Bandwidth Calculator

The Bandwidth Calculator converts between various bandwidth units and calculates file transfer times based on network speeds. Bandwidth represents the maximum data transfer rate of a network connection, typically measured in bits per second (bps) or bytes per second (B/s). Understanding bandwidth is essential for capacity planning, troubleshooting slow transfers, estimating backup times, and choosing appropriate internet plans. This tool helps you convert between different units (Mbps, Gbps, MB/s), account for protocol overhead and network efficiency, and get realistic estimates for data transfers including comparisons with common connection speeds.

Bandwidth is the maximum data transfer rate of a network path, measured in bits per second (bps). Understanding bandwidth units and realistic transfer rates is crucial for network planning.

Bandwidth Units:

• Bits per second (bps): Base unit for bandwidth
  • 1 Kbps (kilobit) = 1,000 bps
  • 1 Mbps (megabit) = 1,000,000 bps
  • 1 Gbps (gigabit) = 1,000,000,000 bps
  • Note: Decimal (SI) units, not binary (1000 not 1024)
• Bytes per second (B/s): Used for file transfer speeds
  • 1 byte = 8 bits
  • 1 KB/s = 8 Kbps
  • 1 MB/s = 8 Mbps
  • 1 GB/s = 8 Gbps
• Common confusion:
  • ISPs advertise in Mbps (megabits)
  • Download speeds shown in MB/s (megabytes)
  • 100 Mbps connection = 12.5 MB/s theoretical max

Bandwidth vs Throughput:

• Bandwidth: Theoretical maximum capacity (highway width)
• Throughput: Actual data transfer rate achieved (cars on highway)
• Factors reducing throughput:
  • Protocol overhead (TCP/IP headers, acknowledgments)
  • Network congestion
  • Latency (RTT affects TCP window size)
  • Packet loss requiring retransmissions
  • Application limitations (disk I/O, CPU)
• Typical efficiency: 70-90% of advertised bandwidth
  • TCP overhead: ~5-10%
  • Ethernet frame overhead: ~5%
  • Real-world congestion: variable

Transfer Time Calculation:

• Formula: Time (seconds) = File Size (bits) / Bandwidth (bps)
• Example: 1 GB file over 100 Mbps connection
  • File size: 1 GB = 1,073,741,824 bytes = 8,589,934,592 bits
  • Bandwidth: 100 Mbps = 100,000,000 bps
  • Theoretical time: 8,589,934,592 / 100,000,000 = 85.9 seconds
  • Realistic (80% efficiency): 85.9 / 0.8 = 107 seconds (~1.8 minutes)

Common Connection Speeds:

• Dial-up: 56 Kbps (obsolete)
• DSL: 1-100 Mbps (older residential)
• Cable: 100-1000 Mbps (common residential)
• Fiber: 100-10,000 Mbps (modern residential/business)
• Mobile 4G LTE: 5-50 Mbps (varies by signal)
• Mobile 5G: 100-1000 Mbps (peak speeds, rarely sustained)
• Ethernet:
  • Fast Ethernet: 100 Mbps
  • Gigabit Ethernet: 1 Gbps (1000 Mbps)
  • 10 Gigabit Ethernet: 10 Gbps
  • 100 Gigabit Ethernet: 100 Gbps (data centers)

Protocol Overhead Examples:

• TCP/IP headers: 40 bytes per packet (20 TCP + 20 IP)
• Ethernet frame: 18 bytes overhead (14 header + 4 CRC)
• Maximum payload: 1500 bytes (MTU) - 40 = 1460 bytes
  • Efficiency: 1460/1538 = 94.9%
• TCP acknowledgments: Every ~2 packets, adds ~2-5% overhead
• TLS/SSL encryption: Additional 5-10% overhead for HTTPS

Bandwidth-Delay Product (BDP):

• Definition: Bandwidth × RTT = Amount of data "in flight"
• Formula: BDP = Bandwidth (bps) × RTT (seconds)
• Example: 100 Mbps × 50 ms RTT = 625 KB in flight
• TCP Window Size: Must be >= BDP for full bandwidth utilization
  • Default TCP window: 64 KB (limits throughput over high-latency links)
  • TCP Window Scaling (RFC 1323) allows larger windows
• Impact: High latency reduces effective throughput if window too small

Asymmetric Bandwidth:

• Most residential connections are asymmetric (download > upload)
• Cable: 100 Mbps down / 10 Mbps up (10:1 ratio)
• DSL: 25 Mbps down / 3 Mbps up (8:1 ratio)
• Fiber: Often symmetric (1 Gbps / 1 Gbps)
• Impact: Slow uploads affect video conferencing, cloud backups, VPN

Real-World Transfer Examples:

• Netflix HD streaming: 5 Mbps required
• Netflix 4K streaming: 25 Mbps required
• Zoom video call: 3-4 Mbps recommended
• 100 GB game download:
  • 100 Mbps: ~2.2 hours (theoretical), ~2.8 hours (realistic)
  • 1 Gbps: ~13 minutes (theoretical), ~17 minutes (realistic)
• 1 TB backup:
  • 10 Mbps upload: ~9.3 days (realistic)
  • 100 Mbps upload: ~22 hours (realistic)
  • 1 Gbps upload: ~2.8 hours (realistic)

Factors Affecting Transfer Speed:

• Server limitations: Web server may limit per-connection speed
• CDN throttling: Content delivery networks may rate-limit
• Disk I/O: Slow hard drive can bottleneck downloads
• CPU: Encryption/decryption can limit speeds on older hardware
• Number of connections: Parallel transfers can increase total throughput
• Time of day: Network congestion varies (peak vs off-peak)

Measuring Bandwidth:

• Speed test websites: Speedtest.net, Fast.com
  • Measure download and upload speeds
  • Test from multiple locations for accuracy
• iperf: Network performance testing tool

  # Server
  iperf -s
  
  # Client
  iperf -c server-ip
  # Output: 940 Mbps (94% of 1 Gbps link)

• Browser downloads: Observe actual transfer rates
• Important: Test with wired connection for accurate results (WiFi adds variables)

Bits vs Bytes Cheat Sheet:

• 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 = 125 MB/s
• 10 Gbps = 1,250 MB/s = 1.25 GB/s

Best Practices:

• Account for 20-30% overhead when estimating transfer times
• Use wired connections for large transfers (more reliable than WiFi)
• Enable TCP window scaling for high-bandwidth, high-latency links
• Use parallel connections/threads for large downloads (when allowed)
• Schedule large transfers during off-peak hours
• Monitor actual throughput during transfers (not just start/end)
• Consider compression to reduce effective file size