Glossary term
Law of One Price (LOOP)
The law of one price says identical goods or financial assets should have the same price across markets after adjusting for currency and transaction costs.
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What Is the Law of One Price?
The law of one price says identical goods or financial assets should sell for the same price in different markets after prices are expressed in the same currency and adjusted for transaction costs. If the same item is meaningfully cheaper in one market, buyers can purchase it there and sell it where the price is higher.
The idea is closely tied to arbitrage. Price differences attract trading activity, and that trading can push prices back toward alignment. In real markets, the law is a benchmark rather than a guarantee.
Key Takeaways
- The law of one price applies to identical goods or assets across markets.
- Prices need to be compared in the same currency and after transaction costs.
- Arbitrage pressure can reduce price differences, but frictions can keep gaps open.
- The concept supports pricing ideas in foreign exchange, commodities, securities, and purchasing power parity.
How Arbitrage Connects Prices
Assume the same bond trades in two markets. If it can be bought for less in one market and sold for more in another, traders have an incentive to buy the cheaper version and sell the expensive version. Their buying can raise the low price, while their selling can lower the high price.
The same logic can apply to goods, commodities, currencies, and securities, but only when the items are truly comparable. Quality differences, taxes, shipping costs, tariffs, capital controls, information gaps, and settlement risk can all prevent easy arbitrage.
Condition | Pricing Role |
|---|---|
Identical item | Similar is not enough; differences can justify different prices. |
Common currency | Exchange rates must be used before comparing prices. |
Low frictions | Transport, taxes, trading costs, and regulations can erase arbitrage profit. |
Market access | Traders must be able to buy and sell in both markets. |
Where Price Gaps Persist
Real-world violations are common because markets are not frictionless. A product may cost more in one country because of sales taxes, shipping, warranties, local regulations, brand positioning, or exchange-rate pass-through. A security may trade at different prices because of liquidity, settlement timing, short-sale constraints, or funding costs.
For investors, the law of one price is useful because it separates a clean pricing principle from messy market reality. When two prices differ, the next question is whether the gap is a true opportunity or compensation for hidden costs and risks.
The Bottom Line
The law of one price is a core pricing benchmark: identical things should not sell for different effective prices in competitive, frictionless markets. It is powerful because it explains arbitrage, but useful analysis starts with the frictions that keep real prices from matching perfectly.