The Quick Answer
Data transfer speed measures how fast data moves between two points, typically expressed in megabits per second (Mbps) for network connections or megabytes per second (MB/s) for file operations.
The key conversion: 1 byte = 8 bits, so divide Mbps by 8 to get MB/s.
100 Mbps = 12.5 MB/s
To calculate download time:
Time (seconds) = File size (MB) x 8 / Speed (Mbps)
Use our data transfer time calculator to compute exact download and upload times for any file size and connection speed.
Bits vs Bytes: The Core Distinction
A bit (b) is the smallest unit of data -- a single 0 or 1. A byte (B) is a group of 8 bits. The capital vs lowercase letter matters:
| Abbreviation | Unit | Size |
|---|---|---|
| b | bit | 1 bit |
| B | byte | 8 bits |
| Kb or kbit | kilobit | 1,000 bits |
| KB | kilobyte | 1,000 bytes (8,000 bits) |
| Mb or Mbit | megabit | 1,000,000 bits |
| MB | megabyte | 1,000,000 bytes (8,000,000 bits) |
| Gb or Gbit | gigabit | 1,000,000,000 bits |
| GB | gigabyte | 1,000,000,000 bytes (8,000,000,000 bits) |
Why the confusion exists: Internet service providers (ISPs) and networking equipment measure speed in bits per second (Mbps, Gbps), following long-standing telecom conventions. Operating systems and file managers show file sizes in bytes (KB, MB, GB). This mismatch means your "100 Mbps" connection downloads files at 12.5 MB/s -- a number that looks much smaller.
Note: Storage manufacturers use decimal prefixes (1 GB = 1,000,000,000 bytes), while some operating systems use binary prefixes (1 GiB = 1,073,741,824 bytes). This guide uses decimal (SI) prefixes throughout.
Key Conversions
| Network Speed (Mbps) | File Transfer Rate (MB/s) |
|---|---|
| 10 Mbps | 1.25 MB/s |
| 25 Mbps | 3.125 MB/s |
| 50 Mbps | 6.25 MB/s |
| 100 Mbps | 12.5 MB/s |
| 200 Mbps | 25 MB/s |
| 500 Mbps | 62.5 MB/s |
| 1 Gbps (1,000 Mbps) | 125 MB/s |
| 10 Gbps | 1,250 MB/s |
To convert in either direction:
MB/s = Mbps / 8
Mbps = MB/s x 8
Download Time Formula
Time (seconds) = File size (MB) x 8 / Speed (Mbps)
Or equivalently:
Time (seconds) = File size (MB) / (Speed (Mbps) / 8)
For real-world estimates, multiply the result by 1.05 to 1.15 to account for protocol overhead and network conditions.
Worked Example 1: Downloading a Movie
Scenario: Download a 4.7 GB movie on a 100 Mbps connection.
Step 1 -- Convert file size to MB.
4.7 GB = 4,700 MB
Step 2 -- Apply the formula.
Time = 4,700 MB x 8 / 100 Mbps
Time = 37,600 / 100
Time = 376 seconds
Time = 6.27 minutes
Step 3 -- Add real-world overhead.
With typical 10-15% overhead: 376 x 1.1 = 414 seconds, approximately 6.9 minutes. In practice, expect roughly 7 to 8 minutes depending on server speed and network congestion.
Worked Example 2: Uploading Photos
Scenario: Upload 500 photos totaling 2 GB on a 20 Mbps upload connection.
Step 1 -- Convert to MB.
2 GB = 2,000 MB
Step 2 -- Apply the formula.
Time = 2,000 MB x 8 / 20 Mbps
Time = 16,000 / 20
Time = 800 seconds
Time = 13.3 minutes
Step 3 -- Real-world estimate.
With overhead: 800 x 1.1 = 880 seconds, approximately 14.7 minutes. Upload speeds often fluctuate more than download speeds, so actual time could range from 14 to 18 minutes.
Common Connection Types and Real-World Speeds
Advertised speeds represent theoretical maximums. Actual speeds depend on many factors. The table below shows typical ranges you can expect in practice:
| Connection Type | Advertised Speed | Typical Real-World Speed | Latency |
|---|---|---|---|
| Fiber (FTTH) | 500 - 2,000 Mbps | 400 - 900 Mbps | 1 - 5 ms |
| Cable (DOCSIS 3.1) | 100 - 1,000 Mbps | 80 - 400 Mbps | 10 - 30 ms |
| DSL (VDSL2) | 10 - 100 Mbps | 8 - 50 Mbps | 15 - 40 ms |
| 5G (mid-band) | 100 - 1,000 Mbps | 100 - 300 Mbps | 10 - 30 ms |
| 4G LTE | 30 - 150 Mbps | 20 - 50 Mbps | 30 - 60 ms |
| WiFi 6 (802.11ax) | Up to 9,608 Mbps | 200 - 600 Mbps | 1 - 5 ms (local) |
| WiFi 5 (802.11ac) | Up to 3,467 Mbps | 100 - 400 Mbps | 1 - 5 ms (local) |
| Satellite (LEO) | 50 - 200 Mbps | 25 - 100 Mbps | 20 - 60 ms |
| Satellite (GEO) | 25 - 100 Mbps | 10 - 30 Mbps | 500 - 700 ms |
WiFi speeds listed are the local link speed between your device and router. Your actual internet speed is still limited by your ISP plan. Source: IEEE 802.11ax specification for WiFi standards, Ookla Speedtest Global Index for median speed data.
Why Actual Speeds Are Lower Than Advertised
Four primary factors reduce throughput below the theoretical maximum:
1. Protocol overhead (5-10%)
Every packet of data includes headers for TCP, IP, and Ethernet framing. These headers carry routing and error-checking information but are not part of your file data. On a 100 Mbps connection, roughly 5 to 10 Mbps is consumed by overhead, leaving 90-95 Mbps for actual data.
2. Network congestion
Cable and DSL connections share bandwidth with neighbors in your area. During peak hours (typically 7 PM to 11 PM), congestion can reduce speeds significantly. Fiber connections are less susceptible because each home typically gets a dedicated line to the provider's equipment.
3. Server-side limits
The server hosting the file may limit your download speed. A server capped at 50 Mbps per connection will not deliver faster regardless of your 500 Mbps plan. Downloads from popular servers (Steam, Windows Update, major CDNs) are usually fast; downloads from small or distant servers may be slower.
4. WiFi signal degradation
WiFi speeds drop with distance and physical obstacles. Every wall between your device and the router reduces signal strength. Interference from neighboring networks and household electronics (microwaves, baby monitors) adds noise. A wired Ethernet connection bypasses all WiFi limitations and delivers consistent full-speed performance.
File Size Reference
Common file sizes for everyday content:
| Content Type | Typical Size |
|---|---|
| Text email (no attachments) | 10 - 50 KB |
| Web page (with images) | 2 - 5 MB |
| Smartphone photo (12 MP) | 3 - 5 MB |
| DSLR photo (RAW) | 20 - 50 MB |
| MP3 song (4 minutes) | 3 - 5 MB |
| FLAC song (4 minutes) | 25 - 40 MB |
| HD movie (1080p, 2 hours) | 3 - 6 GB |
| 4K movie (2 hours) | 15 - 25 GB |
| AAA video game | 50 - 150 GB |
| OS update (major) | 3 - 10 GB |
| Full iPhone backup | 30 - 128 GB |
Using these sizes with the download time formula gives practical estimates. For example, downloading a 100 GB video game on a 100 Mbps connection: 100,000 MB x 8 / 100 = 8,000 seconds = approximately 2 hours 13 minutes (theoretical), or roughly 2.5 hours with overhead.
Storage Units at a Glance
| Unit | Abbreviation | Size | Real-World Equivalent |
|---|---|---|---|
| Kilobyte | KB | 1,000 bytes | A few paragraphs of text |
| Megabyte | MB | 1,000 KB | One smartphone photo |
| Gigabyte | GB | 1,000 MB | About 250 MP3 songs |
| Terabyte | TB | 1,000 GB | About 200 HD movies |
| Petabyte | PB | 1,000 TB | Roughly 500 billion pages of text |
Bandwidth Requirements for Common Activities
| Activity | Minimum Speed | Recommended Speed |
|---|---|---|
| Email, web browsing | 1 - 3 Mbps | 5+ Mbps |
| SD video streaming (480p) | 3 Mbps | 5 Mbps |
| HD video streaming (1080p) | 5 Mbps | 10 Mbps |
| 4K video streaming | 15 Mbps | 25 Mbps |
| Video call (1:1) | 2 - 4 Mbps | 5 Mbps |
| Group video call (5+ people) | 5 Mbps | 10 - 15 Mbps |
| Online gaming | 3 - 5 Mbps | 15+ Mbps (low latency matters more) |
| Working from home (general) | 10 Mbps | 25 - 50 Mbps |
| Large file uploads | 5+ Mbps | 20+ Mbps upload |
For households with multiple users, add the requirements together. A family of four streaming HD video on separate devices needs at least 20 Mbps dedicated to streaming alone, plus headroom for other activities.
Data Transfer Time Calculator
Enter any file size and connection speed to see exactly how long the transfer will take, with adjustments for real-world overhead.
Open Transfer Time CalculatorFrequently Asked Questions
Why is my download slower than my internet speed?
Several factors reduce actual throughput below your advertised speed: protocol overhead (TCP/IP headers consume 5-10% of bandwidth), network congestion from shared infrastructure, server-side rate limits, WiFi signal loss due to distance and obstacles, and the fact that ISPs advertise "up to" speeds which represent the theoretical maximum.
What is the difference between Mbps and MB/s?
Mbps is megabits per second; MB/s is megabytes per second. There are 8 bits in 1 byte, so 1 MB/s = 8 Mbps. To convert Mbps to MB/s, divide by 8. A 100 Mbps connection transfers data at 12.5 MB/s. ISPs use Mbps because the larger number appears more impressive in marketing.
How fast do I need for streaming 4K video?
Most 4K streaming services recommend 25 Mbps per stream. Netflix recommends 15 Mbps minimum for 4K, while YouTube suggests 20 Mbps. If multiple people stream simultaneously, multiply accordingly. A 100 Mbps connection can handle about four 4K streams at once.
Is 100 Mbps fast?
100 Mbps is fast enough for most households. It can download a 1 GB file in about 80 seconds, stream multiple 4K videos simultaneously, and handle video calls without issues. It equals 12.5 MB/s of actual file transfer. For reference, the global median fixed broadband speed was approximately 90-100 Mbps in late 2025 according to the Ookla Speedtest Global Index.
How long does it take to download 1 GB on various connections?
At 10 Mbps: about 13.3 minutes. At 50 Mbps: about 2.7 minutes. At 100 Mbps: about 1.3 minutes. At 500 Mbps: about 16 seconds. At 1 Gbps: about 8 seconds. These are theoretical times; actual downloads take 10-20% longer due to overhead.
Why do ISPs advertise in Mbps instead of MB/s?
ISPs advertise in megabits per second (Mbps) because networking has historically measured bandwidth in bits per second, following telecom conventions going back to telegraph and telephone systems. It also produces a larger number: 100 Mbps sounds faster than 12.5 MB/s, even though they represent the same speed.
What is the difference between bandwidth and throughput?
Bandwidth is the maximum capacity of a connection -- the theoretical upper limit. Throughput is the actual amount of data transferred in practice. Throughput is always less than bandwidth due to protocol overhead, latency, congestion, and other real-world factors. Think of bandwidth as the width of a highway and throughput as the number of cars actually passing through.
How do I calculate how long a file download will take?
Use the formula: Time (seconds) = File size (MB) x 8 / Speed (Mbps). For example, a 500 MB file on a 50 Mbps connection: 500 x 8 / 50 = 80 seconds. Multiply the result by 1.1 to account for approximately 10% real-world overhead. Use our data transfer time calculator for instant results.
What is a good upload speed?
For video calls, 3-5 Mbps upload is sufficient. For streaming live video to platforms, 10-20 Mbps is recommended. For uploading large files regularly, 50+ Mbps is helpful. Most home internet plans have asymmetric speeds, with upload being 10-50% of download speed. Fiber connections often offer symmetric speeds (same upload and download).
Does WiFi reduce my internet speed?
Yes. WiFi adds overhead and loses signal strength with distance and obstacles. A WiFi 6 router (IEEE 802.11ax) can deliver 200-600 Mbps in typical home conditions, but walls, interference from other networks, and distance reduce that. A wired Ethernet connection delivers full speed with minimal overhead. If your plan is faster than your WiFi can deliver, using Ethernet for stationary devices will help.
Related Tools
- Data Transfer Time Calculator -- calculate exact download and upload times
- Data Rate Converter -- convert between Mbps, MB/s, Gbps, and other units
- Data Size Converter -- convert between KB, MB, GB, TB, and binary equivalents