What Is Theory of Constraints?
Also known as: TOC, constraint management
Definition
Theory of Constraints is a management methodology, developed by Eliyahu Goldratt, holding that every system is limited by a small number of constraints, and that overall performance improves only when effort is focused on the constraint.
Theory of Constraints Explained
TOC is operationalized through five focusing steps: identify the system's constraint, decide how to exploit it so not a minute of its capacity is wasted, subordinate every other decision to that choice, elevate the constraint by adding capacity if exploitation is exhausted, and then return to step one because the constraint will have moved. Subordination is the step organizations skip, because it requires non-constraint resources to deliberately run below capacity, which conflicts with utilization-based performance measurement.
The scheduling implementation is drum-buffer-rope. The drum is the constraint's schedule, which sets the pace for the entire plant. The buffer is a time buffer of work placed ahead of the constraint so it never starves despite upstream variability. The rope is the release mechanism that admits material to the floor only at the rate the constraint consumes it. The result is a plant with low WIP everywhere except a deliberate protective queue at one location.
TOC also reframes accounting. Throughput accounting measures throughput as revenue minus truly variable cost, inventory as money tied up in the system, and operating expense as the money spent turning inventory into throughput. Decisions are evaluated by their effect on throughput first, then inventory, then operating expense. This directly contradicts absorption costing incentives that reward building inventory to spread fixed overhead, which is why TOC adoption usually creates a genuine conflict with the standard cost system rather than a technical one.
Constraints are frequently not machines. They can be a single skilled operator, a certification, a supplier, a fixture, an approval step, or a market that does not want more product. TOC distinguishes physical constraints from policy constraints, and in mature plants policy constraints dominate: a batching rule, a minimum order quantity, or a monthly close cadence limits flow more than any equipment does. Buying capacity for a policy constraint spends money and changes nothing.
Why It Matters
- Concentrates limited improvement resources where they change plant output, avoiding the common waste of optimizing non-constraints.
- Drum-buffer-rope delivers large lead-time and WIP reductions through release control alone, with no capital investment.
- Throughput accounting exposes how absorption costing incentives push plants to build inventory that damages cash and lead time.
- Distinguishing policy from physical constraints prevents capital spending on problems that a rule change would have solved.
In Practice
A plant identifies a five-axis machining center as its constraint and finds it idle 90 minutes daily across lunch and shift change, plus 6 hours weekly of setup that could be done offline. Exploitation alone, staggering breaks to keep it cutting and moving setup to a presetter, recovers roughly 13 percent of its capacity at essentially zero cost. Only after exploitation is exhausted does elevation, in the form of a second machine at 1.2 million dollars, become the right conversation.
Frequently Asked Questions
What are the five focusing steps of TOC?
Identify the constraint, exploit it by wasting none of its capacity, subordinate all other decisions to that exploitation, elevate it by adding capacity if needed, and then repeat because the constraint will have moved elsewhere. Goldratt added a warning against inertia: do not let the policies created for the old constraint remain after it moves.
How does TOC differ from lean?
Lean attacks waste everywhere in the value stream and pursues flow through continuous incremental improvement. TOC concentrates almost all effort on one constraint and deliberately accepts idleness elsewhere. In practice they are complementary: TOC is excellent at telling you where to point lean tools, and lean provides the methods for exploiting the constraint once it is identified.
Related Terms
Bottleneck
A bottleneck is the resource in a process whose available capacity is less than the demand placed on it, making it the step that limits total system output. Every other resource can only run as fast as the bottleneck permits.
Throughput
Throughput is the rate at which a process, line, or plant produces good, sellable units over a defined period, expressed as units per hour, shift, or week. Only conforming output that can be sold counts toward throughput.
WIP (Work in Process)
WIP (Work in Process) is inventory that has been released to production and has begun conversion but is not yet a finished good. It includes partially machined parts, subassemblies, and material sitting in queue between operations.
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