“The trade captures a 1% rise. The commission is tiny, so it won't matter.” Before accepting that claim, answer two questions. Is the charge based on profit or on the amount traded? Does the 1% price change apply to the entire account or only to the small portion invested?
Costs also change with the proposed execution. A larger order or a later fill may invalidate the original price and quantity assumptions. We will first reconcile one transparent trade, then ask whether it could actually be executed on those terms. A correctly calculated result still depends on the supplied conditions.
What you will check
- Put commissions, slippage, and turnover on explicit amounts and denominators without counting them twice.
- Compare predeclared idealized, base, stress, and delayed-execution scenarios.
- Identify limitations from capacity, partial fills, and personal taxes before interpreting paper results.
Separate a bill from a price difference
Explicit fees, including commissions, can generally be located in a fee schedule or transaction record. Implicit costs can appear in the difference between a reference price and the price received. The SEC's fee bulletin distinguishes trading and ongoing expenses and directs readers to actual disclosures.[1] Asking only whether commissions are zero leaves the investigation incomplete.
Here, slippage means execution-price deviation from a specified reference price. Actual deviations can help or hurt. For the comparisons below, assume adverse slippage: a higher purchase price and a lower sale price. State the reference time so different price comparisons do not silently become the same cost measure.
If a measured execution deviation already captures the spread or price impact, adding that same component in full again duplicates it. CFA Institute's public overview includes explicit costs, implicit costs, delay, and unfilled trades in implementation analysis.[2] Different labels do not guarantee distinct economic amounts; trace where each enters the calculation.
Freeze one small trade
This independent synthetic account starts with $10,000 and buys 20 units of a fictional asset before selling all 20. The planned purchase reference is $100 and the sale reference $101. Every order fills completely. There are no other trades, deposits, withdrawals, dividends, taxes, borrowing, or cash interest. These are not actual SPY trades or a complete C2 backtest.
Register three conditions before calculating. The idealized reference has zero commission and zero slippage. The base case charges 5 basis points of commission on each trade and applies another 5 basis points of adverse slippage. The stress case uses 15 basis points for each. One basis point is 0.01%; 5 basis points are 0.05%, or 0.0005 as a decimal, not 5%.
Let c denote the commission rate and s the slippage rate. Multiply the purchase reference by one plus s and the sale reference by one minus s. Calculate each commission from that trade's actual execution value times c. Slippage is already in the execution price and must not be deducted again as a cash fee.
Holding the quantity at 20 isolates cost changes in this exercise. Real C2-A sizing instead uses available cash including fees and its specified quantity precision. Do not transplant a fixed 20-unit policy into that strategy. Likewise, an account initially fully invested cannot necessarily buy the same quantity after a fee increase.
Reconcile the base case
The purchase executes at 100 times 1.0005, or $100.05. Twenty units cost $2,001 before commission. Commission is 2,001 times 0.0005, or $1.0005. Total purchase spending is $2,002.0005, leaving $7,997.9995 in cash.
The sale executes at 101 times 0.9995, or $100.9495. Twenty units generate $2,018.99 before commission. Deduct $1.009495, leaving net sale proceeds of $2,017.980505. Subtract the total purchase spending to obtain net profit of $15.980005. Ending account cash is $10,015.980005.
With no costs, the same 20 units earn $20 from the reference-price increase. The base case retains about $4.02 less. That difference already includes both adverse execution-price changes and both commissions. Deducting another general “spread cost” from the result could count a price effect twice.
The reference price rose 1%, but only about $2,000 participated. Net profit of roughly $15.98 divided by the full $10,000 account is about 0.1598%. It is not a 0.798% or 1% return on the entire account. Whenever a percentage appears, identify the particular amount serving as its denominator.
You can also solve for the break-even sale reference. Under this fixed-quantity model with no minimum fee, net sale proceeds must equal purchase spending. The sale reference is multiplied twice by 0.9995, for adverse slippage and commission; the purchase reference is multiplied twice by 1.0005.
Starting from a purchase reference of $100, the required sale reference is approximately $100.20020015, a rise of about 0.2002%. This is the arithmetic threshold under the stated conditions, not a prediction that the price will reach it. A different fee or sizing policy requires a new calculation.
Compare the declared cost cases
In the stress case, the purchase price is $100.15. Twenty units cost $2,003, commission adds $3.0045, and total spending is $2,006.0045. The sale price is $100.8485, producing $2,016.97 before commission. Deduct $3.025455 to receive $2,013.944545. Net profit is $7.940045.
| Declared case | Commission / adverse slippage on each trade | Purchase spending | Net sale proceeds | Net profit | Ending account cash |
|---|---|---|---|---|---|
| Idealized reference | 0 / 0 bps | 2000 | 2020 | 20 | 10020 |
| Base | 5 / 5 bps | 2002.000500 | 2017.980505 | 15.980005 | 10015.980005 |
| Stress | 15 / 15 bps | 2006.004500 | 2013.944545 | 7.940045 | 10007.940045 |
The extra decimal places support reconciliation. They do not claim that real charges are assessed to that precision. Keep internal amounts unrounded to cents in this example and round only for display. Actual brokers' minimums and rounding policies require separate checking.
All three examples profit, but the idealized result is not the selected conclusion. The comparison shows sensitivity for one given transaction. A real long-term strategy needs a recalculation over its full orders. Apply matching assumptions to the benchmark's actual trading policy; do not add the strategy's extra trades to a lower-turnover benchmark.
Turnover needs a stated definition
Base-case purchase value is $2,001 and sale value $2,018.99. Define turnover here as the sum of both traded amounts divided by starting account equity of $10,000. The result is 4,019.99 divided by 10,000, or 40.1999%. This is the exercise's definition, not a universal reporting convention.
Counting purchases only gives 2,001 divided by 10,000, or 20.01%. Using average assets or another convention can produce another number. Retain the purchase and sale amounts, reporting period, and denominator before comparing two figures both called turnover.
With fixed proportional fees, more traded value generally means more total commission. Here the two commissions sum to $2.009995. The $4,019.99 total already includes both sides. Multiplying it by the 5-basis-point commission and then multiplying by two “for the round trip” would charge both sides twice.
The transaction ledger remains the cash authority. Turnover multiplied by an unclear fee rate cannot replace it. Such a shortcut can hide different reference prices, incomplete orders, changing rates, or a sizing response to costs. First determine whether the proposed fee base actually represents the trades.
A one-day delay changes prices too
Now use a separate delayed scenario. The planned purchase was D1 and sale D3. Delay each by one valid trading day: buy on D2 at a reference of $102 and sell on D4 at $100.50. Keep 20 fully filled units, base commission and slippage, and all other small-account assumptions unchanged.
Purchase spending becomes $2,042.04051. Net sale proceeds are $2,007.9905025, giving a net loss of $34.0500075. The original base profit of about $15.98 has become a loss of about $34.05. Much of the difference comes from different trade prices, not an additional fixed charge called a delay fee.
All four prices are synthetic inputs declared for the exercise. They do not show that one day's delay must be unfavorable; another path could help. A stress comparison investigates timing dependence. Selecting the best date afterward and presenting it as the original plan would answer a different question.
For a real partial fill, record fees and holdings using the quantity actually executed. Treat the remainder according to the existing order policy. The account cannot earn a fully invested position's return while paying only the fees and cash requirement for a partial position. Without quote and quantity records, identify the execution assumption rather than claim feasibility.
A larger order may exhaust the best quote
Capacity concerns the scale at which a strategy might still be implemented as intended, including how greater size changes costs and risk. We are not estimating any strategy's real capacity. Consider an independent static sell-side order book: 20 units offered at $100.05, followed by 180 at $100.20. Assume all are available to your order and cash is sufficient.
Buying 20 averages $100.05. Buying 200 crosses both levels. Its amount is $2,001 plus $18,036, or $20,037, giving an average of $100.185. The best quote is unchanged, yet the larger order cannot all execute there. CME's liquidity methodology likewise uses deeper levels when the requested quantity exceeds top-of-book availability.[3]
Actual order books change. Hidden quantities, competing orders, and price reactions after execution also matter. This two-level example illustrates a quantity constraint, not permanent market impact. Nor does a futures-tool methodology supply a complete ETF execution model. Fixed 5-basis-point slippage is a scenario assumption, not market evidence that this cost is available at every size.
A smaller personal account may face fewer scale problems, but still needs checks on order size relative to liquidity, trading time, and charges. A zero-commission account can have spreads, other fees, or currency-conversion costs. Small size is not evidence that all implementation questions have disappeared.
Pretax profit is not yet personal spending money
C2's 5/5-basis-point base case, 15/15 stress case, and delay assumptions are teaching conditions. Personal taxes are excluded. A real task also checks applicable market fees, product and account charges, data services, financing, and conversion. State separately whether each belongs in strategy performance or the research budget.
After-tax results depend on residence, tax status, account, and transactions. Do not invent a uniform tax rate without that information. Also check whether a fund's operating expense is already reflected in its net asset value before subtracting it again. A longer list of cost names does not justify mechanically adding every item.
Test the “tiny cost” claim
Explain why 5 basis points of commission plus 5 basis points of slippage cannot be applied once as 0.1% of the $20 gross profit. Then calculate commission on the base case's $4,019.99 two-sided trade value. What happens if someone doubles the answer for a round trip?
Reference answers · reveal
Answer guide: Commission applies to each actual trade value; slippage changes each execution price adversely. Neither is a single charge on profit. Total commission is 4,019.99 times 0.0005, or $2.009995. Doubling it repeats both trades. The price deviation is already present in the ledger and is not another identical cash deduction.
The reference price ends where it started
Change only the sale reference from $101 to $100. Keep 20 units, $10,000 starting cash, and the base 5/5-basis-point assumptions. Calculate purchase spending, net sale proceeds, and the final account. Explain why your answer is not a guaranteed real-market outcome.
Reference answers · reveal
Answer guide: Purchase spending stays $2,002.0005. The sale executes at $99.95, generating $1,999 before a $0.9995 commission. Net proceeds are $1,998.0005. Ending cash is $9,996, a $4 loss. The loss comes from the declared adverse execution and fees despite equal reference prices. Unknown real quotes, quantities, and charges prevent treating it as an actual-fill guarantee.
Save a cost and execution sensitivity table
Freeze rules and inputs. Record reference prices, execution adjustments, commission bases and timing, quantities, and numerical precision. Compare idealized, base, and stress results side by side; add separate delay and partial-fill conditions. Keep traded values, fees, cash, and unfilled status, not just net returns.
Add the turnover definition, quantity evidence, size assumption, tax boundary, and research-budget boundary. Choose one cost and point to the first price or cash entry containing it. If you cannot, investigate an omission or duplication. End with the most sensitive implementation assumption and the next material needed to check it.
Follow each dollar before scaling the claim
Fix reference prices and fee definitions, handle purchase and sale directions separately, then reconcile trade values and cash. State turnover's denominator. When slippage is already in price, do not charge it again; a zero-cost reference cannot substitute for an implementation judgment.
Change declared cost and timing conditions, then investigate quantity and liquidity. If paper gains depend on an execution assumption you cannot verify, obtain that evidence or narrow the conclusion. More decimal places cannot turn a hypothetical fill into a verified return.
References
- SEC: How Fees and Expenses Affect Your Investment Portfolio, July 23, 2025. Fee types and disclosures, not a current broker price list.
- CFA Institute: Trading Costs and Electronic Markets, 2026 public overview. Explicit and implicit costs, delay, and unfilled trades; the paid reading was not inspected.
- CME: Understanding the CME Liquidity Tool Methodology. Quantity and order-book levels; no endorsement or transfer of its instrument-specific results.
Sources inspected October 30, 2026. AI-assisted draft; independent AI full-text and bilingual review is complete. Human financial review and target-reader testing have not been performed. Prices, order-book quantities, and fills are synthetic. Real long-term C2 cost evidence and execution acceptance remain incomplete.