Why 1 TB Shows as 931 GB (and Other Unit Conversion Surprises)
A 1 TB drive is 0.909 TiB, so 9.1% seems to vanish. And a plain floating-point converter failed to round-trip 1,294 of 10,000 mile-kilometer values in our test.
Why a 1 TB Drive Shows as 931 GB
Storage makers count in powers of 1,000: a terabyte is 1,000,000,000,000 bytes. Many operating systems count in powers of 1,024 (a tebibyte is 1,099,511,627,776 bytes) while still printing “GB” or “TB”. One TB is 0.909 TiB, so about 9.1% of the advertised size seems to vanish, and a 1,000 GB drive shows as 931 GB. Nothing is missing; the two numbers use different units. The IEC names kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) and tebibyte (TiB) exist to tell them apart.
US Units Are Defined in Metric
An inch is exactly 2.54 centimeters, a US gallon exactly 231 cubic inches, and a pound exactly 0.45359237 kilograms; NIST’s quick-reference card rounds these for everyday use and points to its Special Publication 811 for the exact factors. Everything else follows: a foot is 0.3048 m, a mile 1.609344 km, a US fluid ounce 29.5735295625 mL. Recipes and labels round these, which is where small differences between converters come from.
Where Rounding Creeps In
Most online converters multiply by a decimal factor in floating point, which cannot store most decimals exactly. We measured how often converting a two-decimal value there and back fails to return the same number: 1,000 of 10,000 values for inches and centimeters, 1,294 for miles and kilometers, 820 for pounds and kilograms, 559 for gallons and liters. Our Measurement Unit Converter keeps every factor as an exact fraction and rounds only the final display, so there and back always returns what you typed, and its results match NIST’s exact factors.
Temperature Is Different
Temperature scales have different zero points, so they need an offset as well as a factor: °F = °C × 9/5 + 32, and K = °C + 273.15. That is why −40 is the same in Celsius and Fahrenheit and why doubling a Celsius temperature does not double it in Fahrenheit.
Convert units exactly
Length, weight, volume, area, temperature, speed, time, data, pressure, energy.
Frequently Asked Questions
Why does my 1 TB hard drive only show 931 GB?
Is an inch exactly 2.54 cm?
Why do unit converters give slightly different answers?
How do you convert Celsius to Fahrenheit?
Published Oct 14, 2026 by Derek Giordano