Multi-WAN Capacity Calculator
Aggregate up to three WAN circuits and check the capacity that survives the loss of the largest link.
Inputs
Usable Capacity at Target
560.00Mbps
Forwarding Capacity
800.00Mbps
Usable Capacity after Losing the Largest Link
210.00Mbps
Capacity Lost in That Failure
62.50%
Headroom Reserved
240.00Mbps
Step by step
Values used
Circuit 1 = 500 Mbps; Circuit 2 = 200 Mbps; Circuit 3 = 100 Mbps; Design utilisation target = 70 %; Forwarding mode = Active/active per-flow load sharing
Multi-WAN Capacity
active/active capacity = sum of circuits; N−1 capacity = sum minus the largest circuit; usable = capacity × utilisation target.
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
How it works
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
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.