The Mechanics of Realising a Loss
The Mechanics of Realising a Loss
Realizing a loss is mechanically identical to any other securities sale, but the paperwork and tracking afterward are what separate a correct harvest from a tax audit risk. When you sell a security at a loss, you are converting a paper loss (unrealized) into a permanent tax loss (realized) that the IRS will recognize. The broker reports this to both you and the IRS on Form 8949 (Sales of Capital Assets) and Schedule D of your tax return. Getting the numbers right matters.
Key takeaways
- Cost basis is the foundation: you must track your original purchase price (plus fees) to compute the loss accurately.
- Most brokers use "average cost" by default, which can reduce available losses; "specific identification" lets you harvest the most recent (and often highest-cost) shares.
- Capital losses are reported to the IRS automatically; misreported losses trigger audits and penalties.
- Different account types have different tax treatment of losses: taxable accounts generate harvestable losses, tax-deferred accounts do not.
- Wash-sale violations and same-account shenanigans can disallow your loss, so meticulous record-keeping and a 30-day buffer are non-negotiable.
What cost basis means and why it matters
Cost basis is the total amount you paid to acquire a security, including the share price and any commissions or fees. If you bought 100 shares of VTI (Vanguard Total Stock Market ETF) at $220 per share plus a $10 commission, your total cost basis is $22,010, or $220.10 per share. If you sell those shares at $200 per share, your realized loss is $2,010 (100 shares × $20 per share loss, plus the commission is already baked into the basis).
The IRS requires you to report this loss accurately. Understating cost basis inflates your loss, which is tax fraud. Overstating it shrinks the loss you can claim, which is merely leaving money on the table. Brokers are required to track and report cost basis to the IRS, so the IRS already knows your numbers. If your tax return shows a different loss, the discrepancy is a red flag.
Identifying which shares you are selling
When you own multiple lots of the same security (purchased at different times and prices), you must specify which lot you are selling. Most brokers default to "average cost," which blends all purchases into a single per-share price. This often reduces the loss available to harvest because it averages your high-cost and low-cost purchases together.
Suppose you bought VTI on three dates:
- Lot A: 50 shares at $200 (cost basis $10,000)
- Lot B: 50 shares at $215 (cost basis $10,750)
- Lot C: 50 shares at $190 (cost basis $9,500)
The average cost is $201.67 per share. If you sell 50 shares at $200 today and use average cost, you realize a loss of $1.67 × 50 = $83.50. Underwhelming.
But if you specify that you are selling Lot B (the highest-cost shares at $215 per share), your loss is $15 × 50 = $750. Much better. The IRS allows "specific identification," which requires that you tell your broker in writing which lot you want to sell. Many brokers have online tools to select lots at sale time, but you must do it explicitly and confirm it in writing.
Account type and loss harvesting eligibility
A capital loss is only harvestable if it occurs in a taxable (non-retirement) account. Here's the breakdown:
Taxable brokerage account: Capital losses are fully realizable. You can harvest them.
Tax-deferred accounts (401(k), IRA, SEP-IRA, Solo 401(k)): Capital losses cannot be harvested. The account is not taxable during your working years, so losses are meaningless to the IRS. You cannot claim the loss on your tax return now, and if you do, the IRS will disallow it.
Tax-exempt accounts (Roth IRA, HSA): Capital losses are also non-harvestable. The account generates no taxable income or gains, so losses have no offsetting function.
This distinction is crucial. Many investors mistakenly believe they can harvest losses from a Roth IRA; they cannot. If you want to use tax-loss harvesting, you must do it in a taxable account.
Executing the sale and confirming the loss
When you place a sell order, the broker executes it at the current market price. The transaction is recorded in your account statement within one business day. You do not need to do anything special to "lock in" the loss; the sale itself is sufficient.
However, you should immediately verify the cost basis and loss on your statement. Errors happen. If your broker computed the loss incorrectly, contact them to correct it before year-end. This is especially important if you used specific identification; if your broker used the wrong lot, go back and fix the record.
Record retention and IRS reporting
The IRS receives a copy of your capital loss from your broker on Form 8949. Your tax preparer (or tax software) will transcribe this into Schedule D of your Form 1040. If you claim a loss on your tax return that differs from what your broker reported, the IRS computer system will eventually notice and send you a correspondence notice asking for clarification. If the discrepancy is large or repeated, it may trigger an audit.
You should retain all brokerage statements, trade confirmations, and cost basis documentation for at least three years (seven is safer for capital transactions). Digital records are fine; most brokers allow you to download or export your statements as PDFs.
Process flow for loss realization
Related concepts
Next
Once you have realized a loss, the critical rule you must obey is the wash-sale rule. Repurchasing the same security too soon can disallow the loss entirely, negating all the paperwork. We examine this rule next and learn how to navigate it.