How to use it
- Choose How long will it take? or What speed do I need?
- Enter the file size and pick KB, MB, GB or TB.
- Enter your speed in Mbps (or the time you have).
- Add protocol overhead for a realistic estimate and read the result.
The formula
Time (seconds) = file size in bits ÷ effective speed in bits per second
The trick is the units. File sizes are measured in bytes, internet speeds in bits, and 1 byte = 8 bits. Effective speed is your plan speed minus overhead.
Example: a 4 GB game patch at 100 Mbps with 5% overhead.
- File size: 4 GB × 1,000³ = 4,000,000,000 bytes × 8 = 32,000,000,000 bits
- Effective speed: 100,000,000 × 0.95 = 95,000,000 bits/s
- Time: 32,000,000,000 ÷ 95,000,000 ≈ 337 seconds ≈ 5 min 37 sec
Without overhead the same download would take 5 min 20 sec.
Download times at common speeds
| File | 25 Mbps | 100 Mbps | 300 Mbps | 1 Gbps |
|---|---|---|---|---|
| Song, 5 MB | 2 sec | under 1 sec | under 1 sec | under 1 sec |
| HD movie, 4 GB | 21 min | 5 min 20 sec | 1 min 47 sec | 32 sec |
| Game, 100 GB | 8 hr 53 min | 2 hr 13 min | 44 min | 13 min |
| Backup, 1 TB | 3 days 16 hr | 22 hr 13 min | 7 hr 24 min | 2 hr 13 min |
Theoretical times with no overhead and decimal units. The calculator shows the same comparison for your exact file under its result.
Why downloads take longer than the math says
- Protocol overhead: TCP/IP headers, encryption and acknowledgments use part of the bandwidth. About 5% is typical on a good wired connection; Wi-Fi with interference can lose 20% or more.
- The other end: download servers often limit speed per user, especially during busy hours or big game launches.
- Your network: other devices streaming or downloading share the same line.
- Wi-Fi: distance and walls can cut speed far below your plan.
- Storage: very fast connections can outrun an old hard drive.
Measure what your connection actually delivers with the internet speed test, then enter that number here for a realistic estimate.
Mbps, MB/s and the 1000 vs 1024 question
Mbps (megabits per second) is the unit providers advertise. MB/s (megabytes per second) is what download windows show. Divide Mbps by 8 to get MB/s: 300 Mbps ≈ 37.5 MB/s. The calculator accepts both, plus kbit/s and Gbit/s.
File sizes have their own wrinkle. Storage makers use decimal units (1 GB = 1,000,000,000 bytes), while Windows calculates with binary units (1,024³ bytes) but still labels them GB. Choose 1024 as the multiplier if your size comes from Windows File Explorer; choose 1000 for sizes from macOS, game stores and drive packaging. The difference is about 7% at the GB level.
What speed do I need?
Switch to What speed do I need? and enter the size and the time you have, for example a 50 GB game you want ready in 30 minutes. The calculator shows the minimum speed in Mbps and the same in MB/s. With 5% overhead, that example needs at least about 234 Mbps.
This is handy before choosing an internet plan, planning a large cloud upload, or estimating whether a backup can finish overnight. Remember that upload speeds are often much lower than download speeds on cable and DSL plans, so check your upload number for backups and video uploads.
Frequently asked questions
How do I calculate download time?
Convert the file size to bits (bytes × 8) and divide by your speed in bits per second. The calculator does this for you and adds overhead.
How long does it take to download 1 GB at 100 Mbps?
About 80 seconds in theory (8,000 megabits ÷ 100 Mbps), or around 84 seconds with 5% overhead.
Why is my download slower than my internet speed?
Overhead, Wi-Fi conditions, other devices on the network and server limits all reduce the real speed.
What is the difference between Mbps and MB/s?
Mbps is megabits per second; MB/s is megabytes per second. 8 Mbps = 1 MB/s.
Should I use 1000 or 1024?
Use 1024 if the file size comes from Windows; use 1000 for macOS, app stores and drive manufacturers.
Last updated: November 12, 2026