Portfolio Turnover: Measure How Trading Costs Erode Your Edge

Portfolio Turnover: Measure How Trading Costs Erode Your Edge

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Learning objective

Learn to calculate portfolio turnover, translate commissions, spreads and slippage into annual cost, and test whether a strategy still has positive expectancy after execution.

Turnover measures how much capital a strategy buys and sells relative to portfolio size. High turnover is not automatically bad, but every replacement creates another chance for spread, commission, slippage, tax and market impact to consume the gross return.

Step 1: measure turnover consistently

A practical annual measure is:

Turnover = the lower of total purchases or total sales ÷ average portfolio value.

Some platforms use different definitions, so record the formula. For a trading strategy, also track round trips, average holding time and traded notional.

Step 2: build a cost stack

For every entry and exit estimate:

• explicit commission and exchange fees;
• half-spread paid at entry and exit;
• slippage against the decision price;
• borrowing, funding or financing cost;
• market impact for larger orders;
• relevant taxes.

Backtests should use costs from the instrument, venue and period being tested. A current tight spread should not be applied to older or stressed data without evidence.

Worked example

A hypothetical £20,000 strategy turns over its capital six times a year. Each complete turnover therefore trades about £120,000 of purchases and £120,000 of sales.

Assume the average all-in round-trip cost is 0.30% of traded capital. Estimated annual drag is:

6 × 0.30% = 1.80%, or about £360 on £20,000.

If the backtest produced 7% gross return, the simplified net result becomes 5.2% before tax. If stressed spreads and slippage lift round-trip cost to 0.70%, annual drag becomes 4.2%, leaving only 2.8% before tax.

This is an illustration, not a universal cost estimate. Real costs vary by size, venue, timing and liquidity.

Step 3: calculate break-even edge per trade

If average round-trip cost is 0.30%, the signal must earn more than 0.30% per completed trade before contributing to net profit. A strategy with many tiny gross winners can fail even with a high win rate.

Compare:

Net expectancy = gross expectancy − commission − spread − slippage − financing.

Use realised fills for live monitoring, not advertised minimum fees.

Step 4: test turnover sensitivity

Recalculate results at:

• normal costs;
• 1.5 times normal costs;
• twice normal costs;
• lower trade frequency;
• delayed execution.

If a small cost change removes the entire profit, the edge is fragile. Reducing turnover can help, but only if filtering trades does not remove the strongest signals.

Common mistakes

• Counting commission but ignoring spread and slippage.
• Using today's liquidity for historical backtests.
• Treating gross return as investable return.
• Comparing strategies with different turnover before costs.
• Assuming limit orders have no cost when missed fills change results.
• Reducing frequency without re-testing the signal.

Practical checklist

• State the turnover formula and period.
• Track traded notional and round trips.
• Build an instrument-specific cost stack.
• Compare gross and net expectancy.
• Stress costs and delayed fills.
• Reconcile modelled costs with actual fills monthly.
• Reject results that depend on unrealistically perfect execution.

This tutorial was prepared with AI-assisted research and writing under The Money Man editorial workflow. It is educational information, not personalised investment advice. Figures are hypothetical and costs vary materially across instruments and venues.