💾 GiB vs GB Storage Discrepancy
Your "1 TB" SSD shows 931 GB in Windows. Nobody lied to you. Storage marketing uses decimal (powers of 10). Operating systems use binary (powers of 2). The gap is 6.9% at GB scale. At the petabyte level it approaches 10%. Quantify the difference before procurement signs the PO.
📐 Drive Configuration
Input the box-label capacity in marketing GB. Output: what your OS actually reports in GiB, plus the exact deficit in absolute and percentage terms. No surprises when the procurement spreadsheet meets the OS disk manager.
GiB vs GB: The 6.87% Storage Discrepancy Every Engineer Should Know
If you have ever plugged in a "1 TB" drive and seen your operating system report only 931 GiB of usable space, you have encountered the decimal vs. binary measurement gap. Drive manufacturers label capacity using the metric (base-10) system where 1 GB = 1,000,000,000 bytes (109). Operating systems, file systems, and memory allocators operate in binary (base-2) where 1 GiB = 1,073,741,824 bytes (230). The ratio between these two definitions is 109 / 230 ≈ 0.9313, meaning every "gigabyte" on the box translates to approximately 0.9313 GiB of actual usable capacity — a 6.87% deficit that compounds as capacity scales up.
Why 1 TB = 931 GiB
The math is deterministic. A drive labeled "1 TB" contains 1,000,000,000,000 bytes (1012). When the OS divides this by 240 (1 TiB = 1,099,511,627,776 bytes), the result is approximately 0.9095 TiB — or 931.3 GiB. This is not a defect, a formatting overhead, or a hidden partition. It is purely a unit-of-measurement mismatch. The drive contains exactly the number of bytes advertised; the OS is simply using a different unit to count them. For an enterprise storage array with 100 TB of raw capacity, the discrepancy amounts to nearly 7 TB of "missing" space — enough to store an entire additional application dataset.
Impact on Enterprise Procurement
This discrepancy has real financial consequences in enterprise procurement and capacity planning. When an infrastructure team requests "500 TB of usable storage," the vendor responds with a quote for 500 TB of raw (decimal) capacity — which yields only 465 TiB of usable space. The gap of 35 TiB represents roughly $700-$1,400/month in cloud block storage costs, or $8,000-$17,000/year. Multiply this across a fleet of storage arrays, backup targets, and archival tiers, and the aggregate budget error can reach six figures annually. Always specify capacity requirements in binary units (GiB/TiB) when dealing with OS-level provisioning, or explicitly factor in the 6.87% conversion margin when procuring raw storage.
Beyond Marketing: Where Binary Units Are Mandatory
In memory (RAM), the binary convention is universal — a 16 GB DIMM is genuinely 16 GiB (234 bytes). In networking, speeds use decimal (1 Gbps = 109 bits/sec). In storage, the split persists: NAND flash chips are fabricated on binary boundaries (128 GiB, 256 GiB), but consumer products are labeled in decimal. The JEDEC and IEC standards bodies have long recommended using the binary prefixes (kibi-, mebi-, gibi-, tebi-) to eliminate ambiguity, but consumer marketing has resisted the change. As a result, every engineer working at the intersection of hardware procurement and system provisioning must internalize this conversion or risk systematic under-provisioning.
Pricing Basis, Sources & Assumptions
Every rate on this page is a published vendor list price — no negotiated discounts, private pricing, or credit offsets. Totals are in USD and exclude tax. Rates were last checked against the sources below on .
| Pricing input | Basis used on this page |
|---|---|
| Region | Not regional: the GiB/GB definition is fixed by the SI and IEC prefix standards and does not vary by geography. The per-TB cloud storage range quoted in the discussion is drawn from vendors' published US list prices. |
| Currency | USD — on-demand list price, tax excluded |
| Last checked | — The prefix definitions were re-checked against NIST on this date: SI prefixes are powers of 1,000 (1 TB = 1012 bytes) and IEC binary prefixes are powers of 1,024 (1 TiB = 240 bytes). Drive labels use the SI reading; operating systems report the binary reading. |
Modelling assumptions
- The discrepancy is a definitional gap, not a defect: 1012 ÷ 240 = 0.9095, so a '1 TB' drive shows as roughly 931 GiB and a '4 TB' drive as roughly 3.6 TiB.
- Drive vendors are quoting the SI quantity they actually ship. The number on the label is correct under the SI definition — which is why this model does not call it false advertising.
- Capacity loss to formatting, filesystem metadata, and reserved blocks is not included here — that is a second, much smaller effect that stacks on top of the prefix gap.
- The cost figures translate the missing TiB into cloud block-storage spend using vendors' published per-GiB-month list prices, not the price of the physical drive.
- Cloud storage is billed per GiB-month, so the binary quantity is the one that reaches the invoice.
What this model excludes
- RAID parity and replication overhead, which reduce usable capacity further and are covered on the RAID pages instead.
- Snapshot, replica, and backup copies of the same data.
- Filesystem overhead, thin-provisioning headroom, and reserved capacity.
- Enterprise discounts, committed-use pricing, and per-transaction storage charges.
Sources
- NIST (2026). "SI Prefixes for Metric Units." The decimal definition behind the capacity printed on a drive label. nist.gov
- NIST (2026). "Prefixes for Binary Multiples." The IEC binary definitions (KiB, MiB, GiB, TiB) behind what an operating system reports. physics.nist.gov
- AWS (2026). "Amazon S3 Pricing." Per-GiB-month object storage rates used to price the gap. aws.amazon.com
- Google Cloud (2026). "Cloud Storage Pricing." Second vendor's per-GiB-month rate, used to bracket the cost range. cloud.google.com
- Seagate (2026). "IronWolf NAS Hard Drives." Vendor labelling convention for consumer and NAS drives quoted in the examples. seagate.com
Vendor list prices change without notice — re-check the linked pages before committing spend. jslet takes no vendor sponsorship and carries no affiliate links; see about.