term retention for compliance. A typical configuration is 7 daily, 4 weekly, 12 monthly = 23 recovery points.
Storage: Sum of: daily incrementals/differentials (e.g. 7), weekly fulls (e.g. 4), monthly fulls (e.g. 12). Adjust based on compliance and RPO.
- Son (daily): Last 7 days. Usually incremental or differential; allows recovery to yesterday.
- Father (weekly): Last 4 weeks. Weekly full + incrementals; covers month-end recovery.
- Grandfather (monthly): Last 12 months. Monthly full; long-term retention for audit/compliance.
Example storage (100 GB, 5% daily change): 7×0.05×100 + 4×100 + 12×100 = 35 + 400 + 1200 = 1635 GB for a full GFS. Use full-only for monthly to simplify; incremental for daily.
3-2-1 Rule and RTO/RPO
The 3-2-1 rule and RTO/RPO define how many copies to keep, where, and how quickly you must recover. They directly influence backup frequency and retention—and thus storage size.
3-2-1 Rule
- 3 copies: original + at least 2 backups
- 2 different media: e.g. disk + tape, or disk + cloud
- 1 offsite: protects against fire, flood, ransomware
RTO and RPO
RPO (Recovery Point Objective): Maximum acceptable data loss (time). Daily backups = up to 24h loss; hourly = 1h. Drives backup frequency.
RTO (Recovery Time Objective): Maximum acceptable downtime. Influences restore strategy: full-only restores faster; incremental chain takes longer.
Best Practices
These practices help you size storage correctly, protect backups from failure and ransomware, and ensure you can actually restore when needed.
- Follow the 3-2-1 rule: 3 copies, 2 different media, 1 offsite.
- Add 15-25% overhead for compression, metadata, retention growth, and catalog.
- Test restores regularly; a backup you can't restore is worthless. Include full restore drills.
- Database change rates vary: transactional DBs 5-15%, static files 1-2%. Measure yours.
- GFS (Grandfather-Father-Son): keep daily (7), weekly (4), monthly (12) for flexible retention.
- Protect backups from ransomware: immutable storage, air-gapped copies, or WORM.
- Verify backup integrity (checksums, test restores); corruption often goes unnoticed until restore.