4.1.5 Network speeds and transmission calculations
Network Speed Units
Network transmission speed is measured in bits per second (bps) — not bytes per second. This is because data is transmitted one bit at a time over a network link. The key multiples are:
| Unit | Abbreviation | Value |
|---|---|---|
| Kilobit per second | Kbps | 1,000 bits per second |
| Megabit per second | Mbps | 1,000,000 bits per second |
| Gigabit per second | Gbps | 1,000,000,000 bits per second |
Note: network speeds use decimal (SI) multiples (powers of 10), not binary multiples. 1 Mbps = 1,000,000 bps, not 1,048,576 bps. This is a common source of confusion with storage (which uses KiB, MiB etc.).
Also note: file sizes are usually quoted in bytes (B), not bits (b). 1 byte = 8 bits. To use file size in a transmission calculation, convert to bits first by multiplying by 8.
The Transmission Formula
The relationship between file size, transmission rate and time is:
Transfer time (s) = File size (bits) ÷ Transmission rate (bps)
Rearranged: File size = Transfer time × Transmission rate | Transmission rate = File size ÷ Transfer time
How long does it take to download a 50 MB file over a 20 Mbps connection?
- Convert file size to bits: 50 MB = 50 × 1,000,000 bytes × 8 = 400,000,000 bits
- Convert speed to bps: 20 Mbps = 20 × 1,000,000 = 20,000,000 bps
- Time = 400,000,000 ÷ 20,000,000 = 20 seconds
A file takes 5 seconds to download at 100 Mbps. What is its file size?
- File size (bits) = Time × Rate = 5 × 100,000,000 = 500,000,000 bits
- Convert to bytes: 500,000,000 ÷ 8 = 62,500,000 bytes = 62.5 MB
A 200 MB file downloads in 40 seconds. What is the transmission rate?
- File size in bits: 200 × 1,000,000 × 8 = 1,600,000,000 bits
- Rate = 1,600,000,000 ÷ 40 = 40,000,000 bps = 40 Mbps
Key Takeaways
- Network speed is measured in bits per second (Kbps, Mbps, Gbps) — not bytes per second.
- File sizes are in bytes: multiply by 8 to convert to bits before calculating.
- Transfer time = File size (bits) ÷ Transmission rate (bps)
- Rearrange as needed: File size = Time × Rate; Rate = File size ÷ Time.
- Network speed prefixes are decimal (1 Mbps = 1,000,000 bps), not binary.