3.24 USDT or 323.88 USDT of margin, same 24.89 USDT loss at the stop

That gap is the whole reason position sizing starts with the stop and ends with leverage, not the other way round.

The screenshot is the live MEXC BTCUSDT perpetual page at 10:17 AM ET: last 76,701.0, down 6.14%, index 76,729.8, fair price 76,707.6, funding +0.0100% with 01:42:37 left on the countdown, 24h high 81,946.0 and low 76,453.5, and a 15m chart showing the overnight slide. The worked numbers use MEXC's contract data from 10:16 AM ET, when the last price was 76,749.1.

The setup: a 5,000 USDT account, a BTC long at 76,749.1, a stop at 75,600, and a plan to risk 0.5% of equity.

Q: What decides the size?

Two inputs only: how much you are willing to lose and how far away the stop is. 5,000 x 0.5% = 25 USDT of risk. The stop distance is 76,749.1 - 75,600 = 1,149.10 per BTC, about 1.50%. Raw size: 25 / 1,149.10 = 0.02176 BTC.

Q: How does that become contracts?

MEXC's margin guide defines position value as price x contract size x number of contracts. Its own example is 100 contracts at 50,000, which is 500 USDT of value, or 50 USDT of margin at 10x. On BTC_USDT one contract is 0.0001 BTC and the minimum is 1 contract, so 0.02176 BTC is about 217 contracts before fees.

Q: Where do fees fit?

Inside the risk. BTC_USDT showed 0.02% taker and 0% maker. A market entry and a stop-market exit are both taker fills: 0.02% x 76,749.1 + 0.02% x 75,600 = 30.47 USDT per BTC. Effective risk per BTC is 1,149.10 + 30.47 = 1,179.57. 25 / 1,179.57 = 0.02119 BTC, which rounds down to 211 contracts, 0.0211 BTC, about 1,619.41 USDT of value. At the stop: 24.25 price loss plus 0.64 fees, 24.89 total. Skip the fee step and the 217 contracts lose about 25.60, a small but real overshoot.

Q: So what does leverage change?

Only the margin. For 1,619.41 USDT of BTC: 323.88 at 5x, 80.97 at 20x, 16.19 at 100x, 3.24 at 500x. Same 24.89 at the stop.

Q: Then why not always use 500x?

Liquidation. BTC_USDT lists 0.2% initial and 0.1% maintenance margin. At 100x, margin is 1% of value, leaving roughly 0.9% before liquidation, inside a 1.50% stop. At 500x the room is about 0.1%. At 20x it is close to 4.9%, comfortably past the stop. Leverage should be picked so the stop is reached first.

Q: Does MEXC have a tool for this?

The Futures Calculator handles Max Open, liquidation price and entry price. Its help page notes the results leave out fees, funding and slippage, so the fee step above still has to be done by hand.

Q: Is a 1.50% stop sensible today?

That is a trading decision, not a sizing one. For context, the 24h range was 76,453.5 to 81,946, about 7.16% of the price, so this stop is roughly a fifth of a day's swing. A wider stop is fine. It just means fewer contracts for the same 25 USDT.

Common mistakes

• Opening a round number of contracts, then wondering what the risk is.
• Leaving out the closing fee on the stop.
• Rounding 211.6 up to 212.
• Letting the leverage cap become the default.
• Holding for days without counting funding: at 0.01% per 8 hours, 1,619.41 USDT pays about 0.16 USDT per settlement while positive.

Short FAQ

Does this work for ETH, SOL, XRP or LINK? Yes. Swap in that contract's size, minimum and fees.

Should risk always be 0.5%? It is an example input. Pick a number you can live with across a losing streak.

Can slippage break the plan? Yes. A stop-market exit in a fast move can fill worse than 75,600, which is one more reason not to size at the edge.

Takeaway: size from the stop and the risk budget, then pick the leverage that keeps liquidation further away than the stop.