Multi-WAN Capacity Calculator
Aggregate up to three WAN circuits and check the capacity that survives the loss of the largest link.
About Multi-WAN Capacity Calculator
Per-flow load sharing lets every circuit forward at once, so aggregate capacity is the sum, but a single TCP flow is still limited to one circuit. Active/standby forwards only on the primary. Either way the number that matters for design is what remains when the biggest link is gone. Buying a second circuit does not create resilience unless the remaining capacity still carries the busy-hour load — that is exactly the N−1 figure here.
Formula
Multi-WAN Capacity
active/active capacity = sum of circuits; N−1 capacity = sum minus the largest circuit; usable = capacity × utilisation target.
- utilisation target
- Design ceiling, usually 70–80% to leave burst headroom
- N−1
- Capacity remaining after the single worst circuit failure
Worked example
With the default values below, Multi-WAN Capacity Calculator returns Usable Capacity at Target of 560.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Circuit 1accepts 0–400,000 Mbps
- 500 Mbps
- Circuit 2accepts 0–400,000 Mbps
- 200 Mbps
- Circuit 3accepts 0–400,000 Mbps
- 100 Mbps
- Design utilisation targetaccepts 10–100 %
- 70 %
- Forwarding modeone of Active/active per-flow load sharing, Active/standby failover
- Active/active per-flow load sharing
Results
- Usable Capacity at Target
- 560.00 Mbps
- Forwarding Capacity
- 800.00 Mbps
- Usable Capacity after Losing the Largest Link
- 210.00 Mbps
- Capacity Lost in That Failure
- 62.50%
- Headroom Reserved
- 240.00 Mbps
Step by step
- 1Values used
- 2Multi-WAN Capacity
- 3Usable Capacity at Target= 560.00 Mbps
- 4Forwarding Capacity= 800.00 Mbps
- 5Usable Capacity after Losing the Largest Link= 210.00 Mbps
- 6Capacity Lost in That Failure= 62.50
- 7Headroom Reserved= 240.00 Mbps
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Multi-WAN Capacity calculated?
active/active capacity = sum of circuits; N−1 capacity = sum minus the largest circuit; usable = capacity × utilisation target. Per-flow load sharing lets every circuit forward at once, so aggregate capacity is the sum, but a single TCP flow is still limited to one circuit. Active/standby forwards only on the primary. Either way the number that matters for design is what remains when the biggest link is gone.
Why does Multi-WAN Capacity matter?
Buying a second circuit does not create resilience unless the remaining capacity still carries the busy-hour load — that is exactly the N−1 figure here.
What values do I need to enter?
This calculator takes 5 inputs: Circuit 1, Circuit 2, Circuit 3, Design utilisation target, Forwarding mode. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.