Critical Ratio Calculator for Job Dispatch Priority
Critical ratio tells you whether a job can still make its due date at normal pace, and which job to run first when several compete. Enter the numbers for a job and the calculator returns the ratio, slack and expected lateness.
Your numbers
Calendar or working days, but use the same basis as the work days below.
Measure from timestamps if you can; a flat 1 day is only a starting point.
Your results
A planning estimate. It does not check finite capacity, calendars or material. Use it to rank jobs, then confirm feasibility on the schedule.
Critical ratio and slack as the due date moves
Sweeps Days until due date from 1 to 30, holding your other inputs fixed.
Want a daily ranked dispatch list?
We can produce critical ratio for every open job from your ERP data. Optional, the calculator stays free.
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How to use the calculator
Enter the days left until the due date, the remaining run and setup hours, the productive hours per day at the work center and the number of operations still to do. Add the average queue and move allowance per operation. The calculator converts hours to days, adds queue time and divides days left by the result. Compare several jobs by running them one after another, then dispatch in order from the lowest ratio to the highest.
- Below 1: expected to be late.
- Equal to 1: exactly on schedule.
- Above 1: slack available.
The formula
Critical ratio equals time remaining divided by work time remaining. Time remaining is the due date minus today. Work time remaining is the sum of setup, run, queue and move time for all operations still to be done, expressed in the same unit. Slack is time remaining minus work time remaining. The measure is dynamic: ratios change every day, so a job that looked safe yesterday can turn critical when its queue grows. Recalculate daily from the live ERP data.
Worked example
A job has 12 days left, 22 run hours, 4 setup hours, 4 operations left, 7.5 productive hours per day and 1 day of queue and move per operation. Process time is 26 / 7.5 = 3.47 days, plus 4 days of queue and move, giving 7.47 days of work. Critical ratio is 12 / 7.47 = 1.61 and slack is 4.53 days, so the job is comfortable. If the due date were 6 days away the ratio would be 0.80, the job would be about 1.5 days late and would need a speed-up of roughly 25 percent.
Common mistakes
The usual error is understating queue time, which makes every ratio look better than reality. Another is using one due date for all jobs on a shared routing. Mixing calendar days left with working-day estimates distorts the ratio around weekends. Finally, critical ratio is a priority signal and not a feasibility test: if all jobs have a ratio below 1 the problem is capacity, and re-sorting the list will not fix it.
Frequently Asked Questions
What is a good critical ratio?
One means exactly on schedule. Values above 1 mean slack and values below 1 mean the job is expected to be late. Many shops flag anything below about 1.2 for attention because estimates carry error, but pick your own threshold from history.
How is critical ratio different from slack?
Slack is the difference in days between time remaining and work remaining. Critical ratio is their quotient. Ratio scales with job size, so a long job with 3 days of slack is ranked more urgent than a short job with the same slack.
Does the rule work for every shop?
It works best in job shops with varied routings and moving priorities. In flow lines with stable demand, takt, kanban or sequence rules often work better. Its weakness is dependence on accurate remaining lead time estimates.
Should I use calendar or working days?
Either, as long as both sides use the same basis. Working days are more precise for shops that do not run weekends. If you use calendar days for the due date, express work time in calendar days too.
To calculate this for every open job daily from your ERP data, we can build a read-only agent that produces the ranked list.
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