Why AWS Load Balancer Bills Are Driven by LCU, Not Bandwidth

AWS charges for Application Load Balancers (ALB) and Network Load Balancers (NLB) based on Load Balancer Capacity Units (LCUs), not raw throughput, contrary to what many engineers assume. LCUs are calculated across four dimensions — new connections, active connections, bandwidth, and rule evaluations — and billing is determined by whichever single dimension is highest in a given hour. This model can produce dramatically different costs depending on traffic patterns; for example, at 50,000 new connections per second, an ALB can cost over $11,700 per month compared to roughly $308 for an NLB. ALB users also face a hidden cost from routing rule evaluations, where exceeding 10 rules can independently drive up LCU counts even under low bandwidth conditions. Understanding which dimension is the binding constraint is key to optimizing load balancer spending on AWS.