A/B comparison: consider average cost and additional investment together
Assume you hold 10 shares at KRW 70,000 and buy more at KRW 60,000, with no fees.
Buying 10 more shares in A requires KRW 600,000 and gives 20 shares at an average cost of KRW 65,000. Pin A, then change the additional quantity to 20 shares. B requires KRW 1,200,000 and gives 30 shares at an average cost of about KRW 63,333.33.
B has a lower average cost but requires KRW 600,000 more. At a current price of KRW 60,000, both scenarios have an unrealized loss of KRW 100,000. A lower average cost does not erase a loss; further price declines increase losses more when you hold more shares.
How average cost is calculated
The average cost of shares bought in multiple purchases is total purchase cost divided by total quantity(a quantity-weighted average). Instead of averaging prices alone, each purchase is weighted by its Quantityquantity.
Use Existing holdings to enter the average cost and quantity from previous purchases in one row. If you have no holdings, leave it empty and enter your purchases to calculate an average cost for new purchases.
Example
Holding 10 shares at KRW 70,000, then buying 10 more at KRW 60,000:
| Category | Unit price | Quantity | Amount |
|---|---|---|---|
| Existing holdings | 70,000 | 10 | 700,000 |
| Additional purchase | 60,000 | 10 | 600,000 |
| Total | — | 20 | 1,300,000 |
Average cost = 1,300,000 ÷ 20 = 65,000 KRW
Why results may differ from your broker
- Fees / transaction tax: Fees default to zero. Purchase fees increase acquisition cost and average cost. Brokerage apps may differ in fee treatment and rounding.
- Fractional shares: Brokers use different rounding and settlement rules for fractional overseas shares, so displayed averages may differ.
- Exchange rates for overseas stocks: Average cost in KRW depends on exchange rates and conversion methods at purchase. This calculator averages entered prices and does not apply exchange rates.
- Taxes: This tool calculates purchase-based average cost only. Selling capital gains and transaction taxes are not included.
Basis, data and updates
This calculator performs arithmetic only on your inputs. It does not automatically fetch prices, so identical inputs always produce identical results regardless of live prices. Formula and wording changes are listed below.
- 2026-10-06 · Added an A/B example, interpretation and limitations.
- 2026-08-26 · Initial release: weighted average cost, optional fees and unrealized profit/loss.
- 2026-08-27 · Added reset, URL sharing, applying results to holdings and average-cost visualization. Expanded crypto, US stock and fractional quantity guidance.
- 2026-08-27 · Added currency and quantity display units for crypto and US stocks, and amount-based purchases (amount ÷ price = quantity).
- 2026-09-05 · Added Korean stock search with previous closing prices from FSC data for profit/loss and average-cost comparisons. Added transfers between calculators.
- 2026-09-14 · Added average-cost comparison bars on a shared scale, separate from return comparisons.
- 2026-09-14 · Added a result explanation using total cost ÷ total quantity = average cost. Formulas unchanged.
Frequently asked questions
- Is averaging down different from calculating average cost?
- The formula is the same. Enter your current average cost and quantity plus new purchases to model an additional purchase, or leave existing holdings empty to calculate an average from purchase records.
- Can I use this for US stocks, crypto and ETFs?
- Yes. The formula uses only price and quantity, so it works for domestic/overseas stocks, ETFs and crypto, including fractional quantities. Use one currency per calculation; exchange rates and KRW conversion are not included.
- Can I enter an investment amount instead of a quantity?
- Yes. Change the additional purchase mode to By amountto enter an amount for each purchase. Quantity is amount ÷ price, including fractions for crypto and US stocks. The tool does not calculate purchases needed to reach a target average or recommend trades.
- What are the display units (KRW/USD, shares/units)?
- Labels use KRW/shares for Korean stocks, USD/shares for US stocks, and units for crypto. The formula is unchanged and no currency conversion is performed. Use one currency per calculation.
- Do I have to enter fees?
- No. Fees default to zero for a price-only average. Enter your buying fee rate for a closer estimate of acquisition cost.
- Does this tell me how much to buy to break even?
- No. This tool calculates average cost from your inputs. It does not suggest purchase quantities or prices, or recommend trades.
- Are selling taxes included?
- No. Average cost is based on purchases. Selling transaction tax and capital gains tax on overseas stocks are separate.