What it means
Porter's original five forces model analysed competition through rivalry among incumbents, the threat of new entrants, the threat of substitutes, and the bargaining power of buyers and suppliers. It remains a standard way to read an industry's structure.
The six forces version adds complementors. A complement is a product that raises the value of another, such as apps for a phone platform or charging stations for electric vehicles.
Complements sit outside the industry, yet they can decide whether demand for the industry's product grows at all. The idea gained prominence through Intel's Andrew Grove and the academic work of Adam Brandenburger and Barry Nalebuff on co-opetition.
Their research treats complementors as a symmetric counterpart to competitors: competitors divide value, while complementors can enlarge the total value available. The sixth force changes the analysis in both directions, since strong complements make an industry more attractive because customers get more value from the combined offer, while weak or monopolised complements can hold an industry back because a missing piece blocks the whole sale.
The Yale survey of the sixth force notes practical responses: a firm can help complementors cut costs, police complement quality, ally with strong complementors, or enter the complement market itself when coordination fails. Electric vehicle makers building charging networks are the standard illustration.
The five inherited forces still do most of the work, as buyer power rises with concentration and alternatives, supplier power rises when inputs are scarce or unique, entry threats depend on barriers such as capital, regulation and brand, and substitutes cap the price the industry can charge. Like the original model, the six forces version is a snapshot that ages, since buyer and supplier power shift, technologies create new substitutes, and complement relationships change.
The framework needs regular revision to stay useful. Complements deserve the same structured scrutiny as competitors: identify which complements the product depends on, who controls them, and whether their economics encourage investment.
A complement controlled by a monopolist can tax the industry's growth just as a powerful supplier can. Used well, the model guides questions rather than delivering answers, so the analyst lists the forces, assesses each one's direction and strength, then tests whether strategy addresses the weakest point, including complement effects that the original framework does not list as a separate force.
In practice
Real-world examples.
Example
A fictional phone maker maps its industry and finds rivalry fierce but complements thriving. The healthy app ecosystem supports demand for its devices even while competing handsets pressure prices. The strategy team therefore invests in developer tools rather than only cutting prices.
Example
A fictional electric vehicle startup finds buyers willing but charging scarce. The sixth force, not the five traditional ones, is the binding constraint on its market. It partners with property owners to install chargers and treats the cost as part of its market-entry plan.
Example
A fictional software firm polices the quality of third-party plugins for its platform. Better complements raise the platform's value without the firm building them itself. The firm publishes standards and removes plugins that fail them, which protects the value of the whole offer.
Formula
Calculation
There is no formula; the model is a qualitative checklist. A simple scoring approach rates the pressure each of the six forces puts on industry profits, say from 1 for weak pressure to 5 for strong pressure, and studies the pattern rather than the total.
Worked example. A fictional scooter-sharing industry scores rivalry 2, new entrants 3, substitutes 4, buyer power 3, supplier power 2 and complementors 5, because secure parking and charging are scarce. The simple average is (2 + 3 + 4 + 3 + 2 + 5) / 6 = 19 / 6, about 3.2. The average hides the point: the complementor score of 5 is the binding constraint, so strategy should address parking and charging before adding more scooters.
Illustrative reading: high scores on rivalry and substitutes together with a high complementor score suggest a difficult industry, while strong complements can offset moderate rivalry. Scores are judgement aids, not measurements.Case study
Seen in the real world.
This case study is fictional and illustrative. A scooter-sharing startup analyses its city market with the five forces and concludes rivalry is low, so it expands its fleet. Ridership disappoints. A six forces review shows the missing piece: secure parking and charging services are complements, and both are scarce. The market's attractiveness depended on factors outside the industry.
The startup redirects spending from scooters to city partnerships and parking infrastructure. The sixth force turned out to be the one that mattered. The leadership team then repeats the review each year, updating each force's score and asking which complement could become a new bottleneck. The startup and its city are invented for illustration, and the story is not a record of any real operator.
Watch out
Common mistakes.
- Stopping at five forces and ignoring complementors outside the industry.
- Treating the analysis as a one-time exercise when buyer power, supplier power and the other forces shift constantly.
- Scoring forces precisely and mistaking the scores for objective measurement.
Questions
People also ask.
What is the sixth force?
Complementors: products or services that raise the value of the industry's own product, such as apps for a platform or charging networks for electric vehicles.
Who added the sixth force?
The extension is most associated with Intel's Andrew Grove and with Brandenburger and Nalebuff's academic work on complements and co-opetition.
Does the model give numeric answers?
No. It is a qualitative framework for reading industry structure; any scoring of the forces is a judgement aid, not a measurement of attractiveness.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%