WAN Failover Capacity Calculator
Size a backup WAN link for a primary-link outage. Enter peak traffic by priority class, decide how much of each class must survive failover, then check usable capacity, headroom, and which traffic needs throttling.
Inputs
Results
Core formulas: retained Mbps = peak Mbps x retained % x burst factor,
wire load = retained Mbps x (1 + overhead), and
planning capacity = backup Mbps x efficiency x (1 - headroom).
| Priority class | Requested failover load | Allocated within headroom plan | Action |
|---|---|---|---|
| Run a calculation to see traffic allocation. | |||
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How to use this WAN failover calculator
- Start with measured peak traffic. Use business-hour or failover-drill measurements rather than monthly averages.
- Separate traffic by priority. Critical apps and real-time traffic should be modeled before best-effort and bulk workloads.
- Choose retained percentages. Retained traffic should match the QoS, firewall, SD-WAN, or routing policy you can actually enforce.
- Reserve headroom. Backup links need spare room for bursts, routing convergence, retransmits, monitoring, and operational uncertainty.
- Test before relying on it. A controlled failover test should verify path selection, throughput, latency, packet loss, and user-facing behavior.
Methodology and assumptions
This calculator treats WAN failover as a capacity and priority planning problem. It converts each traffic class into retained failover load, applies a busy-hour burst factor and protocol overhead, then compares the result with backup link capacity after usable efficiency and target headroom. The priority table allocates capacity from highest to lowest priority so lower-priority classes expose the first throttling pressure.
The model uses decimal network rate units such as Mbps and Gbps. It does not simulate routing convergence, TCP slow start, packet loss recovery, application retry storms, SD-WAN path scoring, jitter buffers, cloud egress limits, carrier committed information rate, or device throughput caps. Treat the result as a planning estimate and validate it with device configuration, traffic monitoring, and failover drills.
Example failover planning scenarios
| Scenario | Useful inputs | Planning note |
|---|---|---|
| Branch office LTE or 5G backup | Critical apps at 100%, voice at reduced quality, bulk at 0% | Validate carrier shaping, data caps, VPN overhead, and latency-sensitive apps. |
| SD-WAN secondary broadband | Typical SD-WAN profile, business apps at 50-80%, best effort throttled | Policy must steer, shape, or block low-priority classes when the primary circuit fails. |
| Data center backup circuit | Critical apps, replication, monitoring, management, and user access classes | Plan for inbound and outbound constraints separately if traffic is asymmetric. |
| Cloud VPN failover | VPN backup profile, tunnel overhead, business burst factor, realistic efficiency | Check gateway throughput limits as well as the carrier link rate. |
FAQs
What traffic should go in Priority 1?
Use Priority 1 for workloads that must keep operating during a WAN outage, such as identity, point-of-sale, call control, emergency operations, or core business applications.
How much headroom should I reserve?
Many planning exercises start around 20-30% headroom, then adjust based on burstiness, packet loss, provider behavior, and failover test results. Smaller backup links often need more conservative headroom.
Why can a class be partially allocated?
The priority allocator gives capacity to higher-priority classes first. If the next class does not fit inside the planned capacity, it shows the portion that can fit and the traffic that needs throttling or deferral.
Does this include latency or jitter?
No. This is a bandwidth capacity model. Real-time applications also need latency, jitter, packet loss, and queueing validation.
Can I use this for active-active links?
Yes, if you model the remaining capacity available after one path fails. Enter the surviving backup or degraded-path capacity as the backup link.
Is this calculator private?
Yes. It runs locally in your browser and does not submit inputs to a server.
Disclaimer
Use these results for planning and comparison. Validate production WAN failover with network-device limits, carrier service terms, QoS policy, monitoring data, application testing, and documented recovery procedures.