Aggregate up to three WAN circuits and check the capacity that survives the loss of the largest link.
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.
Multi-WAN Capacity
active/active capacity = sum of circuits; N−1 capacity = sum minus the largest circuit; usable = capacity × utilisation target.
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.
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.
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.