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Shop Floor Dispatching Rules Reference (FIFO, SPT, EDD, Critical Ratio and More)

Every dispatch list is sorted by some rule, even when nobody chose it on purpose. This table lists the standard dispatching and sequencing rules, what each one sorts by and where it breaks, so a scheduler can pick a rule deliberately.

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Rule	Abbreviation	Sequence jobs by	Tends to improve	Known weakness
First in, first out	FIFO / FCFS	Arrival time at the work center, earliest first	Fairness, predictable waiting	Ignores due dates and job length
Shortest processing time	SPT	Operation time, shortest first	Average flow time, WIP count, throughput of jobs	Long jobs can starve and go very late
Longest processing time	LPT	Operation time, longest first	Makespan balance across parallel machines	Raises average flow time
Earliest due date	EDD	Job due date, earliest first	Maximum lateness (minimizes it on one machine)	Can still make many jobs late when the load is heavy
Earliest operation due date	ODD	Due date assigned to each operation, earliest first	Due date performance in multi-operation routings	Needs sensible operation due dates
Slack time	STR / SLACK	Due date minus today minus remaining processing time	Avoiding jobs that are about to become late	Ignores how many operations remain
Slack per remaining operation	S/RO	Slack divided by number of operations left	Multi-operation jobs with uneven queue exposure	Sensitive to wrong routing data
Critical ratio	CR	Time remaining until due divided by work time remaining, lowest first	Dynamic priority, flags late jobs (CR below 1)	Needs reliable remaining lead time estimates
Least work remaining	LWR	Total processing time still to be done, smallest first	Finishing jobs that are nearly done, lowering WIP	Delays big jobs
Most work remaining	MWR	Total processing time still to be done, largest first	Protecting long jobs from late finish	Raises WIP and average flow time
Fewest operations remaining	FOPNR	Count of operations still to do, fewest first	Clearing jobs close to completion	Ignores due dates
Next queue	NQ	Length of the next work center queue, shortest first	Keeping downstream resources fed	Needs queue visibility across work centers
Weighted shortest processing time	WSPT	Job weight divided by processing time, highest first	Weighted flow time, customer priority	Weights must be set and maintained
Shortest setup time	SST	Sequence-dependent setup time from the current job, shortest first	Lower changeover loss on a bottleneck	Can push important jobs back
Preferred customer or priority code	PCO	Priority flag, then another rule as tiebreak	Expedite and key account handling	Too many flagged jobs defeats the rule
Random	RAND	Random selection	Used as a simulation baseline	No planning value
Johnson's rule	Johnson	Two-machine flow shop: jobs with shorter first-machine time go first, others go last by longest second-machine time	Minimum makespan for a two-machine flow shop	Only exact for two machines, no sequence-dependent setups
Moore-Hodgson algorithm	Moore	EDD order, then remove the longest job each time a job would be late	Minimum number of tardy jobs on one machine	The removed jobs finish last and can be very late
Apparent tardiness cost	ATC	Weighted index mixing weight, processing time and slack	Total weighted tardiness on one machine	Needs a tuned look-ahead parameter

Rules are heuristics. Which one wins depends on load, due date tightness and setup structure, so test a rule on a copy of your real queue before changing your dispatch policy.

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How to use this reference

Type a rule name, an abbreviation or a goal such as lateness into the search box to filter the table. Read the sort column first, because that is the field your ERP or scheduling tool must hold for the rule to work. Then read the weakness column. A rule that needs remaining lead time cannot be run until routings and queue times are credible. Export the filtered rows to Excel to attach them to a scheduling policy or an ERP configuration request.

Search by goal: lateness, flow time, makespan, setup.
Check the data the rule needs before you configure it.
Use a tiebreak rule, usually EDD, behind any primary rule.

Which rule for which goal

Classic scheduling theory gives a few firm results for a single machine. SPT minimizes average flow time. EDD minimizes maximum lateness. Moore-Hodgson minimizes the number of tardy jobs. For two-machine flow shops Johnson's rule minimizes makespan. Real shops have several machines, setups and changing queues, so these results become guidance rather than guarantees. Most planners end up with a hybrid: critical ratio or slack for the main dispatch list, SPT or SST on a bottleneck where changeover loss is high, and a priority code override for expedites.

Rules inside ERP and APS systems

Most ERP shop floor modules offer a small set of built-in dispatch sort options, typically start date, due date and priority, and only an advanced planning module offers true rule based sequencing. Before assuming your system uses critical ratio, check what the dispatch list or work center queue screen really sorts by. Sorting by operation start date, for example, behaves like a forward looking EDD variant and not like CR. If your rule needs data the ERP does not store, an export to a spreadsheet or script is the usual workaround.

Common mistakes

The most common mistake is changing the rule to fix a capacity problem. No rule creates capacity, it only decides who is late. The second is using SPT everywhere and ignoring starvation: long jobs wait until they are very late. The third is mixing rules between shifts so operators see a different order each time. Publish one rule, one tiebreak and one expedite process, and measure on time delivery and average flow time for a month before changing anything.

Frequently Asked Questions

What is the best dispatching rule?

There is no single best rule. SPT is strong for average flow time, EDD for maximum lateness and critical ratio for dynamic due date control. The right choice depends on your goal, load and setup structure. Compare two rules on a copy of your real queue before deciding.

What does a critical ratio below 1 mean?

Critical ratio is time remaining until the due date divided by work time remaining. Below 1 the job cannot finish on time at normal speed, equal to 1 it is exactly on schedule, and above 1 it has slack. Rank jobs from lowest to highest.

Is FIFO a good rule for production?

FIFO is fair and easy to explain, and it works when due dates are loose and job lengths are similar. It performs poorly under heavy load or mixed priorities because it ignores due dates and operation length entirely.

Can I run these rules in my ERP?

Basic ERPs usually offer only date or priority sorting. True rule based sequencing normally needs an APS or finite scheduling module, or a script working on an export of open operations. Check the actual sort fields on your dispatch screen first.

If your dispatch list does not sort the way your planners need, we can add a rule-based priority score on top of live ERP data, read-only by default.

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