The problem: two standards, one drive, and the 7% that vanishes
You bought a 1 TB drive. You plug it in, Windows says 931 GB. You didn't lose 69 GB to formatting overhead — you lost it to a naming conflict. The drive manufacturer used decimal units (1 TB = 1,000,000,000,000 bytes) and Windows reported in binary units (1 TiB = 1,099,511,627,776 bytes) but labeled them "GB" instead of "GiB." The 931 is correct; the label is wrong. This is the central trap of byte conversion: the same abbreviation means two different numbers depending on who's talking, and the gap is 7.37% at the gigabyte scale and grows from there. The honest move is a converter that knows both standards and shows them side by side.
Fastest path
Open the Byte Converter, type the value, pick the unit, read both columns.
Input: 1 terabyte (decimal)
→ Binary: 0.9095 TiB (931.3 GiB)
→ Decimal: 1,000 GB (1,000,000 MB)
→ Bits: 8 Tb
The tool converts through bytes as the base unit and shows both the binary (IEC, base-1024) and decimal (SI, base-1000) result for every unit simultaneously. Toggle the standard mode to see one or both. The rest of this guide is why two standards exist, where the 7.37% comes from, and why your internet speed is measured in bits but your file size in bytes.
The substance: one base unit, two standards, seven tiers
The base unit and the two-step conversion
Bytes are the base. The tool stores one factor per unit — the number of bytes it equals — and every conversion is value × fromFactor ÷ toFactor. There's no memorized GB-to-MB constant; the tool goes GB → bytes → MB, and because both factors are exact powers, the intermediate rounding is zero. This is why a converter with a base unit beats a lookup table: 14 units need 14 factors, not 14 × 13 = 182 pairwise constants.
Two standards, one abbreviation
The 14 units split into two standards, and the abbreviation is where the confusion lives.
| Tier | Decimal (SI, base-1000) | Binary (IEC, base-1024) | Ratio |
|---|---|---|---|
| 1 | kB (1,000 B) | KiB (1,024 B) | 2.4% |
| 2 | MB (1,000,000 B) | MiB (1,048,576 B) | 4.9% |
| 3 | GB (1,000,000,000 B) | GiB (1,073,741,824 B) | 7.4% |
| 4 | TB (1,000,000,000,000 B) | TiB (1,099,511,627,776 B) | 9.9% |
| 5 | PB | PiB | 12.6% |
| 6 | EB | EiB | 15.3% |
The gap grows with every tier because the 2.4% compounds. At kilobyte scale it's noise. At terabyte scale it's 69 GB — enough to make you think the drive is defective. At petabyte scale it's 125 TB, which is an entire extra rack of storage.
Decimal (SI) uses powers of 1,000 and the symbols KB, MB, GB, TB. Defined by the SI (International System of Units), the same system that says kilo = 1,000. Used by: storage manufacturers (Seagate, Western Digital, Samsung), network bandwidth (Mbps), and most of the marketing copy on the box.
Binary (IEC) uses powers of 1,024 and the symbols KiB, MiB, GiB, TiB. Defined by the IEC in 1998 to resolve the ambiguity. Used by: Windows (but mislabeled as KB/MB/GB), macOS (correctly labeled since Snow Leopard, but using decimal), RAM (always binary, because memory addresses are powers of 2), and Linux ls -lh (KiB when precise, KB when rounded).
The tool's "Both" mode shows both columns side by side so you can see the gap. If you're comparing a drive's advertised capacity to what Windows reports, set the standard to "Both" and read the decimal column for the advertised number and the binary column for the OS number.
Bits vs bytes: the 8× trap
1 byte = 8 bits. The tool's bit units (bit, kilobit, megabit, gigabit, terabit, petabit, exabit) all carry a ÷ 8 in their factor. This is where two practical confusions live:
Internet speed is in bits, file size is in bytes. Your 100 Mbps plan delivers 100 megabits per second, not 100 megabytes. 100 Mbps ÷ 8 = 12.5 MB/s. When you download a 1 GB file on a 100 Mbps connection, the time is not 10 seconds (1 GB ÷ 100 Mbps) — it's 80 seconds (1,000 MB ÷ 12.5 MB/s). The tool's Download Time tab does this division for you: type 1 GB, 100 Mbps, and you get 1m 20s, not 10s.
RAM is in binary bytes, storage is in decimal bytes. 16 GB of RAM is 16 GiB (17,179,869,184 bytes) — RAM uses binary because memory chips are sized in powers of 2. 16 GB of SSD is 16,000,000,000 bytes — storage uses decimal because manufacturers adopted SI. The tool's binary column gives you the RAM number; the decimal column gives you the SSD number. Same "16 GB" label, different byte counts.
The download time calculation
The Download Time tab is one division: time = fileSizeBytes ÷ speedBytesPerSec. The work is in the unit conversion — getting both numbers into bytes per second before you divide.
File: 4 GB (decimal) = 4,000,000,000 bytes
Speed: 100 Mbps = 100,000,000 bits/s ÷ 8 = 12,500,000 bytes/s
Time: 4,000,000,000 ÷ 12,500,000 = 320 seconds = 5m 20s
The tool handles the bits→bytes conversion internally (SPEED_META[speedUnit].bytesPerSec already includes the ÷ 8). The common mistake is dividing gigabytes by megabits without converting either — 4 ÷ 100 = 0.04, which is meaningless. The tool's speed presets (3G, 4G, 5G, 10 Mbps, 100 Mbps, 1 Gbps, USB 2, USB 3) give you realistic reference points: a 4K movie (15 GB) takes 3.5 hours on 10 Mbps, 20 minutes on 100 Mbps, 2 minutes on 1 Gbps.
The bandwidth calculator
The Bandwidth tab is the inverse: data = speed × time. If you stream 8 hours a day at 50 Mbps, that's 50,000,000 bits/s × 0.125 bytes/bit × 28,800 s = 180,000,000,000 bytes = 180 GB. The tool shows the total in bytes, KB, MB, GB, and TB simultaneously, plus a breakdown of what you could download in that time (how many HD movies, MP3s, etc.).
Gotchas
- Windows labels binary as decimal. Windows shows "931 GB" for a 1 TB drive, but it's computing in GiB. The tool's binary column gives you the Windows number; the decimal column gives you the box number. macOS since Snow Leopard reports in decimal, so a 1 TB drive shows as 1 TB on a Mac — the one place the label matches.
- ISPs sell in bits, downloads show in bytes. 100 Mbps is 12.5 MB/s, not 100 MB/s. The ÷ 8 is the only thing standing between you and a very disappointing speed test. The tool's Download Time tab handles it.
- RAM is binary, storage is decimal. 16 GB RAM = 16 GiB = 17.18 billion bytes. 16 GB SSD = 16 billion bytes. Same label, different count. Don't compare them directly.
- The gap compounds with tier. 1 kB vs 1 KiB is 2.4%. 1 TB vs 1 TiB is 9.9%. 1 PB vs 1 PiB is 12.6%. If you're provisioning petabyte-scale storage, the 12.6% is 126 TB — more than a full rack.
- kB vs KB vs KiB. The SI standard uses lowercase
kfor kilo (kB), uppercaseKfor kelvin. The IEC standard usesKifor kibi (KiB). Windows usesKBfor both, which is the root of the confusion. The tool uses the correct symbols: KiB for binary, KB for decimal. - "Megabyte" is ambiguous; "mebibyte" is not. If a spec sheet says "megabyte," it could mean either. If it says "mebibyte," it means binary. When precision matters, use the IEC names.
- USB speeds are theoretical maxima. USB 2.0's 480 Mbps is the signaling rate; real-world throughput is ~280 Mbps after protocol overhead. USB 3.0's 5 Gbps is similarly optimistic. The tool's presets use the theoretical max; expect 60-80% of that in practice.
- Download time ignores overhead and congestion. TCP overhead, protocol headers, and network congestion add 5-15% to real download times. The tool's estimate is the theoretical minimum; multiply by 1.1 for a realistic number.
Summary
- Two standards: decimal (SI, base-1000, KB/MB/GB) and binary (IEC, base-1024, KiB/MiB/GiB). Storage manufacturers use decimal. Windows uses binary but labels it decimal. macOS uses decimal. RAM always uses binary. The tool shows both columns so you can see the gap.
- The gap is 7.37% at GB scale, 9.9% at TB, 12.6% at PB. Your 1 TB drive is 931 GiB — not broken, just two standards. The 69 GB didn't vanish; it was never there in the binary counting.
- 1 byte = 8 bits. ISPs sell speed in bits (Mbps); file sizes and download speeds are in bytes (MB/s). 100 Mbps = 12.5 MB/s. The ÷ 8 is the one conversion that catches everyone.
- Download time is one division:
fileSizeBytes ÷ speedBytesPerSec. The work is getting both numbers into the same unit first. The tool handles the bits→bytes conversion; you just type the numbers. - Bandwidth is the inverse:
speed × time = data. 50 Mbps for 8 hours = 180 GB. The tool shows the total across all units and what it could hold. - Convert at the Byte Converter; for non-digital units use Unit Converter, for time spans use Duration Calculator, and for spelling out numbers use Numbers to Words.