Why a Pool’s Reserves Change the Price of Your Next Swap

A pool’s token reserves set its starting price, and each swap shifts that price by changing the reserves. This matters because the next trader’s price depends on the pool after your trade, not just on the balance before it.

In a simple constant-product pool, the reserves are the amounts of two tokens held for trading. If a pool holds 100 SOL and 10,000 USDC, its starting price is about 100 USDC for one SOL. The reserve ratio gives the marginal price: the rate for a very small trade. Byreal’s overview of how Byreal swaps work covers its trading and pool setup. Its routes can draw on aggregated liquidity and concentrated pools, so one reserve ratio may not describe every swap.

How does a reserve imbalance set the price?

The price in a basic pool comes from the ratio between its token reserves. In a constant-product model, the product of the two reserve amounts stays roughly fixed as traders swap. When someone buys SOL with USDC, USDC goes into the pool and SOL comes out. There is now more USDC and less SOL, so the next SOL costs more.

A pool with 1,000 SOL and 100,000 USDC has the same starting ratio as the smaller example: 100 USDC per SOL. But it has more depth. A trade of 1,000 USDC is a much larger share of the small pool, so it pushes that pool’s price further. The same trade has less effect on the larger pool.

Why can the next swap cost more than the starting price?

A swap moves along the pool’s pricing curve, so the price changes as the trade happens. The marginal price describes the rate at one point on that curve. The average price is what the trader gets across the whole swap. For a trade that buys SOL, the average is usually worse than the starting marginal price because each part of the trade faces a slightly higher price.

Fees also reduce the amount the trader receives. The final quote can differ from the starting price because it reflects the trade size, the pool’s available liquidity, fees and, for a routed swap, the liquidity used along the route. A thin pool or a large trade usually means more price impact, the change caused by the trade itself.

What should you check before swapping?

Use the quote for the size you plan to trade. A displayed spot price alone does not tell you the average price you will receive. Before confirming, check:

• The token amount you will receive after fees.
• The price impact and any slippage limit, which sets how much the final amount may vary.
• The route and pools used, especially if the swap draws from more than one source.

Reserve imbalance is a useful way to understand why prices move, but it is not a complete quote. For a basic pool, compare both its reserve ratio and its depth. For a routed swap, judge the actual quote for your trade size.