Tax-loss harvesting is a strategy that lets you convert paper losses in your investment portfolio into real tax savings — without fundamentally changing your long-term investment approach. Done correctly, it can save investors hundreds or even thousands of dollars per year.
The Core Idea
When a taxable investment is worth less than you paid for it, you have an unrealized loss. Tax-loss harvesting means selling that investment to realize the loss on paper, then using that loss to offset capital gains elsewhere in your portfolio. If your losses exceed your gains, you can even deduct up to $3,000 of net losses against ordinary income each year, with any remainder carrying forward to future years.
How the Math Works
Suppose you have a $10,000 long-term capital gain from selling appreciated stock. Without harvesting, if you are in the 15% capital gains bracket, you owe $1,500 in federal tax.
If you also hold a different investment sitting at an $8,000 loss, you can sell it to harvest the loss. The $8,000 loss offsets $8,000 of your $10,000 gain, leaving only $2,000 taxable. Your tax drops to $300 — a savings of $1,200.
The Wash-Sale Rule
The IRS has a rule specifically designed to prevent you from harvesting a loss while maintaining identical market exposure: the wash-sale rule.
If you sell a security at a loss and buy a “substantially identical” security within 30 days before or after the sale, the loss is disallowed. The disallowed loss is not gone forever — it is added to the cost basis of the repurchased shares — but you lose the current-year tax benefit.
What counts as substantially identical?
- Buying back the exact same stock or fund
- Purchasing call options on the same stock
- Buying the same mutual fund from a different share class
What does not trigger the wash-sale rule?
- Buying a similar but not identical ETF (e.g., selling one S&P 500 ETF and buying a different provider’s S&P 500 ETF is a gray area — selling a large-cap growth ETF and buying a broad market ETF is generally safer)
- Buying shares in a different company in the same industry
- Waiting 31 days before repurchasing the same security
Carry-Forward Losses
If your net capital losses exceed your gains in a given year, you can deduct up to $3,000 against ordinary income (or $1,500 if married filing separately). Any losses beyond $3,000 carry forward indefinitely to future tax years, where they can offset future gains.
This makes tax-loss harvesting valuable even in years when you have no gains — you are building a bank of losses to deploy in future high-gain years.
When to Harvest Losses
The best opportunities arise when:
- Markets have declined significantly, creating paper losses across your portfolio
- You have realized gains earlier in the year and want to offset them before year-end
- You are approaching year-end and want to do an annual portfolio review
December is the most active month for tax-loss harvesting, but opportunities can arise any time during the year. Do not wait until the last week of December — settlement typically takes two business days, so you need to trade by December 29 to settle by December 31.
Long-Term vs. Short-Term Losses
The IRS matches losses to gains by type first: short-term losses offset short-term gains, and long-term losses offset long-term gains. Any excess then crosses over to offset the other type. Since short-term gains are taxed at higher ordinary income rates, short-term losses are generally more valuable to harvest.
Limitations to Keep in Mind
- Tax-loss harvesting only applies in taxable brokerage accounts — not IRAs or 401(k)s, where gains and losses have no current-year tax impact.
- If you are in the 0% capital gains bracket, harvesting losses provides little immediate benefit since you owe no tax on gains anyway.
- Harvesting a loss reduces your cost basis, meaning you will have a larger gain when you eventually sell the replacement investment — so it is a deferral strategy, not a permanent elimination of tax.
Bottom Line
Tax-loss harvesting is one of the few strategies that can reduce your tax bill without requiring you to stay out of the market for long. The key discipline is maintaining your investment exposure through replacement securities while staying clear of the wash-sale rule. For investors in the 15% or 20% capital gains bracket with taxable investment accounts, this strategy is worth executing systematically every year.