KB or KiB? Why file size limits are ambiguous and how to stay safe

Two meanings of one word

In the metric system, kilo means 1000: a kilometre is 1000 metres. By that rule a kilobyte is 1000 bytes and a megabyte is 1,000,000 bytes.

Computer memory, however, comes in powers of two, and 2 to the power of 10 is 1024, close to 1000. For decades a lot of software used kilobyte for 1024 bytes and megabyte for 1024 × 1024 bytes, while still writing KB and MB.

To end the confusion, the International Electrotechnical Commission (IEC) approved separate names for the binary units in December 1998. NIST, the US standards institute, lists them alongside the decimal ones.

• 1 kilobyte (kB) = 1000 bytes; 1 kibibyte (KiB) = 1024 bytes.
• 1 megabyte (MB) = 1,000,000 bytes; 1 mebibyte (MiB) = 1,048,576 bytes.
• 1 gigabyte (GB) = 1,000,000,000 bytes; 1 gibibyte (GiB) = 1,073,741,824 bytes.

Why devices show different sizes

The new names are precise, but they are not used everywhere. Some software still divides by 1024 and labels the result KB or GB. Apple's support article on storage explains that drive makers count in decimal, while some software counts in binary, and it names iOS 10 and earlier and OS X Leopard and earlier as examples of systems that reported capacity in binary. That is why the same drive or file can show different numbers on different systems.

The gap grows with each step: about 2.4 percent at the kilo level, about 4.9 percent at mega and about 7.4 percent at giga.

Why a form's '50 KB' is ambiguous

Most upload forms state a limit in KB without saying which kind. The check behind the form compares an exact number of bytes, and the person who wrote it chose either 1000 or 1024. You usually can't tell which from the page.

Rounding adds a second trap. A file manager that shows '50 KB' may be hiding 50,400 bytes, which fails a 50,000-byte limit. So the displayed figure is not enough when you are close to the edge. Look for the exact byte count in the file's information or properties window where your system shows one.

The safe window for a range

You don't need to know which meaning the form uses. Pick the stricter reading at each end of the range.

• For the maximum, count 1 KB as 1000 bytes. A file under max × 1000 bytes is also under max × 1024.
• For the minimum, count 1 KB as 1024 bytes. A file above min × 1024 bytes is also above min × 1000.

What our tools count

The image compressor's Make every file smaller than setting counts 1 KB as 1000 bytes, so a result under the limit fits both readings of a maximum. The guide on image file size for online forms explains how it gets there.

The Photo and signature resizer handles ranges. It aims above the minimum × 1024 bytes and under the maximum × 1000 bytes, so either meaning of KB passes. It has presets for forms such as the US Diversity Visa photo, IBPS, UPSC, NTA and SSC uploads, plus Custom: my own pixels and KB range for anything else.

Small, clean images can come out below a minimum even at the best quality. In that case the resizer adds an empty comment block inside the JPG to reach it. The picture itself doesn't change.

When you are still rejected

If a file inside the safe window is refused, the size probably isn't the reason. Check the other rules: the format (many forms take JPG only), the pixel dimensions, and the file name. Read the form's instructions again, since no tool can promise that an authority will accept a file.