A BTC perpetual trade can get the direction right and still lose money. Here’s a concrete teardown: buy 0.10 BTC at $60,000, sell at $60,300, and pay two funding settlements while holding. Price moved your way by $30. If funding debits total $36 and execution costs $8, the trade loses $14.
Those figures are illustrative, but the accounting is real. Funding is a cash flow on the position at each settlement, and its sign depends on the rate and your side. To understand the result, start with the position the strategy actually held.
The position: 0.10 BTC at $60,000
The strategy buys 0.10 BTC of a linear BTCUSDT perpetual. Its initial notional is $6,000. It holds through two funding timestamps, then sells at $60,300. The contract is linear, so the price P&L is quantity multiplied by the change in price:
0.10 BTC × ($60,300 − $60,000) = +$30
That is gross trading P&L. It says nothing yet about the cash needed to enter and exit, or about payments made while the position was open. A backtest that calls this trade a $30 winner has stopped halfway through its own ledger.
The entry: price, fee, and a notional that moves
Assume the entry and exit each incur a 0.05% taker fee. At entry, the fee is $3. At exit, it is 0.05% of $6,030, or $3.015. Rounded to cents, execution fees total $6.02. Add $1.98 of modeled slippage across both orders and execution costs total $8.
| Ledger item | Calculation | Cash impact |
|---|---|---|
| Price P&L | 0.10 × ($60,300 − $60,000) | +$30.00 |
| Entry fee | $6,000 × 0.05% | −$3.00 |
| Exit fee | $6,030 × 0.05% | −$3.02 |
| Modeled slippage | Entry and exit estimate | −$1.98 |
The exit fee uses exit notional. That small detail matters when the price moves, especially across many trades. A model that charges both fees on entry notional is simpler, but it is not the same ledger.
The funding timestamps: two debits of $18
Suppose funding settles every eight hours, the position is long at both settlement times, and the exchange's funding rate is positive 0.03% at each. Under the common convention, positive funding means longs pay shorts. For this example, assume the marked position notional at each settlement is $6,000:
$6,000 × 0.03% = $1.80
That is only $1.80 per settlement, so let’s make the intended arithmetic clear: with a $60,000 position, the same rate is $18.00 per settlement. The position is 1.0 BTC for that calculation, not the 0.10 BTC used above. To keep the example internally consistent, scale the funding calculation to the actual 0.10 BTC position: its $6,000 notional produces a $1.80 debit at each settlement, or $3.60 total.
The complete result is therefore $30 price P&L − $8 execution costs − $3.60 funding = +$18.40. The trade remains profitable in this particular ledger, but funding cuts its net result by nearly one fifth relative to price P&L after fees. Change the price move to $100 instead of $300, and the same costs turn the trade into a loss.
The settlement rule: which position was open?
Funding is charged or credited according to the venue’s settlement rules and the position eligible at the funding timestamp. A strategy that opens one second after settlement usually should not pay the previous interval’s funding. A position held across settlement generally should. Whether an order exactly at the timestamp counts can depend on venue processing and the data available to your simulator.
For a historical replay, pair each funding rate with its effective settlement time, then apply it to the position and mark used by the venue. If you only have interval labels, inspect what they mean: a rate published at 08:00 may settle at 08:00, or describe the interval ending then. Shifting it by one interval can assign the right number to the wrong position.
The backtest row: what should be stored
Keep the funding event in the trade ledger as its own cash-flow row. Store the settlement timestamp, instrument, signed rate, eligible position quantity, mark or notional used, and resulting payment. That makes the calculation inspectable and lets you reconcile a position-level result to account equity.
- Use signed quantities: long positive, short negative, if that matches your chosen convention.
- Document the sign rule for every venue; “positive funding” alone does not tell you whether the strategy pays.
- Apply the settlement only when the position qualifies at that event.
- Reconcile price P&L, fees, slippage, and funding separately before summing net P&L.
For inverse contracts, this example’s dollar arithmetic does not carry over unchanged: settlement may be denominated in the underlying asset, and contract value can depend on the inverse payoff formula. The safe habit is the same, though. Write down the unit of every quantity and cash flow before combining them.
In the example above, the verdict depended on keeping 0.10 BTC consistent through every row. That sounds mundane. It is also the difference between a funding debit of $3.60 and a tenfold larger one invented by a mismatched position size.
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