Download Time: File Size, Internet Speed, and Why Mbps Is Not MB/s
October 29, 2026
Internet speeds are quoted in megabits per second (Mbps), while file sizes are given in bytes (MB or GB). A byte is 8 bits, so you divide the speed by 8 to get megabytes per second. A 100 Mbps connection moves at most 12.5 MB per second.
The formula: time in seconds = file size in megabits ÷ speed in Mbps. A 4 GB file is 4 × 1,000 MB × 8 = 32,000 megabits. At 100 Mbps, that takes 32,000 ÷ 100 = 320 seconds, or 5.3 minutes.
Skip the math and get your answer instantly:
Open the Data Converter →Table of Ideal Times for a 4 GB File
The times assume full use of the speed. Real downloads are typically 80% to 95% of the rated speed because of protocol overhead, Wi-Fi conditions, server limits, and other devices on the network. At 80% efficiency, the 100 Mbps time becomes 320 ÷ 0.8 = 400 seconds, or 6.7 minutes. Wired connections are more consistent than Wi-Fi. If a speed test shows much less than the plan's speed, check the router, signal strength, and the number of connected devices.
| Connection speed | Ideal time (4 GB) | At 80% efficiency |
|---|---|---|
| 10 Mbps | 53.3 min | 66.7 min |
| 25 Mbps | 21.3 min | 26.7 min |
| 100 Mbps | 5.3 min | 6.7 min |
| 500 Mbps | 64 sec | 80 sec |
| 1 Gbps | 32 sec | 40 sec |
Streaming Data Use
Video streaming uses data in proportion to its bitrate. HD streaming at about 5 Mbps uses 5 ÷ 8 × 3,600 = 2,250 MB, about 2.25 GB per hour. 4K streaming at 25 Mbps uses 11.25 GB per hour. Under a monthly data cap of 1 TB, 4K streaming for 3 hours a day would use about 1,012 GB a month, near the limit. Several people streaming at once add up: a household with two 4K streams and a video call needs about 60 Mbps at the same time.
Upload Speed Matters Too
Many plans offer much lower upload speeds than download speeds. Uploading a 4 GB video at 10 Mbps takes 53 minutes at best. Cloud backups, video calls, and large file sharing depend on upload speed. Fiber plans often provide symmetric speeds.
Binary vs. Decimal Units
Network speeds use decimal prefixes (1 Mbps = 1,000,000 bits per second), and storage vendors do too. Operating systems sometimes show sizes in binary units, where 1 GiB = 1,073,741,824 bytes. A 4 GiB file is 4.29 GB, adding about 7% to the download time. Check units when comparing a file size to a speed.
Frequently Asked Questions
How many MB per second is 100 Mbps?
12.5 MB/s at most.
How do I calculate download time?
File size in megabits divided by speed in Mbps.
Why is my real download slower than my plan?
Overhead, Wi-Fi quality, server limits, and shared use.
How much data does 4K streaming use?
About 11 GB per hour at 25 Mbps.
Is a GiB the same as a GB?
No. A GiB is about 7% larger.