Storage Overprovisioning Calculator

Work out storage overprovisioning instantly with clear inputs, formula shown and shareable results.

About Storage Overprovisioning Calculator

Raw capacity shrinks at three stages: SSD overprovisioning held back for garbage collection and wear levelling, RAID parity, and filesystem reserved blocks. Each is multiplicative, so 28 percent overprovisioning plus two parity disks in twelve plus 5 percent reserve leaves well under 60 percent of the raw figure as usable.

Formula

Usable capacity

usable = raw x (1 - overprovisioning) x (data disks / total disks) x (1 - filesystem reserve)

Worked example

With the default values below, Storage Overprovisioning Calculator returns Usable capacity of 57.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Raw capacityminimum 0.001 TB: 100 TB
Overprovisioningaccepts 0–100 %: 28 %
Filesystem reserved blocksaccepts 0–30 %: 5 %
Parity disks in the arrayaccepts 0–8: 2

Results

Usable capacity: 57.00 TB
Capacity after overprovisioning: 72.00 TB
Capacity after parity: 60.00 TB
Total overhead: 43.0%

Step by step

1Values used
2Usable capacity
3Usable capacity= 57.00 TB
4Capacity after overprovisioning= 72.00 TB
5Capacity after parity= 60.00 TB
6Total overhead= 43.0

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why do SSDs need overprovisioning?

Spare blocks let the controller perform garbage collection and wear levelling without waiting for host trim, which keeps write latency stable and write amplification low.

Can filesystem reserved blocks be reduced?

On ext4 the default 5 percent can be lowered with tune2fs. It exists to prevent fragmentation and to leave root room to fix a full filesystem, so reduce it cautiously.