How crypto swaps work: the mechanics behind an instant exchange, from deposit to payout

Three ways to turn one coin into another — an order book, an on-chain pool, and an instant swap — and how the third one works in detail: where the rate comes from, why limits and confirmations exist, what 'non-custodial' means here, and what happens when a deposit is late or wrong.

Three ways to exchange one coin for another

All crypto exchanges do the same job — someone who has coin A and wants coin B is matched with liquidity going the other way — but they do it with different machinery:

Model	Where the liquidity is	You need	Coins it can handle
Order book exchange (centralised)	Other traders' limit orders on the exchange's books	An account, a deposit into the exchange	Whatever the exchange lists
Automated market maker (DEX)	A smart-contract pool of both coins on one chain	A wallet on that chain, gas	Only tokens on the same chain — no Bitcoin, no Monero natively
Instant swap (what SyntheticSwap is)	Liquidity providers and market makers the service routes to	A receiving address	Any coin the providers quote, across chains

The first two are where most trading volume happens. The third is what most people actually need when they simply want to convert: it is the only one of the three that works across unrelated chains — Bitcoin to Monero, Ethereum to TRON-USDT — without an account and without wrapping anything.

The lifecycle of an instant swap

Here is what happens, in order, from clicking Swap now to the coins appearing in your wallet.

1. Quote. You choose a pair and an amount. The service asks its liquidity sources for a rate, applies its spread and the network fee for paying you out, and shows the estimate. On SyntheticSwap the routing between sources is automated — the swap engine compares the quotes available for that pair at that moment and picks the best all-in result — and the estimate refreshes about once a minute.

2. Order creation. You give a receiving address (and optionally a refund address). A deposit address is generated that exists only for this order. No account is created; the order is identified by its status link.

3. Deposit. You send coin A from your own wallet to the deposit address. From this moment the order is waiting for the blockchain, not for the service.

4. Confirmation. The service waits for a fixed number of confirmations on coin A's chain — 2 for Bitcoin, 15 for Ethereum, 1 for USDT on TRON, 5 for Monero. Every pair page states the number. Confirmations are not bureaucracy: a Bitcoin transaction with zero confirmations can still be replaced by the sender, and an exchange that paid out against it would be paying against nothing.

5. Rate lock and execution. When the deposit is confirmed, the floating rate is fixed for the amount that actually arrived, and the exchange is executed with the liquidity source.

6. Payout. Coin B is sent to your address. The status page shows the transaction hash; your wallet shows the coins after its own confirmation threshold.

Total time is dominated by step 4. A USDT (TRC-20) deposit confirms in seconds; a Bitcoin deposit takes about 20 minutes at a normal fee.

Where the rate comes from

The rate you see is not a single "market price". It is:

the best quote available from the liquidity sources for that pair and amount at that moment,
plus the service's spread,
minus the network fee for sending you coin B.

Because the payout fee is a fixed amount in coin B, its weight in percentage terms falls as the amount grows. This is why small swaps look proportionally more expensive: a payout fee that is negligible on a $2,000 swap is visible on a $30 one. On every pair page SyntheticSwap compares its all-in quote for a sample amount with the CoinGecko mid-market rate, so you can see the total cost as one number.

Why there is a minimum and a maximum

The minimum exists because the payout fee and the liquidity source's own minimums make very small amounts uneconomical or impossible to route. The maximum reflects how much liquidity is available for that pair right now without moving the price. Both change with the rate and with market depth, which is why the pair page shows live numbers rather than a fixed table.

"Non-custodial" — what it means for an instant swap

The phrase is used loosely across the industry, so here is what it means on SyntheticSwap specifically:

There is no account and no balance kept for you.
Your coins are in your wallet until you send the deposit.
The service handles your funds only between the deposit confirming and the payout leaving — minutes, not days.
After payout, the coins are in your wallet, and the service holds nothing.

It does not mean the swap is executed by a smart contract you can audit — that model exists only for tokens on a single chain. For cross-chain swaps involving Bitcoin or Monero, an instant swap is the practical way to keep custody at a minimum.

Floating vs fixed rates

The estimate you see is a floating rate: it can move between the moment you see it and the moment your deposit confirms. The rate you actually get is the one at confirmation. A fixed rate freezes the number at order creation, in exchange for a wider spread that pays for the risk. SyntheticSwap currently quotes floating rates; the floating vs fixed guide explains when each is worth it and how to keep drift small.

When something goes wrong

Deposit below the minimum: it cannot be routed; it is returned to your refund address minus the network fee. Without a refund address, it waits for a support case.
Deposit above the maximum: the part above the limit is handled the same way; give a refund address.
Amount different from the order: the swap is recalculated for the amount that arrived; nothing is lost.
Deposit sent on the wrong network: the most serious case — a deposit address on TRON cannot receive Ethereum tokens. Recovery depends on whether the chain and address type make the funds reachable at all. Check the network label before sending; it is the single most important check.
Payout address invalid: the form validates format, but a valid-looking address on the wrong network passes validation on EVM chains (the same 0x address exists on Ethereum, BNB Chain, Polygon and Base). Make sure your wallet is on the chain you selected.

Frequently asked questions

Is an instant swap the same as a bridge? No. A bridge locks a token on one chain and mints a wrapped version on another; you end up with a derivative. A swap gives you the native coin on its own chain.

Why does the service need my address before I send? Because there is no account to attach the payout to. The receiving address is the order's identity.

Can the rate change after I send? Yes, until the deposit confirms — that is what floating means. Fast-confirming deposit coins (USDT on TRON, Solana) keep the window to seconds.

What data does the service keep? The order: both addresses, amounts, rate, transaction hashes — the same data that is already public on the two blockchains, plus the link between them. No identity data exists because none is collected.

Which pair should I start with? The pairs page lists every core pair with its current limits; BTC → ETH and USDT (TRC-20) → BTC are the most common first swaps.