What is Queue Time? Definition & Manufacturing Examples

What is Queue Time?

Queue time is the time a job spends waiting in front of a work center before processing begins. When a job arrives at a CNC mill but the machine is still running the previous job, the waiting period is queue time. It is the single largest component of manufacturing lead time, typically accounting for 80 to 90 percent of the total time a job spends on the shop floor. Despite being non-productive time that adds no value, queue time is often the most overlooked opportunity for manufacturing improvement.

How Queue Time Works in Manufacturing

Queue time exists because manufacturing resources are shared. Multiple jobs compete for the same machines, and when demand exceeds capacity at a work center, jobs must wait their turn. The length of the queue depends on how many jobs are waiting, how long each takes to process, and how efficiently the work center turns over between jobs.

In most MRP systems, queue time is a fixed planning parameter — the system assumes every job waits a standard number of hours or days at each work center, regardless of actual shop conditions. A planner might set queue time at 2 days for the CNC department. This works as an average but is inaccurate for any specific job. Some jobs sail through with zero wait, while others sit for a week.

Finite capacity scheduling eliminates this problem by calculating actual queue times based on the schedule. If CNC Mill 3 finishes its current job at 2:00 PM and the next job in queue is scheduled to start at 2:00 PM, the queue time is zero. If the next job is not scheduled until the following morning due to a higher-priority job running first, the queue time is calculated precisely.

The relationship between utilization and queue time is exponential, not linear. At 70 percent utilization, average queue time might be manageable. At 90 percent, it doubles. At 95 percent, it can quadruple. This is why bottleneck resources with utilization near 100 percent develop massive queues while underutilized resources have minimal wait times.

Queue Time Example

A shop tracks actual queue times at its busiest work center, a 5-axis CNC mill:

• Average jobs in queue: 6
• Average processing time per job: 4 hours
• Average queue time: 18 hours (over 2 shifts)

By implementing finite capacity scheduling and resequencing jobs to reduce setup time, the shop increases effective capacity by 12 percent. The average queue drops from 6 jobs to 4 jobs, and average queue time falls from 18 hours to 11 hours. For a job with 5 operations, this 7-hour reduction per operation translates to 35 hours less time on the shop floor — almost a full week of lead time saved.

Why Queue Time Matters for Production Scheduling

Queue time reduction is arguably the highest-impact benefit of modern scheduling software. Since queue time dominates manufacturing lead time, even modest reductions translate into significant improvements in delivery performance and WIP inventory.

Resource Manager DB (RMDB) reduces queue time by scheduling jobs against finite capacity, ensuring work arrives at each work center close to when it will actually be processed. Instead of dumping all work onto the floor and letting it pile up in queues, the system controls the flow of work to match resource capacity. This pull-based approach to scheduling is what enables lean manufacturers to achieve short, predictable lead times.

Related Terms

• Lead Time — The total time from order to delivery, of which queue time is the largest component
• Cycle Time — The total production time including queue time, setup, run, and move time
• Bottleneck — The constraint resource where queue times are typically longest

Frequently Asked Questions

Learn more in our complete manufacturing glossary or production scheduling guide.