Btrfs Capacity Calculator
Estimate usable storage for a Btrfs pool based on equal-size drives, data profile, and free-space reserve planning.
Note: This calculator assumes equal drive sizes and provides planning estimates, not exact on-disk chunk accounting.
What this Btrfs calculator helps you estimate
Btrfs is flexible, but that flexibility can make capacity planning confusing. This calculator gives you quick estimates for raw capacity, usable space after your selected data profile, and a practical planning value after reserving free space for snapshots, balancing, and healthy operation.
It is especially useful when deciding between profiles like raid1, raid10, raid5, and raid6, where redundancy changes usable storage significantly.
How the calculations work
1) Raw capacity
Raw capacity is straightforward:
- Raw = number of drives × size per drive
2) Usable data by profile
For equal-size drives, these formulas are used:
- single / raid0: usable = raw
- raid1 / raid10: usable = raw ÷ 2
- raid5: usable = raw × (n−1)/n
- raid6: usable = raw × (n−2)/n
3) Planned usable after reserve
Btrfs generally behaves better when you keep free space available. This tool applies your reserve percentage to produce a planning number:
- Planned usable = usable × (1 − reserve%)
Btrfs profile quick guide
- single: no redundancy; maximum capacity, minimum safety.
- raid0: striped performance, no redundancy; any drive failure can destroy the array.
- raid1: two copies of data across devices; common home NAS choice.
- raid10: striped mirrors; good performance and redundancy, requires at least 4 drives.
- raid5: single-parity layout; can survive one drive failure.
- raid6: dual-parity layout; can survive two drive failures.
Metadata profile matters too
Btrfs stores file data and metadata in separate block groups, and metadata resilience is critical. Even when data profile is tuned for capacity, metadata should still be protected appropriately.
- dup keeps two copies on one device (good for single-disk setups, not a substitute for multi-disk redundancy).
- raid1 / raid1c3 / raid1c4 increase metadata redundancy across devices.
- Higher metadata replication improves resilience but consumes more physical space.
Practical planning tips
Leave headroom
Avoid running a Btrfs filesystem nearly full. Keeping 10–20% free helps write allocation, balances, and snapshot-heavy workloads.
Snapshots are not free
Snapshots start cheap but grow as blocks diverge. If you keep frequent snapshots, plan for additional headroom beyond your normal working set.
Backups are still required
RAID and checksumming improve availability and integrity, but they are not a backup strategy. Keep offline or offsite backups for important data.
Example scenario
Suppose you have 4 drives × 8 TB:
- Raw: 32 TB
- raid1 usable: ~16 TB
- With 20% reserve: ~12.8 TB planned
That planned value is often a more realistic “day-to-day” ceiling than the raw usable number.
Final note
This calculator is designed for fast, practical decision-making. Real-world Btrfs behavior can vary due to mixed drive sizes, chunk allocation history, balance status, compression ratio, and workload pattern. Use these estimates as a planning baseline, then validate with your actual system before committing production data.