Enter Values
Key Formulas
MR = MC → Qsr = (a − v) / (2b + 2c)
LR Tangency: Qlr = √(FC / (b + c))
a = Demand intercept |
b = Demand slope |
v = Base MC |
c = Cost curvature |
FC = Fixed cost
Model Assumptions
- Linear demand curve for each firm (P = a − bQ)
- Quadratic total cost: TC = FC + vQ + cQ² (rising MC = v + 2cQ)
- U-shaped ATC: ATC = FC/Q + v + cQ
- Free entry and exit drives LR profit to zero
- Product differentiation gives each firm market power
- LR entry/exit shifts only the demand intercept (a), not the slope (b)
- For educational purposes. Not financial advice. Market conventions simplified.
Short-Run Equilibrium
SR Quantity
20.00
SR Price
$60.00
SR ATC
$40.00
SR Profit
$400.00
Long-Run Equilibrium (Zero Profit)
LR Quantity
11.55
LR Price = ATC
$43.09
LR Profit
$0.00
LR Marginal Cost
$31.55
Required LR Demand Intercept (alr)
$54.64
Efficiency Analysis
Efficient Scale
20.00
Minimum ATC
$40.00
Excess Capacity
8.45 units
Markup over MC ($)
$11.54
Markup over MC (%)
36.6%
Formula Breakdown
SR: MR = MC → a − 2bQ = v + 2cQ
Step-by-step calculation with your inputs
Short-Run vs Long-Run
| Measure | Short Run | Long Run |
|---|---|---|
| Price vs ATC | P > ATC | P = ATC |
| Price vs MC | P > MC | P > MC (markup) |
| Profit | Positive | Zero |
| Entry/Exit | Entry | Equilibrium |
Understanding Monopolistic Competition
What is Monopolistic Competition?
Monopolistic competition is a market structure where many firms sell differentiated products with free entry and exit. Each firm faces a downward-sloping demand curve (due to product differentiation) but earns zero economic profit in the long run (due to free entry).
Key Equations (Mankiw Ch. 16)
Cost: TC = FC + vQ + cQ² → MC = v + 2cQ
SR Demand: P = a − bQ → MR = a − 2bQ
SR Optimum: MR = MC → Qsr = (a − v) / (2b + 2c)
LR Tangency: P = ATC and slopes equal → Qlr = √(FC / (b + c))
The LR tangency point also satisfies MR = MC for the shifted demand curve.
SR Demand: P = a − bQ → MR = a − 2bQ
SR Optimum: MR = MC → Qsr = (a − v) / (2b + 2c)
LR Tangency: P = ATC and slopes equal → Qlr = √(FC / (b + c))
The LR tangency point also satisfies MR = MC for the shifted demand curve.
Short-Run vs Long-Run Equilibrium
Short Run
Profit or loss possible
Firms maximize profit at MR = MC. Number of firms is fixed. Positive profit attracts entry; losses trigger exit.
Long Run
Zero economic profit
Entry/exit shifts demand until P = ATC (tangency). Firms still mark up P above MC, creating excess capacity.
Excess Capacity & Markup
Two key inefficiencies distinguish monopolistic competition from perfect competition:
- Excess capacity: Firms produce below efficient scale (Qlr < Qeff). They could lower average cost by producing more, but don't because it requires cutting price.
- Markup over MC: P > MC means some consumers willing to pay more than the cost of production are priced out. This creates deadweight loss.
- Zero profit paradox: P = ATC (zero profit) but P > MC (markup exists) because firms operate on the declining portion of ATC where ATC > MC.
Key insight: Textbook treatments often compare the markup and excess-capacity costs of monopolistic competition with the benefit of product variety. This calculator reports the cost-side metrics of that comparison without making a policy recommendation.
Further Reading
Explore related topics to deepen your understanding of market structures and firm behavior:
- Monopolistic Competition — Full article on SR/LR equilibrium, excess capacity, and product differentiation
- Costs of Production — Understanding TC, MC, ATC, and the U-shaped cost curve
- Monopoly & Market Power — Compare monopoly pricing with monopolistic competition
Try these related microeconomics calculators:
Frequently Asked Questions
Monopolistic competition features many firms selling differentiated products with free entry and exit, driving long-run profit to zero. Monopoly has one firm with barriers to entry and positive long-run profit. Both face downward-sloping demand and mark up price above marginal cost, but free entry in monopolistic competition eliminates supernormal profits over time.
Positive short-run profits attract new firms (entry), which shifts each incumbent's demand curve leftward as customers spread across more options. Entry continues until price equals average total cost, driving profit to zero. Conversely, short-run losses trigger exit, shifting demand rightward for remaining firms until P = ATC.
Excess capacity is the gap between a firm's efficient scale (the output that minimizes average total cost) and its actual long-run output. Monopolistically competitive firms produce below efficient scale because expanding output requires cutting price. This means each firm could reduce per-unit costs by producing more, but chooses not to.
The markup is the difference between price and marginal cost (P − MC). It exists because downward-sloping demand gives each firm some market power. In the long run, P = ATC (zero profit) but MC < ATC because firms operate on the declining portion of ATC. So P > MC persists even with zero economic profit. The markup creates an incentive for firms to seek additional customers at the posted price.
Zero profit means P = ATC, not P = MC. In monopolistic competition, firms operate on the downward-sloping portion of the ATC curve where ATC > MC. Because price equals average total cost (covering all costs including fixed costs), price automatically exceeds marginal cost. The markup P − MC exactly compensates for the excess capacity inefficiency — the firm needs to charge above MC to cover its fixed costs spread over fewer units than the efficient scale.
Relative to perfect competition, the textbook model is not fully efficient. Two gaps appear: (1) the price markup above MC creates deadweight loss, and (2) excess capacity means firms do not operate at minimum ATC. Textbook analysis often weighs those costs against the presence of product differentiation and variety; this calculator reports the market-structure mechanics without taking a policy position.
Disclaimer
This calculator is for educational purposes only and assumes a simple linear demand curve with quadratic costs. Real-world monopolistically competitive markets involve more complex demand functions, dynamic entry/exit processes, and heterogeneous firms. This tool should not be used for business decisions.
