Why is the real download slower than the estimate?

Real-world throughput is reduced by protocol overhead, network congestion, server limits and Wi-Fi loss. Use the overhead field (commonly 10–20%) for a closer estimate, but actual time still varies — this tool gives an approximation, not a guarantee.

Why is a 1 GB file not 1024 MB here?

The calculator uses decimal (SI) units, where 1 GB = 1000 MB = 1,000,000,000 bytes, matching how storage and ISPs quote sizes. That is why a 1 GB file works out to exactly 8 billion bits.

Why are speeds in bits but file sizes in bytes?

ISPs advertise speed in bits per second (Mbit/s), while files are measured in bytes. Since 1 byte = 8 bits, an 'up to 100 Mbit/s' line transfers about 12.5 MB per second — the factor of 8 is the most common source of confusion, and this tool converts it for you.

Is my file or speed data sent anywhere?

No. The calculator is privacy-first and runs entirely in your browser — nothing you enter is uploaded or stored anywhere.

Download Time Calculator

Q: How is download time calculated?

Download time equals file size divided by connection speed, after converting both to bits. The formula is: time (seconds) = file size in bits / speed in bits per second. Since 1 byte = 8 bits, a 1 GB file is 8 billion bits, so on a 100 Mbit/s line it takes about 80 seconds (plus overhead).

Download Time Calculator

Estimate how long a file will take to download from its size and your connection speed. It is for anyone planning a large download, comparing broadband plans, or sanity-checking why a transfer is taking so long.

How it works

The maths is straightforward once the units line up. File size is measured in bytes (1 byte = 8 bits) using decimal units (1 GB = 1000 MB), while connection speed is advertised in bits per second (Mbit/s). The calculator converts both to bits and divides:

time (seconds) = file size in bits ÷ speed in bits per second

Real downloads are slower than this theoretical best because of protocol overhead, congestion, server limits and Wi-Fi loss. The optional overhead field (commonly 10–20%) inflates the estimate to reflect that, while still showing the theoretical best for comparison.

Example

A 1 GB file on a 100 Mbit/s connection:

File size in bits = 1 × 1,000,000,000 bytes × 8 = 8,000,000,000 bits
Speed = 100 Mbit/s = 100,000,000 bits/s
Time = 8,000,000,000 ÷ 100,000,000 = 80 seconds (theoretical)
With 15% overhead ≈ 92 seconds

File size	10 Mbit/s	100 Mbit/s	1 Gbit/s
100 MB	~80 s	~8 s	~0.8 s
1 GB	~13 m 20 s	~80 s	~8 s
10 GB	~2 h 13 m	~13 m 20 s	~80 s

Everything is calculated locally in your browser — nothing is sent anywhere.