📁 File Size Calculator

Convert file size units, estimate transfer time at a given connection speed, and work out how many files fit on a drive.

Convert File Size

ℹ Decimal (SI): 1 KB = 1,000 bytes — used by storage manufacturers & network speeds.   Binary (IEC): 1 KiB = 1,024 bytes — used by operating systems & RAM specs.
Conversion Results
Decimal (SI) Binary (IEC)
Exact byte count:

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Download / Upload Time Calculator

0% (theoretical)50%
Theoretical Time At full rated speed with no overhead
Realistic Time (% overhead) Accounts for TCP/HTTP protocol losses
Speed Comparison

Time to transfer at common connection speeds

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Decimal units, binary units and the transfer formula

Two different unit systems are in everyday use. The decimal SI units advance in powers of 1,000: 1 KB is 1,000 bytes, 1 MB is 1,000,000 bytes, 1 GB is 1,000,000,000 bytes. The binary IEC units advance in powers of 1,024: 1 KiB is 1,024 bytes, 1 MiB is 1,048,576 bytes and 1 GiB is 1,073,741,824 bytes. Storage manufacturers and network operators quote decimal units, while Windows reports binary sizes but labels them GB, which is the root of most apparent discrepancies.

Transfer time depends on a second conversion, between bits and bytes. There are 8 bits in a byte, and connection speeds are quoted in bits per second while file sizes are quoted in bytes. The calculation is: time in seconds = file size in bytes x 8, divided by speed in bits per second. When a speed is entered in bytes per second, such as MB/s, the factor of 8 drops out and the tool divides size by speed directly.

The storage planner uses a simpler relation: total = size per file x number of files, and files that fit = capacity / size per file, rounded down.

Worked example: a 4.7 GB file on a 100 Mbps line

Take a 4.7 GB file, the capacity of a single-layer DVD, and a 100 Mbps connection.

First convert the file to bits. 4.7 GB in decimal units is 4,700,000,000 bytes. Multiplied by 8, that is 37,600,000,000 bits. A 100 Mbps line carries 100,000,000 bits per second, so the transfer takes 37,600,000,000 / 100,000,000 = 376 seconds, which is 6 minutes and 16 seconds.

The same file on a 20 Mbps upload link takes five times as long: 1,880 seconds, or 31 minutes and 20 seconds. Entering the speed as 12.5 MB/s instead gives the same 376 seconds, because 12.5 MB/s is simply 100 Mbps expressed in bytes.

For the storage planner, suppose you are archiving 4.7 GB video files onto a 1 TB drive. 1 TB is 1,000 GB in decimal units, so 1,000 / 4.7 = 212.7, meaning 212 complete files fit with about 3.4 GB left over. Note that the drive reports 1 TB as 931 GiB in Windows, since 1,000,000,000,000 / 1,073,741,824 = 931.3.

Why real transfers are slower than the calculation

The figure this tool produces is the theoretical minimum at the speed you entered. Real transfers are slower for several reasons, and it is normal to see 80 to 90 per cent of the calculated throughput at best. Protocol overhead from TCP and IP headers consumes part of the raw link rate. Latency and packet loss limit the achievable window size, which matters on long-distance connections. The server at the other end may be rate-limited or busy, and on shared connections your neighbours or other devices in the house take their share.

Wi-Fi adds another gap. An advertised wireless link rate is the radio's peak, not the sustained throughput, and walls, interference and older devices commonly halve it. For a realistic estimate on Wi-Fi, run a speed test and enter the measured figure rather than the number on your broadband contract.

Two mistakes recur. The first is confusing Mbps with MB/s: divide by 8 to convert, so 100 Mbps is 12.5 MB/s, not 100. The second is comparing a drive's advertised decimal capacity against the binary figure your operating system reports, and concluding that space has gone missing.

Frequently Asked Questions

MB is a decimal megabyte equal to 1,000,000 bytes, while MiB is a binary mebibyte equal to 1,048,576 bytes, about 4.9 per cent larger. Manufacturers, ISPs and most file listings use decimal MB. Windows displays binary sizes but labels them MB and GB, which is why the same file can appear to have two different sizes.
Because the drive is sold as 1,000,000,000,000 decimal bytes, while Windows divides by 1,073,741,824 to display gibibytes and then labels them GB. The arithmetic gives 931.3, so nothing has been lost. Formatting and the file system also reserve a small amount, which accounts for a further slight reduction in usable space.
Multiply 4 GB by 8 to get 32,000,000,000 bits, then divide by your connection speed in bits per second. On 100 Mbps that is 320 seconds, or about 5 minutes 20 seconds, under ideal conditions. Expect roughly 10 to 20 per cent longer in practice because of protocol overhead, latency and server-side limits.
Both answers are in circulation. Under the SI decimal convention used by storage makers and network operators, 1 GB is 1,000 MB. Under the binary convention, 1 GiB is 1,024 MiB. The units are genuinely different, so this calculator lists both side by side rather than picking one and hiding the discrepancy.
Mbps is megabits per second and MB/s is megabytes per second. Since a byte is 8 bits, divide Mbps by 8 to get MB/s: a 100 Mbps connection delivers at most 12.5 MB/s. Broadband is advertised in bits to make the number look larger, while download managers usually report progress in bytes.