Input Rate / Demand
The rate at which new work arrives at the process, expressed in units per time period.
Also known as: Arrival Rate, Demand Rate, Inflow Rate, Customer Demand
Input rate represents the demand side of the equation. It’s what the customer, the market, or the upstream process sends you. The relationship between input rate and capacity determines everything else: throughput, utilization, backlog, cost per unit, and flow time.
When input rate is below capacity, throughput equals demand, all work gets processed. When input rate exceeds capacity, throughput is capped at capacity and the excess accumulates as backlog.
Example
A permit office receives 50 applications per day (input rate). The office can process 40 per day (capacity). Throughput = 40. Each day, 10 applications go unprocessed and join the backlog. After a week, 50 applications are waiting.
If demand drops to 30 per day, throughput = 30, the office has spare capacity, and the existing backlog can be worked down.
Why It Matters
Input rate is the variable you often can’t control, it comes from customers, regulations, or upstream processes. Process design must account for variability in demand: peak periods, seasonal patterns, and unexpected spikes. The difference between average demand and peak demand drives staffing strategy and capacity planning decisions.