What Is Critical Ratio in Production Scheduling?

Critical ratio is a scheduling priority rule that ranks each job by dividing the time remaining to its due date by the work content still required to finish it. A job whose ratio is below 1.0 cannot finish on time at normal pace and gets the highest priority; a job with a large ratio has slack and can wait. In one number, critical ratio weighs urgency against workload, which a due-date-only rule cannot do.

This entry is part of the EDGEBIC by User Solutions glossary series; for the broader vocabulary of planning, see the manufacturing glossary.

How Critical Ratio Works

The formula is time remaining divided by work remaining: (due date minus now) divided by the processing hours still ahead. The result reads as a cushion. A ratio of exactly 1.0 means the job will finish precisely on the due date if nothing goes wrong. Above 1.0 there is room to spare. Below 1.0 the job is already losing the race and needs to move to the front of the queue.

Because both the deadline and the outstanding work feed the ratio, the rule reacts to two things at once. As a job's due date approaches, the numerator shrinks and its priority climbs. As work gets completed and the remaining content falls, the denominator shrinks and the ratio grows, so a job that is almost done naturally drifts back down the queue. That self-correcting behavior is why critical ratio is one of the classic dispatching rules taught alongside earliest due date and shortest processing time.

The trade-off is that critical ratio needs an honest estimate of remaining work. If the routing hours are wrong, the ratio is wrong, and the sequence it produces is only as good as the underlying work content.

A Concrete Example

Suppose it is Monday morning and three jobs share the same machine queue:

Job A has a ratio below 1.0: it cannot finish in the available window at normal pace, so it is the most critical and runs first. Job C, at 1.20, has a thin cushion and runs second. Job B, at 4.00, has plenty of room even though it shares Job A's deadline, so it runs last.

A pure due-date rule would have treated A and B as tied on Friday and might have run B first, sending the job that was actually in trouble to the back. Critical ratio prevents that by seeing that A still has 50 hours to go.

How EDGEBIC Uses It

EDGEBIC exposes critical ratio as one of the ordering heuristics inside its multi-run schedule optimizer. When the optimizer runs, it does not commit to a single sequencing rule. It tries several global job orderings, including critical ratio calculated as time-to-due divided by total work content ascending, plus earliest due date, shortest processing time, longest processing time, priority-then-due-date, and a set of seeded random shuffles.

Each ordering is fed through the unmodified scheduling engine to produce a complete, finite-capacity schedule, and every one of those schedules is scored. The optimizer keeps the best result and is clamped so the answer is never worse than the engine's default greedy plan. Critical ratio, in other words, is a contender rather than a fixed setting: the engine adopts the critical-ratio sequence only when the full schedule it generates actually scores best against your goals.

This matters because sequencing rules interact with capacity. A rule that looks smart in isolation can produce a mediocre schedule once real shift hours, setup times, and shared machines are applied. By generating a complete schedule for each rule and comparing outcomes, EDGEBIC lets the numbers decide rather than trusting any single rule on faith. You can read more about how the presets and goals steer that choice in the optimizer goals and presets guide.

Related Reading

Critical ratio is a cousin of the pure due-date rule; see earliest due date scheduling for the simpler date-only version, and the generic priority rules glossary entry for the wider family of dispatching rules. For how ordering fits into the larger picture of planning, start with what production scheduling is.