What Is Critical Ratio in Scheduling? Formula and Examples

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.

The Critical Ratio Formula

The formula divides two quantities measured in the same units, hours or working days:

Critical ratio = time remaining until due date  /  work remaining

Time remaining is the working time between now and the due date. Work remaining is the processing time still left in the routing, summed across every operation that has not yet finished. Read the result like a fuel gauge:

The lowest ratio is the most urgent, so a critical-ratio queue runs the smallest numbers first.

How Critical Ratio Works

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.

Example 1: Three Jobs in Hours

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 run C first, because it is due soonest, and then treated A and B as tied and might have run B next, 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.

Example 2: Working Days, and the Clock Moving

Take three jobs competing for the same milling machine on a Monday morning. Friday is 4 working days out.

A critical-ratio sequence runs MO-202 first (0.67), then MO-203 (1.20), then MO-201 (2.00). A plain due-date sort would run MO-201 and MO-202 in some tied order before MO-203, and could put MO-201, which has plenty of margin, ahead of MO-202, the only job actually behind pace.

Now let the clock advance. Say MO-202 runs Monday and Tuesday and its work remaining drops to 4.0 days, while nothing else moves. On Wednesday, with 2 working days left to Friday, MO-202's ratio is 2 / 4.0 = 0.50: still the most urgent, and now clearly unable to finish by Friday on one machine. Meanwhile MO-201's ratio has fallen to 2 / 2.0 = 1.00, on pace with no margin. The ratio told the planner on Monday that MO-202 was at risk and tells them again on Wednesday that it needs overtime, a second machine or a new date. The numbers updated themselves because both inputs changed.

Critical Ratio vs Other Priority Rules

For the simpler date-only rule, see earliest due date scheduling, and for the wider family, the priority rules glossary entry.

Why Sequence Matters Most in a Finite Plan

Critical ratio matters most in a finite capacity plan, where contested machine time is the whole game. When a machine can take only one job at a time, the sequence decides who is late. Putting the nearly-finished job ahead of the barely-started one wastes the scarce slot on a job that had margin, and starves the job that did not. Critical ratio spends the machine on the job with the least room to spare.

When Critical Ratio Is the Wrong Tool

Critical ratio shines when due dates and work content vary widely across the queue, which is the normal state of a job shop. It is a weaker choice in a few situations:

• Setup-heavy lines. Running strictly by critical ratio can scatter similar jobs and pile up changeover time; see how a scheduler trades off setup against due date.
• Bad routing data. If routing hours are wrong, the ratio is wrong, and the rule confidently prioritizes the wrong job. Clean routing data is the price of admission.
• Hopeless jobs. A job at 0.3 may be unrecoverable at normal pace. Running it first can make two more jobs late without saving it. That calls for a decision (overtime, split, new date), not just a sort.

How EDGEBIC Uses It

The standard EDGEBIC engine does not sort by critical ratio as its default. It sorts every order deterministically by priority, then start date, then due date, so the same inputs always produce the same sequence; see how a scheduler decides which job runs first.

Critical ratio enters through the multi-run schedule optimizer. When the optimizer runs, 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, and nothing changes until you accept it.

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. See the optimizer goals and presets guide, and for a step-by-step on running and reading a critical-ratio candidate, critical ratio in the EDGEBIC optimizer.

There is also a manual path. Because critical ratio is just time remaining over work remaining, you can compute it from fields you already maintain (the due date and the routing hours) and use it to set the priority integer or the start date. The engine then honors your ordering directly.

Related Reading

For how ordering fits into the larger picture of planning, start with what production scheduling is, and for the rule-by-rule comparison in a job shop, see priority rules in the job shop.