Costs of a Round Trip

The short answer

A round trip is two fills, and you are charged on both. You cross the spread going in and again coming out, the venue takes a fee on each fill, the book gives you whatever slippage it has that day, and a position still open at the financing cut-off accrues a holding cost on top.

None of that depends on being right. It means price must travel a specific, calculable distance before the trade is merely level — and that distance is knowable before you click anything.

The four components

Spread

The gap between the best bid and the best ask. If you buy at the ask and would have to sell at the bid, you start the trade underwater by the width of that gap, multiplied by your size. This is a cost even on venues that advertise zero commission, which is part of why zero commission is not the same as free.

The spread is not fixed. It widens when liquidity thins — overnight, around scheduled announcements, in smaller instruments — and it is widest exactly when you are most likely to want out.

Commission or per-fill fee

The venue's own charge. It may be a flat amount per order, a percentage of notional, a per-contract or per-share figure, or a maker/taker split where resting liquidity is charged less than liquidity you remove. Some venues apply a minimum, which makes small trades disproportionately expensive.

Two things to check in your venue's published schedule: whether the fee is charged per fill (a partially filled order can be billed more than once) and whether your order type routinely makes you the taker.

Slippage

The difference between the price you expected and the price you got. A market order takes whatever the book offers, so if depth at the top level is smaller than your order, you fill through several levels at progressively worse prices. A stop that triggers into a fast move can fill some distance past its trigger.

Slippage is the one cost you cannot look up in advance. You can only estimate it from your own filled orders, which is a good reason to record intended price alongside actual price from your very first trade.

Overnight financing or funding

If the position uses borrowed money or is a derivative with a periodic funding mechanism, holding it across the cut-off costs money. Margin interest, futures carry and perpetual funding payments all sit in this category, and they accrue whether the position is winning or losing. Funding and carrying costs covers the mechanics in detail.

A per-night figure that looks trivial is a different number when the position is held for three weeks.

Break-even is a distance

Costs are easier to reason about as a distance than as a dollar amount:

round-trip cost  = spread cost + entry fee + exit fee + slippage + financing
break-even move  = round-trip cost ÷ position size
break-even %     = round-trip cost ÷ notional × 100

A worked example

The numbers below are invented for teaching. They describe no real instrument, venue or fee schedule.

• Quote: 100.00 bid / 100.04 ask
• Size: 200 units, so notional is roughly $20,000
• Venue fee: 0.02% of notional per fill
• Held two nights, financing 0.01% of notional per night
• Slippage on exit: 0.01 per unit

Component	Working	Cost
Spread	0.04 × 200	$8.00
Entry fee	0.02% × $20,008	$4.00
Exit fee	0.02% × $20,000	$4.00
Slippage	0.01 × 200	$2.00
Financing	0.01% × $20,000 × 2	$4.00
Total		$22.00

Break-even move is $22.00 ÷ 200 units = $0.11 per unit, or about 0.11% of notional.

That sounds small, and in isolation it is. The point is what happens when you repeat it. At one round trip a week the annual cost on this size is roughly $1,150. At five a day it is roughly $27,500 on the same $20,000 of exposure — which is no longer a detail, it is the main term in the equation. FINRA's own day-trading disclosure makes this arithmetic explicit for frequent traders: the commissions alone set a profit target before anything else is considered.

You can run these numbers for your own venue with the trading fee calculator.

Why something fine before costs is negative after

Consider an approach whose average winning trade and average losing trade are close together — many short-horizon approaches look like this. The gross expectancy might be a small positive number per trade. Subtract a fixed round-trip cost from every single repetition and that number can go through zero without anything about the approach changing.

Three specific ways this happens quietly:

1. The test was gross. Backtests that do not deduct spread, fees and a slippage allowance are measuring an instrument nobody can trade. See backtesting and forward testing.
2. Frequency crept up. The cost per trade stayed the same; the number of trades tripled. Nobody re-ran the arithmetic.
3. Conditions changed. The approach was measured in a liquid window and is now being traded in a thin one, where spread and slippage are both larger.

The uncomfortable implication is that cost is not a friction you optimise later. It is a hurdle the approach has to clear from the first trade, and the size of that hurdle is partly your choice — through instrument, venue, order type and how often you trade.

Where people underestimate

• Zero-commission venues. The cost moved into the spread or the order routing. It did not disappear.
• Partial fills. One intended trade can become several billable fills.
• The exit. People model entry carefully and assume the exit is free. It is the same cost again, often in worse conditions.
• Small accounts. A minimum per-order fee is a much larger percentage of a small notional.
• Wide stops. A wider stop does not raise fees, but it usually means a longer hold, which raises financing.

What to do with this

Before your first trade, do three things.

Pull your venue's published fee schedule and write the numbers down rather than remembering them approximately. Compute the break-even distance for the size you actually intend to trade, and keep it visible — it belongs next to your entry and stop in the same way position size does.

Then record, for every trade you place, the price you expected and the price you got. After thirty trades you will have a slippage estimate specific to you, your venue and your order types, which is worth considerably more than any figure someone else quotes.