An online broker might charge a $10 commission to execute a stock trade. Your cost to buy 100 shares of a $50 stock would be $5,000 plus $10.

Trade commissions reduce before-tax gain

The commission would amount to only 0.2% of the principal invested. A few months later, you sell the stock for $60. As before, you would incur a $10 commission.

After subtracting the commission costs, you net $980 before taxes. The commission has reduced your net gain by a full 2 percent.

Short-term capital gains tax far exceeds commission cost

A short-term gain brings on a tax that lowers its return by 35 percent if you are subject to the top marginal tax rate. The capital gains tax on a $980 short-term gain would total $343.

This tax cost is 17 times greater than the round-trip commissions cost.

Long-term tax rate still takes a big bite

Had you held the stock more than one year before selling, the capital gain would be long-term. Such a long-term gain is taxed at only 15 percent.

For a $980 long-term gain, the capital gains tax would total $147. Waiting to realize a long-term capital gain results in a tax cost more than 7 times that of the trade commissions.

Delay capital gains tax by delaying a stock sale

A trivial way to delay the tax cost is to delay selling the stock. While the gain remains unrealized, it incurs no costs. Unless your asset allocation plan dictates that you dispose of the position, holding an unrealized gain is the best method for lowering your investment costs in the current year.

A dollar today is worth more than a dollar next year. Postponing the tax leaves you with more cash to invest now and discounts the tax cost in a future year.

Avoid the tax by offsetting the capital gain with a capital loss

Actually, capital gains tax is not calculated individually for each stock sale. If you must sell a stock with a short-term capital gain, you might look for another holding that has an unrealized short-term capital loss.

Selling another stock with a $1,000 short-term loss will more than offset the $980 short-term gain. This would leave you with zero net capital gains tax.

Rather than waiting until you have a gain that you want to realize, you may want to employ tax loss harvesting to recognize those losses as they arise. No investor sets out to lose money after buying a stock. Those who hold onto those losses when they could use them to offset gains miss a certain way to lower their tax.

In the preceding post we noted the risk of an unrealized capital loss becoming long-term due to a wash sale. Tax loss harvesting also creates a situation constrained by the wash sale rule.

Tax loss harvesting

As in the preceding post, suppose you bought 100 shares of BP (BP plc ADR) at $50 per share on June 15, 2009. Less than a month ago on May 26, 2010, BP stock was at $43 per share. You had an unrealized short-term capital loss of about $700.

Rather than buying more BP stock, you sold 100 shares for $4,300. This action is also known as harvesting a tax loss. This loss would offset any short-term capital gains in your taxable portfolios.

Two weeks later on June 9, BP closed at $29.20. To rebalance your asset allocation, you were anxious to restore your holding in BP. On June 10 you bought 100 shares for $3,100. This purchase triggered a wash sale and negated the whole purpose of harvesting the tax loss.

Wash sale effect on cost basis

The short-term loss that you recognized on May 26 would be disallowed as a wash sale, because you bought substantially identical stock within 30 days after the sale.

To account for the wash sale, you must add the disallowed loss to the cost basis of the purchase that triggered the wash sale. In this case, the tax lot for the June 10 purchase had its cost basis raised by $700 to $3,800. You also change the beginning of the holding period of the tax lot to the purchase date of the stock sold.

The tax lot now has an unrealized loss of $700, and its holding period began on June 15, 2009. On June 16, this loss became long-term.

Tax loss harvesting starts a 30-day wash sale window

Recognizing a capital loss in a taxable portfolio sets up the potential for a wash sale during the next 30 days. To avoid a wash sale that would disallow the prior capital loss, you have two choices.

You can wait 31 days after the loss before buying replacement stock. While simple, this choice has a major drawback. During the period when the portfolio lacks the holding that had been sold, your asset allocation may differ from your chosen allocation.

Buy and hold

You have decided to buy and hold a portfolio of 10 stocks in a $100,000 portfolio. After four months, the total value of the portfolio has risen to $103,000. One stock, Bank of America (BAC), has dropped 30% to just $7,000, but most of the other holdings have gains.

Given your belief that BAC will bounce back from its loss, you hold onto it. Six months later, the portfolio value is $110,000. The BAC has recovered all of its loss and has a value of $11,000.

Replace a losing stock with a similar stock

Rather than holding onto the BAC after it had dropped, assume that you sold it to realize a $3,000 short-term loss. With the proceeds from the sale, you purchased $7,000 of Wells Fargo (WFC). Replacing the BAC with a similar financial stock helped to maintain the portfolio diversification.

As before, the portfolio value rose to $110,000 six months later. At that point, the WFC position had also risen to a value of $11,000.

Tax loss harvesting

Capital gains tax works in reverse for a capital loss: for the benefit of the taxpayer. You can exclude from your taxable income up to $3,000 in capital losses. If you are subject to the top 35% marginal tax rate, this exclusion lowers your tax by $1,050.

By proactively selling a stock and harvesting a loss, you gained a tax savings. The simple buy and hold approach left $1,050 on the table. If you chose to invest the savings, tax harvesting would have raised your portfolio value to $112,050.

Almost one year ago, on September 24, 2008, an investor bought 200 shares of stock at a price of $60. Today, the latest quote for the stock is $45. This drop of $15 gives her an unrealized short-term loss of $3,000.

Tax loss selling at year-end

Many investors review their portfolios before the end of each year. Because capital losses offset gains, some sell stock at that time to realize losses for the current tax year. In fact, some academic studies credit this practice of selling for part of the so-called January effect.

She may hope that between today and the end of the year, the stock price may rise above $45. Then again, it may not. What is certain is that 10 days from now (September 25, 2009) the basis of the stock will become long-term. If she sells the stock for a loss in December, she will realize a long-term capital loss for the 2009 tax year.

Tax loss harvesting year-round

An earlier post showed the tax savings advantage of a short-term loss relative to a long-term loss. By waiting until December to sell, she will have missed the chance in September to realize a short-term capital loss.

Investors purchase stock at all times of the year. This means that the holding period for stock purchased within the last twelve months will also change from short-term to long-term throughout the year.

By making a habit of taking capital losses in December, an investor risks having a number of long-term capital losses. Taking those losses earlier in the year will help avoid that mistake.