You have 2,000 USDT and want to long ETH on BloFin with a 2% stop.

The size that keeps the loss at the stop inside 20 USDT is 42.5 contracts, about 942 USDT of ETH. Not 45.1 contracts, and not whatever the leverage slider suggests.

The screenshot is the live BloFin ETHUSDT perp page at 10:17 AM ET: last 2,214.71, down 5.02% on the day, index 2,215.56, mark 2,214.32, 24h high 2,378.47 and low 2,172.74, funding +0.0085% with 01:42:36 left on the countdown, a 1h chart and the live order book. The math below uses the 10:16 prices.

Before: sizing from the stop alone

Step 1. Risk in USDT: 2,000 x 1% = 20 USDT.

Step 2. Stop distance per ETH: 2,216.53 - 2,172.20 = 44.33 USDT, which is 2.00%.

Step 3. Raw size: 20 / 44.33 = 0.4512 ETH. One BloFin ETH-USDT contract is 0.01 ETH, so that is 45.1 contracts, roughly 1,000 USDT of position value.

That looks finished, but it skips fees. BloFin's regular tier charges 0.06% taker on position value, on the way in and on the way out. A market entry plus a stop-market exit on 0.4512 ETH costs about 1.19 USDT, so the real loss at the stop is about 21.18 USDT. That is roughly 6% over budget, and the overshoot grows as stops get tighter.

After: sizing with fees inside the risk

Step 4. Add the round-trip taker fee per ETH to the stop distance: 0.06% x 2,216.53 + 0.06% x 2,172.20 = 2.63 USDT. Effective risk per ETH: 44.33 + 2.63 = 46.96 USDT.

Step 5. Size: 20 / 46.96 = 0.4259 ETH. The lot size is 0.1 contract, so round down to 42.5 contracts, which is 0.425 ETH or about 942.03 USDT.

Step 6. Check it: price loss 0.425 x 44.33 = 18.84, fees about 1.12, total 19.96 USDT. Inside the 20.

Step 7. Only now choose leverage. Margin at 5x is 188.41 USDT, at 10x 94.20, at 20x 47.10. The loss at the stop is 19.96 in all three. Leverage decides how much USDT gets parked as margin. It does not decide how much you lose when the stop fills.

Step 8. Check the tier and the liquidation distance. Tier 1 for ETH-USDT isolated runs up to 9,000 contracts with a 0.4% maintenance margin rate and up to 150x, so 42.5 contracts is far inside it. The trap is the other direction: at 50x the margin is only 2% of position value, and after 0.4% maintenance there is roughly 1.6% of room, which is less than the 2% stop. At 50x this trade gets liquidated before the stop ever triggers. At 10x the room is close to 10%.

One more piece of context. The stop distance here is about a fifth of the last 24 hours range. On a day like today a tight stop gets tagged more often. Widening it is fine, as long as the size shrinks with it so the 20 USDT stays 20 USDT.

Common mistakes

• Choosing leverage first and backing into a size. Size comes from risk and stop distance. Leverage comes last.
• Leaving fees out. Here it was a 6% overshoot. With a 0.5% stop the same fees would be a far bigger share of the loss.
• Rounding up to a cleaner number. 45.1 contracts felt neater than 42.5. Round down.
• Using high leverage because it is available, which can put liquidation inside the stop.
• Ignoring funding on longer holds. At 0.0085% per 8 hours, 942 USDT of ETH pays about 0.08 USDT a settlement while funding stays positive.

The same eight steps work on BTC, SOL, XRP or LINK perps. Only the contract size and lot size change.

Short FAQ

Does cross margin change the size? No. Stop and size set the loss. Cross only means the rest of the balance backs the position, which makes the stop more important, not less.

What if I enter with a limit order? BloFin's regular maker fee is 0.02%, so the fee per ETH drops and the size can rise slightly. Run step 4 again with the lower rate.

Is there a built-in tool? The order panel has a Calculator link for margin and PnL checks. It still needs your stop distance from step 2.

Try it on your own numbers: write down equity, risk percent, entry and stop, run steps 1 to 8, then compare the answer with the size you were about to type in.