Calculator Input
Enter WAN characteristics, TCP behavior, and operating hours. The calculator estimates a realistic throughput ceiling using capacity, window, and loss limits.
Example Data Table
| Scenario | Bandwidth | Utilization | Overhead | RTT | Window | Loss | Flows | Notes |
|---|---|---|---|---|---|---|---|---|
| Branch Office MPLS | 100 Mbps | 85% | 6% | 40 ms | 512 KB | 0.001% | 1 | Typical business traffic with moderate reserve. |
| Regional SD-WAN | 250 Mbps | 78% | 7% | 55 ms | 1024 KB | 0.002% | 2 | Balanced policy with parallel application sessions. |
| Data Replication Link | 1 Gbps | 70% | 8% | 80 ms | 2048 KB | 0.0001% | 4 | Long-haul replication with larger windows. |
Formula Used
The calculator combines line capacity, payload efficiency, TCP window behavior, and packet loss constraints. It then chooses the smallest limit as the realistic throughput estimate.
- Raw link speed: Provisioned bandwidth converted to bits per second.
- Usable link: Raw speed × utilization × (1 − QoS reserve).
- Payload efficiency: (Packet size − header bytes) ÷ packet size.
- Network-limited throughput: Usable link × (1 − protocol overhead) × payload efficiency.
- Window-limited throughput: (TCP window bytes × 8 ÷ RTT seconds) × parallel flows.
- Loss-limited throughput: (1.22 × MSS × 8) ÷ (RTT × √loss probability), adjusted by flows.
- Estimated throughput: Minimum of network limit, window limit, and loss limit.
- Monthly transfer: Estimated throughput × active hours × active days.
How to Use This Calculator
- Enter the provisioned WAN speed and pick the correct unit.
- Add realistic utilization instead of assuming full saturation.
- Enter protocol overhead and reserved bandwidth percentages.
- Set packet size and header bytes to reflect framing behavior.
- Provide RTT, TCP window size, loss percentage, and parallel flows.
- Enter active hours and days to estimate monthly transferable data.
- Click Calculate Throughput to show the result above the form.
- Use the CSV and PDF buttons to export the current analysis.
FAQs
1. What does WAN throughput mean?
WAN throughput is the actual data delivery rate you can achieve across a wide area network after utilization, overhead, latency, TCP behavior, and packet loss reduce raw link speed.
2. Why is throughput lower than bandwidth?
Bandwidth is the provisioned ceiling. Throughput is the real transfer rate after headers, reserved capacity, congestion, RTT, window size, and packet loss limit performance.
3. Why does RTT matter so much?
Higher RTT increases the time needed for acknowledgments. If the TCP window is small, long RTT can sharply cap throughput even when the physical link is fast.
4. What does the packet loss limit represent?
The packet loss limit estimates how retransmissions and congestion control reduce TCP transfer speed. Even tiny loss rates can hurt long-distance WAN performance.
5. Should I use one flow or many flows?
Use one flow for a single session, backup stream, or file transfer. Use multiple flows when traffic is spread across parallel TCP sessions.
6. Is this suitable for SD-WAN planning?
Yes. It helps compare policy reserve, overhead, latency, and loss effects. It is useful for branch sizing, replication planning, and application capacity estimates.
7. What packet size should I enter?
Use the average payload size you expect on the link. Ethernet traffic often uses values near 1500 bytes, while special workloads may use different sizes.
8. Are the monthly transfer results exact?
No. They are planning estimates based on your assumptions. Real usage changes with burst patterns, protocol mix, application design, and transient network conditions.