Last Updated: 28 July 2023

What is a Crypto Flash Loan and How to use it

Discover what a crypto flash loan is, how they work, learn what are the main use cases, and how people make money with it.

Understanding Flash Loans: A Game-Changer in the Cryptocurrency World

Flash loans are a unique feature in decentralized finance (DeFi) that allows users to borrow any available amount of assets from a designated smart contract pool with no collateral requirement. They have become essential building blocks in the DeFi ecosystem, enabling various financial activities like arbitrage trading, collateral swapping, and self-liquidation.

The catch with flash loans is that they must be borrowed and repaid within the same blockchain transaction. In the context of blockchain technology like Ethereum, a transaction represents a set of operations that must be executed atomically – either all steps are completed successfully, or the transaction is rolled back, and none of the steps take place.

To better understand this concept, let's draw a parallel with a database transaction. In a database, if we have two update statements to subtract $500 from Alice's balance and add $500 to Bob's balance, we need to ensure that both updates either happen entirely or none of them happen at all. We achieve this by using transactions in the database, ensuring that the operations are atomic and leave the database in a consistent state.

Similarly, in Ethereum, transactions are grouped together and included in blocks. A single Ethereum transaction can consist of multiple steps, such as sending ERC-20 tokens, interacting with smart contracts, or swapping assets on different protocols like Compound, Uniswap, etc. If any of these steps result in an error, the entire transaction is rolled back, and none of the steps take place.

Flash loans are executed within a transaction scope, and multiple steps can be combined into a single Ethereum transaction. For example, a flash loan user could borrow DAI, swap it for USDC, provide liquidity to a pool, and perform other actions in one transaction. If any step fails, the entire transaction is reverted, ensuring the loan amount is not retained by the borrower.

To execute a flash loan, users need to find a flash loan provider. Different platforms offer smart contracts that enable users to borrow various coins from a designated pool under the condition of repayment within the same Ethereum transaction. There is typically a fixed cost associated with using flash loans, which includes a fee paid by the borrower.

The borrowed amount from the lending pool can be used for any other arbitrary actions as long as it is repaid at the end of the chain of different steps in the same transaction. Since the loan must be repaid within one transaction, there is no risk of borrowers defaulting on the loan. The only risks involved are associated with smart contract vulnerabilities or platform risks.

Flash loans have gained popularity, with some users borrowing significant amounts of cryptocurrency, such as 14 million DAI, through these decentralized financial services. They offer unique opportunities for profit through the efficient and automated execution of complex financial strategies within a single transaction. However, they require technical skills and an understanding of DeFi protocols to utilize effectively.

How Do Flash Loans Work?

Flash loans operate on blockchain platforms, such as Ethereum, through the use of smart contracts. These contracts enable users to borrow funds for a brief period, typically lasting only a few seconds. During this time, borrowers can perform various financial operations, but they must repay the entire loan amount plus fees before the block's validation is complete.

The key to flash loans lies in their instantaneous nature, making them attractive for various applications, especially in the decentralized finance (DeFi) space. Since all transactions occur within a single block, the entire process is risk-free for the lender, as they receive their funds back promptly. However, if the borrower fails to return the borrowed amount, the smart contract automatically reverses the entire transaction, ensuring that the lender does not lose any money.

To better understand how flash loans work, let's break down the process step-by-step:

  1. Smart Contract Execution: Flash loans operate within smart contracts on blockchain platforms like Ethereum. These contracts contain the logic and rules for the flash loan service.
  2. Borrowing: A user initiates a flash loan request by interacting with the smart contract. They specify the amount of funds they want to borrow and the type of asset they want to borrow (e.g., ETH, DAI, USDC).
  3. Validation: The smart contract verifies if the requested loan amount is available in the lending pool. If there are sufficient funds, the flash loan is approved, and the borrower can proceed with the next steps.
  4. Execution Window: The borrower has a brief execution window, typically lasting only a few seconds, to use the borrowed funds for their intended purpose. During this time, they can perform various financial operations, such as trading, arbitrage, or providing liquidity.
  5. Repayment: Before the execution window ends, the borrower must repay the entire loan amount plus fees back to the lending pool. The repayment must occur within the same blockchain transaction in which the flash loan was initiated.
  6. Automatic Reversal: If the borrower fails to repay the loan within the allotted time, the smart contract automatically reverses the entire transaction, undoing all the operations performed during the execution window. This ensures that the lender's funds are returned, and there is no financial loss for the lending pool.

Three Main Use Cases for Flash Loans

1. Trading Arbitrage: Trading arbitrage is a strategy employed by traders to profit from price differences of a cryptocurrency between different exchanges. In the traditional financial world, arbitrage opportunities are often exploited using substantial capital and complex trading algorithms. However, in the world of cryptocurrencies, flash loans have introduced a game-changing approach to arbitrage.

Flash loans allow traders to borrow significant amounts of cryptocurrency instantly without the need for collateral. With this borrowed capital, traders can swiftly execute multiple transactions across various exchanges to capitalize on price disparities. For example, if a cryptocurrency is priced at $100 on one exchange and $101 on another, traders can exploit this difference by purchasing the asset at the lower price and selling it at the higher price, making a quick profit.

While flash loan-based arbitrage was initially a lucrative opportunity, it has become less common due to the emergence of automated trading bots. These bots can execute arbitrage opportunities at an incredible speed, making it challenging for individual traders to compete.

2. Collateral Swap: Collateral swap is another practical use case for flash loans within the decentralized finance (DeFi) ecosystem. In DeFi lending users often deposit cryptocurrencies as collateral to borrow other assets. However, if a user wants to change their collateral position without going through a series of transactions, a flash loan can provide a seamless solution.

Let's consider an example: User A has deposited Ethereum as collateral on a lending platform and borrowed stablecoins to invest in other assets. Over time, User A becomes bullish on a different cryptocurrency and wishes to change their collateral position from Ethereum to the new asset. Instead of repaying the existing loan, converting Ethereum to the new asset, and then borrowing again with the new asset as collateral, User A can utilize a flash loan.

With a flash loan, User A can temporarily borrow the required amount of stablecoins to repay the original loan. Once the original loan is settled, User A can immediately deposit the new asset as collateral and borrow stablecoins once more. This way, the user can change their collateral position in a single transaction, saving time and gas fees on multiple operations.

3. Self-Liquidation: Self-liquidation is a valuable application of flash loans for users who have deposited assets as collateral for loans and wish to access their funds without selling the collateral. In the traditional financial world, users who need to access their collateral would have to repay the loan in part or in full. However, DeFi platforms, powered by flash loans, offer a more efficient solution.

Imagine User B deposited 100 Ethereum as collateral for a loan on a DeFi platform a year ago when Ethereum's price was $200 each. At that time, User B took out a loan in stablecoins to meet their financial needs. Now, Ethereum's price has surged to $2,000 each, and User B wants to retrieve their collateral without selling it.

With a flash loan, User B can temporarily borrow the required amount of stablecoins to repay the original loan. After repaying the loan in the same transaction, User B can withdraw their original 100 Ethereum as collateral, effectively self-liquidating the loan. The flash loan enables them to access their collateral without needing additional capital or repaying the loan over time.

Flash Loan Attacks: The Risks and Ethical Dilemmas

While flash loans have opened up new avenues for decentralized finance, they have also given rise to potential risks, particularly in the form of flash loan attacks. In some instances, attackers have exploited vulnerabilities in smart contracts to exploit platforms for financial gain. One example involved the pancake bunny protocol, where attackers borrowed a significant amount, exploited a bug in the system, profited, and then repaid the loan in a single transaction. The ethical implications of such actions remain a matter of debate among developers and the cryptocurrency community.

In conclusion, flash loans represent a revolutionary financial tool that has transformed the landscape of decentralized finance. These quick, no-collateral loans have enabled users to engage in trading arbitrage, collateral swapping, and self-liquidation with unparalleled efficiency. However, their potential risks, as demonstrated by flash loan attacks, underline the importance of robustly auditing smart contracts to avoid vulnerabilities. As the cryptocurrency ecosystem continues to evolve, flash loans will undoubtedly remain a topic of interest and scrutiny, shaping the future of decentralized finance.

Difference from Traditional Crypto Loans with Collateral

In traditional crypto lending users can deposit their cryptocurrencies into a smart contract and earn interest on these deposits. This process is similar to depositing money into a bank savings account and earning interest on it. The users providing their cryptocurrencies as deposits are known as lenders, and they earn interest as borrowers utilize their funds.

Borrowers, on the other hand, can access these deposited funds by taking out a loan. However, in the world of crypto lending, borrowers need to be over-collateralized. This means they must provide more cryptocurrency as collateral than the value of the loan they wish to borrow. For example, if a user wants to borrow $15,200 worth of cryptocurrency, they would need to provide $19,000 worth of cryptocurrency as collateral.

The over-collateralization serves as a safety net for the lender. If the value of the borrowed cryptocurrency drops significantly, the lender can automatically liquidate the collateral to recover their funds. This way, lenders are protected from potential losses, and borrowers can access loans without a credit check.

The main difference between traditional crypto loans with collateral and flash loans lies in the collateral requirement and the repayment timeline. While traditional crypto loans require borrowers to provide collateral and have a more extended repayment period, flash loans offer unprecedented flexibility but must be repaid within the same block.

How to make money with Flash Loans

Making money with flash loans involves taking advantage of price discrepancies in the cryptocurrency market through a rapid and automated process. Here's how people can potentially profit from flash loans:

  1. Identifying Arbitrage Opportunities: Crypto enthusiasts monitor various cryptocurrency exchanges to spot price differences for specific assets. For example, if Ethereum is priced at $3,050 on one exchange (e.g., Binance) and $3,100 on another exchange (e.g.,, there is a $50 price discrepancy.
  2. Utilizing Flash Loans: Instead of using their own capital, users can borrow a significant amount of cryptocurrency through a flash loan from lending platforms. In this example, the user borrows $300,000 worth of Ethereum.
  3. Executing the Arbitrage: The borrowed Ethereum is immediately used to purchase Ethereum on the cheaper exchange (e.g., Binance) and sell it on the higher-priced exchange (e.g., for a profit of $50 per Ethereum.
  4. Repaying the Loan: To avoid losing the borrowed funds, the entire process must be completed within the short window of an Ethereum block validation time (approximately 13 seconds). The borrowed $300,000 is returned to the platform, and the user keeps the $5,000 profit.
  5. Automation: In practice, flash loan-based arbitrage is performed by automated algorithms or bots, not human traders. These algorithms can execute the necessary transactions within the limited timeframe, ensuring efficiency and maximizing potential profits.
  6. Limitations: While flash loans offer the potential for significant profits, they require technical skills and coding expertise to implement effectively. Additionally, the fast-paced nature of flash loans and the need for precise timing make manual execution virtually impossible.

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