Floating rate vs fixed rate crypto swaps: what you actually pay, and when each one wins

A floating rate follows the market until your deposit confirms; a fixed rate freezes the number and charges you for the certainty. Which is cheaper depends on the deposit coin's confirmation time and on volatility — here is the arithmetic, with the practical ways to keep drift small on a floating swap.

The difference in one sentence

With a floating rate the amount you receive is calculated at the moment your deposit is confirmed; with a fixed rate it is calculated at the moment you create the order, and the service takes the risk of the market moving in between.

SyntheticSwap currently quotes floating rates, so this guide is written from that side — what floating means in practice, how much it can move, and what you can do about it.

Why floating rates exist

An instant swap has a gap between two moments: when you see the price, and when your coin actually arrives and confirms. Nobody controls that gap — it depends on your wallet's fee, the blockchain's block time, and network congestion. During the gap, the market moves.

A floating-rate service passes that movement to you in both directions: if the market moves in your favour before confirmation, you receive more; if against, less. Because the service carries no price risk, it can quote a tighter spread.

A fixed-rate service absorbs the movement. To do that profitably it must quote a wider spread — enough to cover the average adverse move plus the cases where people deliberately delay a deposit when the market runs against the locked rate. That protection is what you pay for.

How big is the gap, really?

The gap is the confirmation time of the coin you send. On SyntheticSwap:

Deposit coin	Confirmations	Typical gap	What that means for drift
USDT (TRC-20), USDC (Solana / Polygon), SOL	1	seconds	negligible
Gram (TON), USDC (Base)	2	under a minute	negligible
Ethereum, ERC-20 tokens	15	~3 minutes	small — a few tenths of a percent in normal markets
Zcash, Monero, Litecoin	6 / 5 / 5	8–12 minutes	noticeable on a volatile day
Bitcoin	2	~20 minutes at a normal fee; hours at a low fee	the case where floating vs fixed matters most

So the question "floating or fixed?" is mostly a question about what you are sending. Sending a stablecoin on a fast chain, the two would give almost identical results and the floating spread is simply cheaper. Sending Bitcoin with a low fee on a volatile day, the drift can exceed the extra spread a fixed rate would have charged.

The arithmetic

Say a fixed-rate quote is 1% wider than the floating one for the same pair. Over one Bitcoin confirmation window (~20 minutes), Bitcoin's typical move is well under 1%; on a very volatile day it can be more. The floating rate is the better expected value; the fixed rate is insurance against the tail. If you cannot afford to receive less than a specific amount — you are paying an invoice, for example — insurance is rational. If you are converting savings and would be equally happy with slightly more or slightly less, the cheaper spread wins on average.

Keeping drift small on a floating swap

Most of the drift on a floating swap is avoidable. It comes from the deposit waiting longer than it needs to.

Pay a fee that confirms in the next block or two. On Bitcoin this is the single biggest lever: a "low" fee can leave the deposit in the mempool for hours, and the rate you saw is long gone by the time it confirms.
Send from a fast chain when you have a choice. If you hold USDT on both TRON and Ethereum, swapping from TRC-20 confirms in seconds.
Send immediately after creating the order. The estimate refreshes every minute; creating the order and sending an hour later makes the estimate meaningless.
Avoid the minutes around major news if the amount is large — the spread is fine, the volatility is not.
Watch the direction of the payout fee. The payout fee is fixed in the coin you receive; the rate can float, the fee does not. On small amounts the fee, not the drift, is what moves your percentage.

What "the rate is locked at confirmation" means exactly

When the required confirmations arrive, the swap is executed at that moment's rate for the amount that actually arrived. Two consequences:

If you sent slightly less than the order amount (a wallet deducted its fee), you are paid at the live rate for what came in — the order is not cancelled.
If you sent within seconds and the deposit coin confirms in seconds, the rate you got is the rate you saw, to within the one-minute refresh.

The pair page shows, for a sample amount, the current all-in cost against the CoinGecko mid-rate. That number already includes spread and payout fee; a floating-rate swap's final cost is that number plus or minus whatever the market did during confirmation.

When a fixed rate is the right choice

You must deliver an exact amount of the output coin (an invoice, a purchase).
You are sending Bitcoin and cannot pay a high-priority fee.
The market is unusually volatile and the amount is large enough that a 1–2% move matters more than a 1% wider spread.

In every other case a floating rate, with the drift-reduction steps above, is the cheaper instrument.

Frequently asked questions

Can the floating rate move in my favour? Yes — it moves both ways. Over many swaps the moves average out; the spread does not.

Is the estimate I see a promise? It is the rate right now, refreshed each minute. The promise is the rule: rate at confirmation for the amount received.

What if Bitcoin moves 5% while my deposit is unconfirmed? You receive 5% more or 5% less. This is exactly the scenario a priority fee prevents: at a normal fee the window is ~20 minutes, and a 5% move in 20 minutes is rare.

Does the confirmation count change? Not per swap. The numbers above are what the service waits for; each pair page shows them for its deposit coin.

Which pairs are most sensitive? Anything you fund with Bitcoin: BTC → USDT (TRC-20), BTC → XMR, BTC → ETH. Anything funded with a fast stablecoin — USDT (TRC-20) → BTC — is nearly drift-free.