Stock Return Calculator — Average Cost

What it is

After you sell a stock, commissions and taxes make the real result differ from the price chart. This stock return calculator takes the buy price, sell price and quantity, and the commission and tax rates that you type in, and returns the net profit, the return, the average cost including commission and the break-even sell price. The average cost mode lets you enter several purchases to find your averaging-down cost, and a current price adds the unrealized profit. Everything is calculated in your browser.

How to use

1. In Return mode, enter the buy price, sell price and quantity.
2. Pick the market to fill in the sale tax rate, and add your buy and sell commission rates from your broker statement.
3. Read the net profit, return, average cost and break-even sell price.
4. In Average cost mode, enter the price and shares of each purchase, plus a commission rate and current price if you like.
5. Use the add button to enter up to 10 purchases.

How it works

• Total cost = buy amount + buy commission, where commission is buy amount × rate.
• Net proceeds = sell amount − sell commission − transaction tax and special tax. KOSPI is 0.20% (0.05% + 0.15%), KOSDAQ 0.20% and KONEX 0.10%, effective 2026-01-01.
• Net profit = net proceeds − total cost, and return = net profit ÷ total cost.
• Average cost = total cost ÷ quantity, rounded to the smallest unit.
• Break-even sell price = total cost ÷ (quantity × (1 − sell commission rate − tax rate)).
• Averaging-down cost = sum of all purchase amounts ÷ total shares, with a commission-inclusive figure when you enter a rate.
• The broker commission rate is a value you type. Major-shareholder capital gains tax, dividends and exchange rates are not included.
• Amounts are computed as whole numbers in the smallest unit.

Examples

Buying at 70,000 and selling 10 shares at 80,000 on KOSPI. The 0.015% commission is a sample value for the illustration only, and the tax is the statutory KOSPI 0.2%.

Item	Amount
Total cost	700,105
Net proceeds	798,280
Sale tax 0.2%	1,600
Net profit	98,175
Return	14.02%

For averaging down, buying 10 shares at 10,000 and 10 at 5,000 gives an average cost of 7,500. With a current price of 9,000 the unrealized profit is +30,000, a 20% return.

Common mistakes

Looking only at the sell price and ignoring commission and tax makes the return look higher than it is. Computing profit from an average cost that leaves out commission is another common slip. The average after averaging down must be weighted by shares, not a simple average of prices. The sale tax is the 2026 statutory rate and may change yearly, so check that it matches your trade date, and choose manual entry for foreign stocks. This is not investment advice.