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If a 48 TB two-bay NAS contains two 24 TB drives and uses a mirror—such as RAID 1 or two-drive SHR-1—you get about 24 TB of pool capacity, equivalent to roughly 21.8 TiB, before the NAS reserves space for its system and filesystem. The space available for files will be lower. RAID 0 or JBOD can expose capacity closer to the combined 48 TB (about 43.7 TiB), but neither provides drive-failure redundancy.
What does “48 TB” mean in a two-bay NAS?
For the usual interpretation of this question, “48 TB” is the sum of the advertised capacities of two installed drives: 24 TB each. It describes raw drive capacity, not necessarily the amount of space available for files. The NAS may use some capacity for a storage pool, system partitions and filesystem metadata, and the selected drive arrangement determines how much of the raw total remains for storage.
It also matters whether you mean pool capacity or file space. Pool capacity is the space the storage layout makes available before all NAS- and filesystem-specific reservations. File space is what the NAS ultimately reports as available for data after those reservations and any volume setup. Without a particular NAS model, filesystem and configuration, there is no exact formatted-capacity figure to promise.
TB and TiB are different units
Drive makers advertise capacity in decimal terabytes: 1 TB is 1,000,000,000,000 bytes. Many computer interfaces calculate in binary units, where 1 TiB is 1,099,511,627,776 bytes, but label the result “TB.” That difference explains why a value around 21.8 or 43.7 may appear on screen even before considering NAS overhead.
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The conversions are arithmetic: 24,000,000,000,000 bytes is about 21.83 TiB, and 48,000,000,000,000 bytes is about 43.66 TiB. These are equivalent ways to describe the same byte counts, not capacity lost to RAID. Synology notes that its RAID calculator uses binary calculations, which helps explain the lower-looking display in that calculator; other NAS interfaces may present units differently.
It helps to keep the sequence straight: first identify the byte capacity printed on the drives, then account for the storage layout, and finally consider NAS and filesystem reservations. A display near 21.8 TiB for a 24 TB mirror is principally a unit conversion; it is not evidence that RAID has taken an additional 2.2 TB. Conversely, a number shown after volume creation can be lower still because the NAS has reserved space.
How much capacity does each two-drive layout provide?
The table assumes two equal 24 TB drives, before system partitions or filesystem metadata. The drive-failure descriptions reflect Synology’s documentation; terminology and implementation details can vary by NAS vendor.
| Layout | Approximate pool capacity | Drive-failure protection | What the trade-off means |
|---|---|---|---|
| RAID 1 | 24 TB decimal, about 21.8 TiB | Mirroring allows the pool to continue operating if one drive remains healthy. | Each drive holds the same data, so about half the raw capacity is used for the second copy. |
| Two-drive SHR-1 | About 24 TB decimal, about 21.8 TiB | One-drive fault tolerance for this two-drive arrangement. | Like RAID 1 here, the second drive supplies a mirror rather than another 24 TB of file capacity. |
| RAID 0 | 48 TB decimal, about 43.7 TiB | None. Synology says a single member-drive failure loses the pool’s data. | Capacity is combined across drives, without a second copy for recovery. |
| JBOD | Approximately 48 TB decimal, about 43.7 TiB | None, according to Synology’s documentation. | The drives are combined for capacity rather than mirrored; it is not a substitute for redundancy. |
Synology defines RAID 1 as writing identical data to each drive, with available capacity equal to the smallest member drive. Its official DSM documentation says, “RAID 1 provides data protection as long as one drive is operating normally.” With two equal drives, that means capacity equivalent to one 24 TB drive, not the combined 48 TB.
The smallest-drive rule matters if the drives are not equal. A mirror cannot make the larger drive’s extra capacity available as mirrored space beyond what the smaller member can match. The headline estimate here assumes a matched pair of 24 TB drives; if you use a different pair, check the NAS maker’s calculator for that combination rather than assuming both labels add together in a mirror.
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For a typical two-drive mirrored setup, the practical headline is therefore “about 24 TB before overhead,” not “48 TB usable.” If you instead prioritize capacity and select RAID 0 or JBOD, the pool can approach the full raw total, but that extra capacity comes without drive-failure redundancy.
Why is file space lower than pool capacity?
A NAS needs storage for more than files you deliberately copy to it. System partitions and filesystem metadata consume some of the installed capacity, and the amount depends on the NAS and how its volume is configured. Those deductions come after the RAID layout has determined the pool’s approximate size; they are a reason not to treat the pool figure as an exact file-space promise.
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Those metadata percentages should not be mistaken for a complete deduction formula. The calculator also accounts for system space, and its notes say actual available capacity is lower than the estimate. A final number depends on the NAS model and volume setup, so simply subtracting 4% or 2% from 24 TB would not establish the exact space available for files.
Synology’s published calculator example reports 1.8 TB available from two 2 TB drives and says it uses binary storage calculations. That example illustrates why a displayed figure can be lower than the advertised total, but it is not a 24 TB-drive test or a direct estimate for your device. The larger figures in the table are arithmetic based on drive-label capacity and documented layout rules; the final file-space figure depends on the actual NAS.
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How to estimate the space your NAS will report
- Confirm what “48 TB” refers to. If it is the total of two 24 TB drives, use the calculations in the table. If it describes something else—such as a model’s maximum supported raw capacity—check the installed drives before estimating storage.
- Choose the intended layout. For a mirror, begin with one drive’s capacity: 24 TB decimal, or about 21.8 TiB. For RAID 0 or JBOD, begin with the combined total: 48 TB decimal, or about 43.7 TiB.
- Check the NAS maker’s calculator and model specifications. Use the calculator for your model, drive sizes, RAID or storage-pool arrangement, and filesystem if those inputs are offered. A calculator estimate is more relevant than a general conversion, but it is not a guarantee of exact file space.
- Look at the configured pool and volume in the NAS interface. After setup, distinguish the capacity assigned to the pool or volume from the volume’s available space. The latter is the useful figure for estimating how much room remains for files.
Check what each calculator result means before comparing it with the NAS interface: an estimate of pool capacity and an estimate of available capacity after reservations are different figures. Synology’s calculator offers HDD inputs through 24 TB, but that input range does not mean every Synology model supports every drive size. The model’s compatibility information and specifications still govern.
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How to choose between capacity and protection
Choose a mirror when continued access after one drive fails matters
RAID 1 or a two-drive SHR-1 arrangement dedicates the equivalent of one drive to a second copy, leaving about 24 TB decimal of pool capacity. That is a substantial reduction from the raw 48 TB, but the mirror is designed to keep data available when one drive fails and the other is still operating normally. It does not protect against every cause of data loss.
In practical terms, the mirrored estimate answers a capacity question before it answers a file-space question: you retain a second copy across the pair, but you do not gain the ability to store 48 TB of distinct files. The reserved mirror capacity is the cost of the layout’s drive-failure tolerance. After that, the NAS and filesystem still take their own share.
Choose RAID 0 or JBOD only with the trade-off understood
RAID 0 and JBOD give more of the two drives’ combined capacity to the pool. In exchange, neither layout provides redundancy according to Synology’s documentation. RAID 0 is especially consequential: Synology says that failure of one member drive loses the pool’s data. JBOD is also not a mirror; its lack of redundancy means a failed drive does not have a second copy supplied by the layout. The exact behavior and recovery options depend on how a particular NAS implements JBOD.
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Do not read “more capacity” as “more protected storage.” On these two-disk layouts, the near-48 TB pool figure describes how much capacity can be combined, not how much can be recovered after a disk problem. If preserving access to the data after a single drive failure is a requirement, compare the mirrored estimate instead.
Keep a separate backup regardless of layout
A mirror is not an independent backup. It keeps a second copy within the same storage arrangement, but it does not create a separate recovery copy if data is deleted, corrupted, or otherwise lost for a reason that affects both copies. RAID selection is about the balance between capacity and drive-failure tolerance; backup planning is a separate decision.
What to verify before buying or configuring the drives
- Drive support: Check the exact NAS model’s specifications and compatibility list for 24 TB drives. A two-bay chassis or a calculator accepting an input up to 24 TB does not establish that every model supports that drive size.
- Pool and volume limits: Confirm that the model and its software support the intended pool size and volume configuration. Drive support alone does not establish every volume limit.
- Filesystem and overhead: Find the filesystem options for the NAS and how its calculator accounts for system partitions and metadata. Do not transfer Synology’s Btrfs or ext4 reservation percentages to another vendor’s product.
- Displayed units: Check whether the interface uses decimal TB or binary TiB, or labels binary calculations as “TB.” A lower displayed number may reflect unit conversion as well as reserved space.
- Capacity expectations: Decide whether the goal is roughly 24 TB before overhead with mirroring, or capacity closer to 48 TB before overhead without redundancy. That choice determines the basic pool estimate before model-specific deductions.
Compatibility is a purchase check, not just a setup detail: do not buy two 24 TB drives on the assumption that any two-bay NAS can use them. Confirm both that the precise model supports the drive capacity and that the intended storage pool and volume are within its supported limits. The available figures are useful for planning, but the model-specific specification is what determines whether the proposed configuration is supported.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How much space is left after RAID on two 24 TB drives?
With RAID 1 or two-drive SHR-1, start from about 24 TB decimal, or 21.8 TiB, before NAS and filesystem overhead. With RAID 0 or JBOD, start from about 48 TB decimal, or 43.7 TiB, before overhead. These are pool-level estimates, not a promise of formatted file space. For an exact figure, check the calculator and specifications for the specific NAS, then use the volume’s reported available capacity after setup.
Frequently Asked Questions
How much space is left after RAID 1 on two 24 TB drives?
A two-drive RAID 1 mirror provides about 24 TB decimal of pool capacity, or roughly 21.8 TiB, before system partitions and filesystem metadata. The exact space available for files depends on the NAS model, filesystem and volume configuration.
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Why does my NAS show about 21.8 instead of 24 TB?
24 TB decimal equals about 21.83 TiB. Some calculators and interfaces use binary calculations, sometimes while labeling the result “TB,” so the display can look lower even before NAS reservations are counted. System partitions and filesystem metadata can reduce available file space further.
Can I get nearly all 48 TB from two 24 TB drives?
RAID 0 or JBOD can provide pool capacity close to the combined 48 TB decimal, about 43.66 TiB, before overhead. Neither layout provides drive-failure redundancy in Synology’s documentation; RAID 0 loses the pool’s data if one member drive fails.
Does a 24 TB drive work in every two-bay NAS?
No. Check the exact NAS model’s specifications and compatibility list, as well as its supported pool and volume limits. A calculator that accepts a 24 TB input does not establish support for that drive on every model.
Is RAID 1 a backup?
No. A mirror provides a second copy within the storage arrangement and can keep the pool operating if one drive remains healthy, but it is not an independent backup against deletion, corruption or other events that affect both copies.
The Bottom Line
With two 24 TB drives, expect about 24 TB (21.8 TiB) before overhead in RAID 1 or two-drive SHR-1. RAID 0 or JBOD can approach 48 TB (43.7 TiB), but without drive-failure redundancy. The precise file space depends on your NAS model, filesystem and volume setup; verify drive compatibility and check the configured volume’s available capacity.
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