Cost basis vs market value: why both matter

When you log into your brokerage app, you are instantly greeted by the "Green Number"-your total return and current market value. Right next to it, usually buried in a secondary tab, is your total cost basis.

Many self-directed investors stare at their all-time P&L and mix these two numbers up. Worse, they use their cost basis as a "break-even" line to judge if they are a good investor.

If you want an accurate picture of your wealth, you need to stop treating your brokerage dashboard like a casino scoreboard. These numbers represent two entirely different things. Market value is for your net worth (and your ego); cost basis is strictly for the tax authorities. The tax man does not care about your feelings or your all-time high-he only cares about your tax lots. Knowing how these numbers interact gives you complete control over your portfolio.

The Two Scorecards: Tax vs. Net Worth

What is market value?

Market value is exactly what your shares are worth if you liquidated them today. You calculate it simply by multiplying the number of shares you own by the current share price on the stock exchange.

This number goes up and down every second the market is open. It tells you your current exposure and dictates your total net worth. When you are calculating your retirement timeline or measuring your financial flexibility, market value is the scorecard you use.

What is cost basis?

Cost basis is a historical record. It represents the original value of your investment, plus the specific incidental costs associated with buying it (such as brokerage fees and trading commissions).

Unlike market value, cost basis does not change with daily market movements. It only changes if you buy more shares, sell some shares, or if a corporate action (like a stock split or a return of capital) directly affects your holdings. Keeping accurate records of your cost basis is a strict requirement from tax authorities globally. Without these records, you cannot calculate your capital gains accurately when you eventually sell.

Why cost basis does not equal performance

If you spend any time on Bogleheads or financial independence forums, you will inevitably see someone post a screenshot of their portfolio panicking because their "Total Cost Basis" is almost identical to their "Market Value." They think they haven't made any money in five years.

This happens because they assume their cost basis is simply the cash they transferred from their bank account. It isn't. Cost basis does not equal performance.

Your cost basis drifts away from your actual cash deposits whenever automatic actions occur in your account. The biggest culprit is the DRIP (Dividend Reinvestment Plan).

When you set your ETFs to automatically reinvest dividends, the brokerage uses that cash to buy more shares. To the tax authorities, every single one of those automated purchases is a brand new transaction, which adds to your total cost basis.

Here is how the math tricks you: Imagine you deposit $10,000 of your own cash. Over five years, the shares pay out $3,000 in dividends. Because you have DRIP turned on, the brokerage automatically uses that $3,000 to buy more shares. Your "Total Cost Basis" on your dashboard increases from $10k to $13,000.

If your Market Value today is $15,000, you might look at your dashboard and see a $15k value against a $13k cost basis. You look at that $2,000 gap (your unrealized gain) and think you only made two grand in five years. You forget that you only ever deposited $10k of your own cash out-of-pocket, meaning your actual return is $5,000.

You aren't a terrible investor; your cost basis was just inflated by years of dividend reinvestments. This is why you should never use cost basis to calculate your overall performance.

How unrealized gains connect the two numbers

The mathematical difference between your market value and your cost basis represents your unrealized capital gains or losses.

An unrealized gain simply means your investment is worth more today than its historical cost basis. However, this gain is only on paper. It does not become actual cash, and it does not trigger a taxable event, until you hit the sell button.

When you do sell, capital gains tax (CGT) applies. You calculate your actual capital gain by subtracting your cost basis from your capital proceeds (the money you receive from the sale minus any selling costs). Because every single purchase-including every automated dividend reinvestment-creates a separate "tax lot" with its own cost basis, you can often choose exactly which shares to sell to optimize your tax outcome.

Tracking market value and tax lots in WealthScout

Spreadsheets often fail to capture the complexity of a growing portfolio over many years. When you rely on a single total line item to represent your investments, you lose the detailed tax lot history required for accurate reporting.

WealthScout is designed to track both scorecards simultaneously.

When you import your trade history, WealthScout builds each holding from its individual dated trades. This means your cost basis stays perfectly connected to the original purchase, accounting for all brokerage fees and distinct tax lots.

The household view in WealthScout uses the current market value to show your total wealth today. At the same time, the parcel history retains the exact cost basis for every share you own. When you plan to sell, you can view the cost basis of each tax lot and understand the potential tax impact before you execute the trade. This approach separates your current wealth from your investment history, ensuring you never confuse your taxes with your performance.