Average Cost Calculator (averaging down / up)

Understanding the average cost calculator — concept and principle

When you buy more of a stock, fund or coin you already own, your average cost per share changes. Buying below your current average is called averaging down; buying above it, averaging up. This calculator gives the new average cost, the total invested, today's market value, the unrealized profit or loss, the current return and the rise needed to break even, so you can see the effect of an additional purchase before you make it.

How it is calculated

• Total shares Q = shares held + shares to buy
• Total invested = shares held × current average + shares to buy × buy price
• New average cost = total invested ÷ Q
• Market value = Q × current price; unrealized P/L = Q × (current price − new average)
• Return now = current price ÷ new average − 1
• Break-even rise = new average ÷ current price − 1 (0 if already in profit)

Fees and taxes are not included. The calculator works with any currency.

The principle

The average cost is a weighted average: each purchase counts in proportion to how many shares it added. That is why buying 50 shares cheaply moves the average less than you might hope when you already own 100. The break-even figure is larger than the loss percentage because gains and losses are measured from different bases: after a 14.29% fall, the price must rise 16.67% to recover, since the rise is measured from the lower price.

Worked example

Default: 100 shares at $50 average, buy 50 more at $40, current price $40.

1. Total shares: 100 + 50 = 150.
2. Total invested: 100 × 50 + 50 × 40 = 5,000 + 2,000 = $7,000.00.
3. New average: 7,000 ÷ 150 = $46.67 (averaging down).
4. Market value: 150 × 40 = $6,000.00; unrealized P/L: −$1,000.00.
5. Return now: 40 ÷ 46.667 − 1 ≈ −14.29%; break-even rise: 46.667 ÷ 40 − 1 ≈ 16.67%.

How to read the result

Averaging down lowers the price you need to break even, but it also puts more money into the same position, so a further fall hurts more. A useful question is whether you would buy this asset at today's price if you did not already own it. Keep position size in mind: a single holding that grows to a large share of your portfolio adds concentration risk.

Common mistakes

• Averaging the prices instead of weighting by shares (here (50 + 40) ÷ 2 = 45 is wrong).
• Forgetting commissions and spreads, which raise the real cost basis.
• Assuming the broker or tax authority uses the same average; tax rules may track each lot separately.

Frequently asked questions

What is my new average if I buy 50 shares at $40 on top of 100 at $50?

$46.67 per share.

How many shares do I need to buy to reach a target average?

Shares = held × (current average − target) ÷ (target − buy price), when the buy price is below the target.

Why is the break-even rise bigger than my loss?

Because the rise is measured from the lower current price, not from your average.

Sources: CFA Institute, investment foundations: weighted average cost and holding period return