Why a stablecoin swap shows 0.99 instead of 1.00 on a DEX

05 September 2026

A stablecoin swap shows 0.99 instead of 1.00 because the automated market maker (AMM) that powers the DEX charges a small fee built into the swap price. The 0.99 reflects that fee - typically 0.01% to 0.30% per trade - plus temporary imbalance in the liquidity pool that pushes the price slightly away from parity.

The mechanism works as follows. A DEX does not use an order book. It relies on a liquidity pool containing reserves of two stablecoins, for example USDC and USDT. The pool's smart contract defines a constant product formula: the product of the reserves must remain unchanged after a swap (ignoring fees). When you trade one stablecoin for the other, you remove tokens from one side of the pool and add them to the other. Removing tokens changes the ratio. The formula calculates a new price that is always slightly worse than 1:1 because the act of trading shifts the balance. The fee is then added on top.

Consider a concrete example. A pool holds 10,000,000 USDC and 10,000,000 USDT. The spot price is exactly 1.00. You swap 10,000 USDC for USDT. The constant product formula (reserve_A * reserve_B = k) means that after depositing your USDC, the pool has 10,010,000 USDC. To keep k constant, the pool must release exactly 9,990.01 USDT. The price you receive is 0.999001 USDT per USDC. The DEX then applies a fee, say 0.05%, deducted from the output amount. Your final receive: roughly 9,985.01 USDT. That is a price of 0.9985. Displayed as 0.99 after rounding, the true rate is a few decimal places deeper.

The displayed 0.99 is a user-interface simplification. Most DEX front ends truncate or round to two decimal places for stablecoin pairs, because showing 0.998501 would imply a precision that does not matter for small trades. For large trades, the slippage becomes visible even with rounding. A $100,000 swap might yield $99,500, clearly showing 0.995. The 0.99 figure is a floor: it tells you that you are getting less than a 1:1 rate without cluttering the screen.

Why does the pool ever drift from 1.00 in the first place? Liquidity providers deposit stablecoins and earn fees. If the pool becomes imbalanced - say more USDC than USDT - arbitrageurs step in. They buy the cheaper stablecoin and sell it elsewhere until balance returns. But arbitrage is not instantaneous. Large swaps, network congestion, or sudden demand can leave the pool skewed for minutes or hours. During those windows, the swap rate visibly deviates from 1.00.

The fee structure is not hidden. Every DEX publishes its fee tier. Common tiers for stablecoin pools are 0.01%, 0.05%, or 0.30%. The displayed 0.99 includes both the fee and the pool imbalance. You cannot separate them from the quoted rate alone.

If you are moving between volatile assets and stablecoins, or between stablecoins on different chains, the 0.99 figure is normal. It is not a sign of error or manipulation. It is the mathematical consequence of how AMMs work. The hub page "Swapping into and out of stablecoins" covers the broader strategy of choosing which stablecoin to use and when the 0.99 matters more than the 1.00. That page is worth reading if you find yourself repeatedly paying this spread and wondering whether a different route - like a centralized swap or a cross-chain bridge - would cost less.

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