Planning a Synology NAS and wondering how much of your drive capacity you will really be able to use? A Synology RAID calculator takes your drive sizes and your protection level and returns the usable space. This guide shows the maths behind it, explains Synology Hybrid RAID (SHR), and points you to our free RAID calculator for standard RAID levels.
Synology uses standard RAID plus SHR
Synology’s DSM software supports the classic RAID levels, such as RAID 0, 1, 5, 6 and 10, and adds its own option called SHR. Which choices appear depends on your model and how many drives you install. QNAP and most other NAS brands also use standard RAID, so the same capacity formulas apply across brands.
| Level | Usable capacity | Survives | Example: 4 x 8 TB |
|---|---|---|---|
| RAID 5 | (N - 1) x smallest drive | 1 drive failure | 24 TB |
| RAID 6 | (N - 2) x smallest drive | 2 drive failures | 16 TB |
| RAID 10 | (N / 2) x smallest drive | 1 drive per mirrored pair | 16 TB |
| SHR-1 | Roughly total minus the largest drive | 1 drive failure | 24 TB |
| SHR-2 | Roughly total minus the two largest drives | 2 drive failures | 16 TB |
What SHR (Synology Hybrid RAID) does
Standard RAID treats every drive as if it were the size of the smallest one, so a bigger drive wastes its extra space. SHR lets you mix drive sizes and still use space efficiently. SHR-1 tolerates one drive failure and SHR-2 tolerates two. SHR-1 can run on as few as two drives, while SHR-2 needs four or more.
The table below compares a standard RAID 5 with SHR-1 on mixed drives. The SHR figures are estimates using the rule of thumb above, so confirm the final number in DSM or with Synology’s own tools before you buy.
| Drives installed | RAID 5 usable | SHR-1 usable (approx.) |
|---|---|---|
| 8 + 8 + 8 + 8 TB | 24 TB | 24 TB |
| 4 + 4 + 8 + 8 TB | 12 TB | 16 TB |
| 4 + 8 + 8 TB | 8 TB | 12 TB |
When SHR makes sense
- You already own drives of different sizes and want to reuse them
- You plan to upgrade one drive at a time over the years
- You want simple setup and do not need a specific RAID level
- You buy matching drives and want a particular level such as RAID 10: use standard RAID instead
How to estimate usable space step by step
- Write down the size of every drive you plan to install.
- Choose how many failures you want to survive: one (RAID 5 or SHR-1) or two (RAID 6 or SHR-2).
- Apply the formula, or enter the drive count and size into the RAID capacity calculator.
- Convert terabytes to tebibytes, because your system may display the smaller binary figure.
- Leave about 20 percent free so the volume stays fast and manageable.
Drive makers use decimal terabytes, so 24 TB is about 21.8 TiB in binary units. The system also keeps a small partition on each drive for itself, which means real usable space is a little lower than any formula result.
Picking a level for common bay counts
- 2 bays: RAID 1 or SHR-1, which both give you one drive of usable space.
- 4 bays: RAID 5 or SHR-1 for balance, RAID 6 or SHR-2 for large drives, RAID 10 for speed.
- 6 or more bays: RAID 6 or SHR-2 is the sensible default for data you care about.
If you are unsure how many bays to buy, read how many bays you need. For a deeper comparison of levels, see RAID explained.
Common mistakes
- Counting raw capacity as usable capacity.
- Expecting RAID 5 to use the full size of larger drives in a mixed set.
- Treating RAID as a backup. It protects against a dead drive only. Follow the 3-2-1 backup rule.
- Choosing SMR drives for an array. Check our guide to NAS hard drives first.
Plan for growth and expansion
Usable space is only half of the plan. Think about how you will expand later. With SHR you can often replace drives one at a time with larger ones, and the extra space becomes available once every smaller drive has been swapped out. With standard RAID, you generally need to replace every drive in the array to gain capacity. Either way, check your model’s documentation for the expansion options it supports before you buy, and keep a spare bay free if you can.
Synology RAID calculator FAQ
Does SHR give more usable space than RAID 5?
With drives of different sizes, usually yes, because SHR can use the extra space on larger drives. With identical drives the two give the same capacity.
How many drives can fail with SHR?
SHR-1 tolerates one drive failure and SHR-2 tolerates two.
Can I use this RAID calculator for QNAP?
Yes for standard RAID levels. QNAP and most other brands use the same formulas for RAID 0, 1, 5, 6 and 10.
Why is my usable space lower than the calculator result?
The file system and a small system partition on each drive take some space, and operating systems often display binary units (TiB), which look smaller than the labelled TB.
