How it works
Cost = buy amount × (1 + buy fee). Proceeds = sell amount × (1 − sell fee − tax). Net profit = proceeds − cost, and return = net profit ÷ cost.
Break-even price
Break-even sell price = buy price × (1 + buy fee) ÷ (1 − sell fee − tax). You need to sell above it to make money after costs.
About the defaults
The 0.015% fee and 0.20% tax are examples based on typical Korean brokerage accounts. Transaction tax rates change by market and year, so use your actual rates.
Percentage return versus money
A 10% return on 1,000,000 is 100,000; the same 10% on 10,000,000 is a million. Enter the share count so you see the money as well as the percentage, otherwise a large return on a tiny position reads as more than it is. If you bought in several lots, get your average cost from the average-down calculator first and use it as the buy price here.
Where the tax applies
Korean stocks are taxed on sale through a securities transaction tax, and dividends are taxed separately when paid. Foreign stocks are different: gains are added up over the calendar year and declared the following May. Rates and thresholds change, so confirm the current ones for your own situation.