How a fixed rate works
With a fixed rate the exchange promises an exact amount. You receive what you saw, whatever the market does, as long as your deposit arrives within the time shown on your order page. The exchange sets that window, usually about 15 minutes.
Because the exchange takes the risk of the price moving against it, fixed offers cost more. On Lethe the difference is often about one percentage point, though it varies by pair, amount and exchange; the percentage on the Fixed rate button shows it for your amount.
How a floating rate works
With a floating rate, the amount you receive is set when your deposit confirms, at the market rate of that moment. The console shows what you would get at today's price, and the final number can end up a little higher or lower.
There's no window to beat, and the cost is usually lower. Some exchanges only make floating offers, so there are often more of them to choose from.
What the percentages mean
Next to each rate type you see a percentage, such as “Fixed rate (1.4%)”. It's how much less you receive than at the market price, with the exchange's fee and Lethe's 1% included and the network fee left out. The dollar values under the two amount fields show the same gap in money.
Before you compare offers, the percentage is an estimate from typical costs and starts with “~”. Once you press Compare offers, it's the real cost of the best offer of that type. Lower is better.
When fixed is the better choice
- The market is moving fast. On volatile days, a few minutes of price movement can cost more than the extra fee.
- You need an exact amount, for example to pay someone a set sum.
- Your deposit will arrive quickly. Solana, TRON and Ethereum deposits usually confirm within minutes, well inside the window.
When floating is the better choice
- The market is calm and you want the lowest cost.
- Your deposit may be slow. Bitcoin sent with a low fee, or a withdrawal from an exchange that batches its payouts, can miss a fixed-rate window.
- You're not sure exactly how much will arrive, for example because the sending platform takes its fee out of the amount.
- You're using the private route, which always uses a floating rate: its second swap only receives its coins after the first has finished, so a fixed window could run out in between.
If you send late, too little or too much
This is where the two types differ most, and the exchange you chose sets the rules:
- Floating rate: most exchanges simply swap whatever arrives, at the rate when it confirms.
- Fixed rate, late deposit: after the window the promise no longer holds. The exchange may swap at the current rate or refund you, minus the network fee.
- Fixed rate, different amount: the locked rate may not apply to another amount. Again, the exchange may recalculate or refund.
Refunds go to the refund address you gave, which is why it should be an address from a wallet you control. The safest habit is the same for both types: send exactly the amount on the deposit card, in one transaction.
Offers expire too
Separate from the fixed-rate window, Lethe keeps a list of offers for 5 minutes. Prices move, so after that the page asks you to compare again. When the swap starts, the exchange's reply is checked: if it promises less than the offer you picked, the trade isn't used and you're told.
The short version
Pick fixed when certainty matters more than a percentage point: fast markets, exact amounts, quick deposits. Pick floating when cost matters more and your deposit may take a while. Either way, the buttons show the full cost before you start. Compare both for Bitcoin to Monero.
