Critical Ratio
Definition
Critical ratio is a dispatching priority index calculated as time remaining until due date divided by work time remaining; a ratio below 1.0 means the job is behind schedule, and lower ratios get worked first.
In Practice
Critical ratio dynamically re-ranks shop orders as conditions change. A job due in 10 days with 5 days of work remaining has a ratio of 2.0 and can wait; a job due in 4 days with 8 days of work left is at 0.5 and needs expediting. Because both the numerator and denominator shrink as time passes and operations complete, the rule continuously rebalances priorities without manual intervention.
It is one of the most widely used dispatching rules in job shops precisely because it blends due-date urgency with remaining workload, unlike simpler rules that look at only one.
Frequently Asked Questions
How is critical ratio calculated?
Divide the time remaining until the due date by the work time remaining on the job, including setup, run, and normal interoperation time. A ratio of 1.0 means exactly on schedule, above 1.0 means ahead with slack to spare, and below 1.0 means the job is late-trending and should be prioritized.
What is the difference between critical ratio and slack time?
Slack subtracts work remaining from time remaining, giving an absolute buffer in days; critical ratio divides them, giving a relative index. The ratio scales better across jobs of different sizes — one day of slack is comfortable for a 4-hour job but alarming for a 10-day job.
Related Calculators
Related Terms
Priority planning is the function of determining what to make and when — setting and maintaining valid due dates and quantities for orders — as distinct from capacity planning, which determines whether the resources exist to do it.
SlackSlack is the amount of time an order or activity can be delayed without missing its due date, calculated as time remaining until the due date minus the remaining work and setup time.
Job Shop SchedulingJob shop scheduling is the task of sequencing many different jobs across shared work centers in a job shop, where each job follows its own routing, so that due dates, queue times, and machine utilization stay in balance.
Lead TimeLead time is the elapsed time between initiating a process, such as placing an order, and its completion, such as receiving the goods. It is one of the most important inputs to inventory, planning, and customer promise decisions.