Averaging down is an investment strategy where an investor purchases additional shares of a stock at a lower price than their original purchase. This reduces the weighted average cost basis per share. For example, if you bought 10 shares at $100 and later 20 shares at $80, your average cost drops to $86.67 per share. This strategy can improve your break-even point but also increases your total exposure to the stock.

The weighted average cost basis is calculated by dividing the total amount invested by the total number of shares owned. The formula is:

Average Cost = (P₁ × S₁ + P₂ × S₂ + ... + Pₙ × Sₙ) / (S₁ + S₂ + ... + Sₙ)

Where P is the purchase price and S is the number of shares for each batch. This gives more weight to larger purchases, accurately reflecting your true cost basis.

While averaging down lowers your average cost, it also increases your total investment in a stock that may continue to decline. Key risks include:

• Throwing good money after bad — investing more in a losing position
• Concentration risk — over-allocating to a single stock
• Opportunity cost — capital tied up that could be deployed elsewhere
• Emotional bias — reluctance to accept a losing investment

Always evaluate the company's fundamentals before averaging down, rather than acting solely on price movements.

Cost basis is the original value of an asset for tax purposes, usually the purchase price adjusted for splits, dividends, and return of capital distributions. When you sell shares, your capital gain or loss is calculated as:

Capital Gain/Loss = Sale Price − Cost Basis

An accurate cost basis is essential for correct tax reporting. In the U.S., brokers are required to report cost basis to the IRS on Form 1099-B. Using the average cost method (especially for mutual funds and DRIPs) simplifies record-keeping.

Averaging down may be appropriate when:

• The company's fundamentals remain strong despite a temporary price drop
• The decline is due to market-wide factors, not company-specific problems
• You have additional capital you're willing to risk
• The stock is undervalued based on your analysis
• You're investing for the long term and can tolerate short-term volatility

It's generally not advisable to average down when the company faces deteriorating fundamentals, mounting debt, or industry headwinds.

Using this calculator is straightforward:

1. Enter your purchase batches — add each buy transaction with its price and number of shares
2. Add more batches by clicking the "+ Add Batch" button for multiple purchases
3. Optionally enter the current market price to see your unrealized profit or loss
4. View your total shares, total investment, and average cost basis instantly
5. Check the batch summary table to see the weight of each purchase

The calculator updates automatically as you type, giving you real-time feedback for investment decisions.