The LCU Math That Decides API Gateway vs ALB
An Application Load Balancer's cost isn't just its ~$16.43/month fixed charge (730 hours × $0.0225/hour) — it's that fixed fee plus $5.84 per Load Capacity Unit (LCU) consumed, and LCU consumption is set by whichever of four independent dimensions is highest, not their sum: new connections/sec ÷ 25, active connections ÷ 3,000, processed bytes (MB/s) ÷ 1, and rule evaluations/sec ÷ 1,000. This calculator computes all four from your rps, payload size, and rule count inputs and takes the max — exactly how AWS bills ALB.
Which Dimension Usually Wins
At moderate RPS with small payloads, the rule-evaluations dimension often dominates because complex routing configurations multiply avgRps × ruleCount before dividing by only 1,000. A service at 500 RPS evaluating 10 host/path rules per request hits 5,000 rule-evals/sec — 5 LCUs from rules alone — while that same traffic's connection and byte dimensions might stay under 1 LCU. Simplifying ALB listener rules can cut cost independent of raising or lowering actual traffic volume.
Why the Crossover Point Exists at All
- API Gateway REST APIs bill a flat $3.50 per million requests with zero fixed monthly fee, which is why they win at low or spiky traffic — there's no baseline charge to amortize.
- ALB's $16.43 fixed cost means it's never free even at near-zero traffic, so API Gateway wins by default below a certain volume.
- Above roughly 150 steady RPS, ALB typically pulls ahead by 60–80%, because API Gateway's cost climbs linearly per request while ALB's LCU cost grows in discrete steps that flatten out relative to request volume.