Tax-Loss Harvesting: When It Works, When It Doesn't, and How to Do It Right

TL;DR

• Tax-loss harvesting sells a losing position, realizes the loss for tax purposes, and immediately reinvests in a similar (not identical) fund — maintaining market exposure while creating a tax deduction.
• The benefit is deferral, not elimination. The deferred tax liability follows the replacement fund at a lower cost basis. But deferring $7,000 in taxes at 7% growth for 20 years is worth $27,100 — the time value is real.
• The wash sale rule (IRC §1091) disallows the loss if you repurchase the same or substantially identical security within 30 days before or after the sale. Swap to a different-but-similar fund (VTI → ITOT) and hold 31 days before switching back.

What tax-loss harvesting is — and isn’t

What it is: You hold VTI purchased at $200/share. It drops to $170. You sell, realizing a $30/share capital loss. Immediately, you buy ITOT (iShares Total Market — similar but not identical to VTI) to maintain your market exposure. You are not out of the market for a single day.

The loss offsets any capital gains you have this year, dollar-for-dollar. If you have no capital gains, up to $3,000 of net losses can offset ordinary income (IRC §1211). Any remaining loss carries forward to future years indefinitely (IRC §1212).

What it is not: It is not market timing. You do not leave equities. You do not bet on the position falling further. You simply swap into a comparable fund and continue holding.

The time-value math

At a 35% marginal rate, $20,000 in harvested losses generates $7,000 in deferred taxes today. That $7,000 stays invested rather than leaving to pay taxes.

What $7,000 of deferred tax grows to at 7% annual return:

The deferred liability is eventually owed — when you sell the replacement fund, your cost basis is lower, creating a larger taxable gain. But you earned compounding returns on the deferred amount for years or decades. The longer the deferral, the greater the net benefit.

Step-up in basis at death eliminates the deferred liability entirely. For investors who plan to hold long-term or pass appreciated assets to heirs, the deferral can become permanent tax elimination.

The wash sale rule: the most common mistake (IRC §1091)

You cannot buy back the same or substantially identical security within 30 calendar days before or after the sale at a loss. Doing so disallows the loss — it does not disappear, but it gets added to the cost basis of the replacement security instead of benefiting you now.

What counts as substantially identical:

• The exact same ETF (sold VTI, bought VTI within 30 days)
• Options or contracts to buy the same security
• Two ETFs tracking the exact same index (VOO and SPY both track S&P 500 — potentially identical)

What is generally considered safe:

• VTI (Vanguard Total Market) → ITOT (iShares Core Total Market)
• VXUS (Vanguard Total International) → IXUS (iShares Core Total International)
• VB (Vanguard Small Cap) → IJR (iShares Core Small Cap)

The IRS has not issued definitive guidance on when two index-tracking ETFs are “substantially identical.” Conservative practice: use funds tracking different indexes, not just different providers tracking the same index.

The 30-day rule runs both directions. If you bought VTI 20 days ago and now want to sell at a loss — you cannot buy VTI back for 30 days after the sale either. The window is 30 days before AND 30 days after the sale.

Dividend reinvestment is the hidden trap. If you sell VTI at a loss and your taxable account automatically reinvests VTI dividends 15 days later, that reinvestment purchases trigger the wash sale rule on the reinvested shares. Turn off automatic dividend reinvestment in taxable accounts during active harvesting periods.

The IRA wash sale trap. Selling VTI at a loss in your taxable account and buying VTI in your IRA within 30 days also triggers the wash sale — even though it is a different account. The rule applies across all accounts you hold.

When to harvest — and when not to

Best times to harvest:

• Market down 10%+ and you have meaningful unrealized losses in taxable positions
• Year-end review before December 31 to lock in losses for the current tax year
• Before a large capital gain event (property sale, RSU vesting, business sale) — pre-harvested losses offset those gains
• After receiving a large capital gain distribution from a mutual fund (typically November–December)

Skip harvesting when:

• You are in the 0% capital gains bracket — losses have minimal current value when LTCG are already untaxed
• The loss is very small relative to transaction costs and complexity
• You cannot identify a suitable replacement fund to maintain exposure
• You are within 30 days of a prior purchase of the same security
• The position is held at a long-term gain — selling resets the holding period clock (new position is short-term for 12 months)

Short-term vs long-term losses: order matters

The IRS applies losses in a specific sequence:

1. Short-term losses offset short-term gains first (both taxed at ordinary rates)
2. Long-term losses offset long-term gains first (both at LTCG rates)
3. Net short-term losses can then offset long-term gains (and vice versa)

Prioritize harvesting short-term losses (held under 1 year). A short-term loss offsets a short-term gain at your 35–37% ordinary rate — far more valuable than a long-term loss offsetting a long-term gain at 15–20%.

Use the calculator

The Tax-Loss Harvesting Savings Calculator models:

• Immediate tax savings from harvesting at your rate
• Future extra tax owed when you eventually sell the replacement fund
• Net present value of the deferral over your holding period
• Deferral benefit table across 1, 3, 5, 10, 20, and 30-year horizons
• Wash sale rule warning banner
• Signal: whether harvesting is worth doing at your specific numbers

What most content gets wrong

“Tax-loss harvesting saves you money.” It defers money — it does not permanently save it (unless you hold until death). The future tax liability is embedded in the replacement fund’s lower cost basis. Never harvest without understanding you are trading a current tax bill for a future one.

“Robo-advisors are best for TLH.” Robo-advisor TLH is automatic but often small — triggered by minor daily fluctuations. For most investors, the biggest harvest opportunities come from genuine market drawdowns of 10%+. Manual review at those moments often captures far more value than daily automated monitoring of small losses.

“You can’t be penalized for the wash sale — you just lose the deduction.” True for federal taxes. Some states (like California) disallow the wash sale loss permanently if the replacement security is sold outside the state in a subsequent year. State treatment varies.

Decision checklist

• Do you have unrealized losses in taxable positions?
• What is your marginal rate — is the tax benefit material?
• Have you identified a suitable replacement fund that is similar but not substantially identical?
• Is dividend reinvestment turned off in the taxable account during the 30-day window?
• Are you tracking wash sale exposure across all accounts (taxable + IRA)?
• Do you have capital gains to offset this year — or will the loss carry forward?
• Have you run the Tax-Loss Harvesting Calculator with your actual numbers?

When to call a CPA

• You have large carryforward losses and want to model an optimal multi-year drawdown strategy
• You are considering a large capital gain realization (property sale, business sale) and want to pre-harvest losses to offset it
• You own individual securities with complex lot accounting (multiple purchases at different prices)
• Your state has different wash sale treatment than federal

Sources

• IRC §1091 — Wash sale rule
• IRC §1211 — Capital loss limitation ($3,000 ordinary income offset per year)
• IRC §1212 — Capital loss carryforward (indefinite)
• IRS Publication 550 — Investment Income and Expenses
• IRS Rev. Rul. 2008-5 — IRA wash sale treatment across accounts