Convert between bytes, KB, MB, GB and TB instantly. Switch between binary (1024) and decimal (1000) units.
This calculator converts between file size units — bytes, kilobytes (KB), megabytes (MB), gigabytes (GB) and terabytes (TB) — and clarifies the difference between decimal (1000-based) and binary (1024-based) definitions, which is a common source of confusion when file sizes don't match what you expect.
There are two competing definitions for units like "megabyte". The decimal definition (used by storage manufacturers and some software) defines 1MB as 1,000 kilobytes and 1KB as 1,000 bytes — consistent with the metric system. The binary definition (used by operating systems like Windows when showing file sizes) defines 1MB as 1,024 kilobytes and 1KB as 1,024 bytes, based on powers of 2 which are natural for computing. This is why a drive sold as "1TB" shows as roughly 931GB in Windows — the manufacturer used the decimal definition, but Windows displays the binary equivalent.
The difference is small for small files but grows significant at larger scales — at the terabyte level, the difference between decimal and binary definitions is roughly 10%.
Many services state size limits in MB or GB without specifying which definition they're using, which can lead to confusing situations — a file that your operating system shows as "24.9 MB" might be reported as exactly "25,000,000 bytes" elsewhere, and whether that fits a "25MB limit" depends on which definition the limit-setter intended. In practice, most upload limits have enough margin that this rarely causes actual failures, but if you're working with a file that's extremely close to a stated limit, understanding that your file manager might be showing a slightly different number than the service's limit checker explains why a file that "looks" like it should fit sometimes gets rejected as too large, or vice versa.
The percentage difference between decimal and binary units is the same at every scale (about 2.4% per unit step, compounding), but the absolute difference becomes much more noticeable as file sizes grow. At the kilobyte level, the difference between 1KB (1000 bytes) and 1KiB (1024 bytes) is just 24 bytes — completely negligible. At the gigabyte level, the difference between 1GB and 1GiB is about 74 million bytes — still small relative to the whole, but now a number large enough to show up as a visible difference in a displayed file size. By the terabyte level, this compounds to roughly a 10% difference, which is substantial enough that it's the main reason storage devices appear to have "less space than advertised" — the space is actually there, it's just being measured and displayed using a different definition than the one used in marketing.
This is useful when checking whether a file will fit within an upload limit stated in a different unit than your file manager shows, understanding why a downloaded file's size doesn't match what a website listed, or converting between units when comparing storage plans or specifications. If you're trying to reduce a file's actual size rather than just understand the units, our Image Compressor and PDF Compressor can help reduce image and PDF file sizes respectively.
This is often due to the decimal vs binary definition difference — what one system calls 1MB (1,000,000 bytes) another might display as approximately 0.95MB using the binary definition (1,048,576 bytes per MiB).
Windows traditionally displays binary (1024-based) values while using decimal unit labels (KB, MB, GB). macOS more recently switched to decimal (1000-based) values for file size displays.
No, this is a unit conversion calculator. To reduce file size, use Image Compressor for images or PDF Compressor for PDFs.
Bytes, kilobytes, megabytes, gigabytes and terabytes, with both decimal and binary calculations.
Manufacturers typically use the decimal definition (1TB = 1,000,000,000,000 bytes), while operating systems often display the binary equivalent, showing approximately 931GiB for the same physical capacity.