SQL SERVER · BACKUP CAPACITY PLANNING

Make room for recovery.
Estimate before storage fills.

Model backup sizes and retained copies with measurable inputs and explicit assumptions.

Backup profile

01 / INPUTS

Uncompressed full backup size (GiB)Use backup_size from history, not allocated database files. Example values below are placeholders.

Retention window (days)Full backup every (days)

Full compression ratio (X:1)3:1 means stored size is one-third of uncompressed size. Use 1 for no reduction. Include differential backups

Typical differential (GiB)Every (days)

Differential compression ratioUse a measured average across a full-backup cycle, or a larger observed size for conservative planning. Differential backups are cumulative.

Include transaction log backups

Uncompressed logs (GiB/day)Log interval (minutes)

Log compression ratioUse daily total log backup_size from history. Existing .ldf size does not predict backup volume.

Stored copiesCapacity headroom (%)

Reserve one extra full cycle for retention dependenciesAdds one full interval to the modeled storage window. This is a planning allowance, not restore-chain validation.

◇ Calculations stay in your browser.

Retention storage plan

02 / RESULTS

CAPACITY ESTIMATE · NOT A RECOVERY GUARANTEE

Planned capacity · all copies + headroom

—GiB

FullDifferentialLog

Estimate summary

Read-only backup history query

SELECT TOP (100) database_name, type,
  backup_start_date, backup_finish_date,
  is_copy_only,
  backup_size / 1073741824.0 AS uncompressed_gib,
  compressed_backup_size / 1073741824.0 AS stored_gib,
  backup_size / NULLIF(compressed_backup_size, 0)
    AS compression_ratio
FROM msdb.dbo.backupset
WHERE type IN ('D', 'I', 'L')
ORDER BY backup_finish_date DESC;

D = full, I = differential, L = log. Filter to the intended database and representative period. Query is read-only; this tool never connects to SQL Server.

UNDERSTAND THE ESTIMATE

Backup history beats
guessing from disk size.

Use measurements from your own database and workload. Compression varies with data, encryption, algorithms and server configuration.

The retention model

Modeled days equal retention days plus one full-backup interval when the extra-cycle allowance is enabled. Full backup count is rounded up from modeled days ÷ full interval. Differential count is rounded up from modeled days ÷ differential interval; overlapping full-backup days are included conservatively. Log volume equals daily uncompressed volume × modeled days.

Divide each backup type by its compression ratio, sum for one copy, multiply by stored copies, then add headroom. Log interval estimates file count and average file size only; it does not change daily volume. The model assumes stable sizes and schedules and does not model database growth or a calendar of actual backup times.

Keep the restore chain intact

Differentials depend on their base full backup. Logs require the appropriate recovery model and a continuous log sequence. An age-based cleanup rule can remove a needed base or log; retain backups by dependencies and test restores. The extra-cycle allowance estimates storage only and cannot prove recoverability or an RPO/RTO.

What capacity excludes

Each stored copy is assumed to have the same retention policy. The aggregate capacity is not the requirement on each volume. Figures exclude filesystem overhead, staging and temporary preallocation, failed backups, immutable retention overlap, growth over time, catalog storage and cloud replication charges. Choose headroom using actual operations and storage behavior.

Microsoft: backup compression Microsoft: differential backup dependencies Microsoft: backup history fields

Guidance checked October 5, 2026. One database per estimate; sum database estimates for a shared repository.

Make room for recovery.Estimate before storage fills.