FIFO Stock Sales vs. Specific Identification: Which Tax Lots Should You Sell?

The highest-basis lot is not always the cheapest lot to sell. Suppose you own two 100-share lots of the same stock. You bought the older lot at $40 per share and the newer lot at $70. The stock is now $100. Selling the $70 lot creates only a $3,000 gain, versus $6,000 on the $40 lot. Now assume the old lot is long-term at 15% and the new lot is short-term at 32%. The old lot produces about $900 of simplified current federal tax, while the newer lot produces about $960. The gain is smaller, but the modeled tax is higher.

How FIFO and specific identification work

First-in, first-out (FIFO) and specific identification do not use different tax formulas. They decide which shares count as the shares you sold when you own several lots of the same stock.

With specific identification, you choose the eligible shares you want treated as sold. For broker-held stock, the federal timing rule uses the earlier of two cutoffs: the settlement date or the settlement time required by the applicable securities rule. A standing lot instruction can also qualify. Keep the broker’s written confirmation; the required record may be electronic.

If you do not identify the shares under these rules, the oldest shares you own supply the basis and holding period unless another permitted basis method applies. That is the FIFO fallback discussed here. For ordinary FIFO stock sales, the oldest eligible shares are treated as sold first. Mutual funds and some dividend-reinvestment-plan shares can use average basis, so this guide stays with ordinary stock lots where FIFO and specific identification are the relevant choices.

Before comparing lots, make sure the inputs are real. Use adjusted basis, not just the original purchase price, and confirm the holding period and the number of shares available in each lot. Stock splits, spin-offs, return-of-capital adjustments, wash sales, and transferred positions can affect or complicate basis records. If a lot looks incomplete or unusual, reconcile it before using that number in the comparison.

Why the highest-basis lot can lose

Go back to the two 100-share lots from the opening. Both are sold at $100 per share. The older lot has a $40 adjusted basis and is long-term. The newer lot has a $70 adjusted basis and is short-term.

The higher-basis lot looks better if you stop at the gain. It has only half as much realized gain. But the rate difference reverses the answer: in this example the lower-basis long-term lot is cheaper by $60. That is why “always sell the highest-basis lot” is not a reliable rule.

Try the same comparison with your own assumptions. The calculator below uses the same positive-gain current-sale model as the example.

How to use it: enter the modeled federal rate that applies to each lot after you have determined its holding period and your relevant tax assumptions. The calculator compares the two lots; it does not determine your tax bracket for you.

Where the answer flips

So where does the newer, higher-basis lot finally become the cheaper one to sell? Keep the example simple: use equal share quantities, assume each share is sold for $100, use a $40 long-term basis, and apply assumed rates of 15% and 32%. Compare the gain on each lot after applying its assumed rate.

The same-rate case is simpler. If both compared lots use the same modeled marginal rate and the same sale quantity, the higher adjusted basis produces the lower simplified current-tax amount. With both rates set to 20% in the example above, break-even occurs when the two lots have the same basis.

This comparison ranks the current sale only. Selling one lot leaves a different basis and holding-period mix in the shares you keep, so lower tax today can still lead to a different lifetime tax result.

When this comparison stops working

The break-even model is useful because it isolates one question: basis versus holding-period rate on positive-gain lots. Stop using it as the answer when other tax rules change the problem.

• Loss lots and wash sales: a realized loss can be disallowed or deferred when replacement shares trigger the wash-sale rules, and the replacement shares’ basis can change.
• Annual gain-and-loss netting: short- and long-term gains and losses interact on the annual return, so a single-lot calculation is not your final federal tax bill.
• Rate crossings, net investment income tax (NIIT), and state or local tax: the example uses fixed federal rates. Your actual marginal result can differ.
• Different sale quantities or portfolio goals: normalize the quantities before comparing the math, and do not let a tax shortcut override diversification, cash needs, or risk constraints.

If a loss or wash-sale issue drives the trade, use the tax-loss harvesting guide instead of forcing this model to answer it. For the broader gain, basis, and rate sequence, use the capital gains tax guide.

Can your broker actually identify the lot?

A tax-preferred lot is useless if the broker does not record the instruction correctly. Check the current lot-selection screen, confirm the basis and share count, and make the specific-lot identification by the applicable settlement deadline. Then keep the broker’s written or electronic confirmation.

Do not assume you can repair the lot assignment after the trade has settled. Major brokers generally impose a settlement cutoff for changing or assigning lots, but the exact screen, default method, and workflow vary by platform. A standing instruction may also control the lot selection, so check your account’s current default before you trade.

For broader account setup outside this lot-choice task, use the brokerage account settings guide.

Which tax lot should you sell?

The best lot is not automatically the oldest lot or the highest-basis lot. Before you sell, make sure four things are true:

1. You know which lots and share quantities are actually available.
2. The adjusted basis and holding period for those lots are correct.
3. Losses, wash sales, annual netting, or other tax constraints do not invalidate the simple comparison.
4. Your broker can identify and confirm the lot you chose within its current workflow.

Then compare basis and holding period together. Use specific identification when a chosen lot better fits the result you want and you can document it. Use FIFO when the oldest eligible shares already fit.

Before you place the trade, verify the lot, the tax constraint that matters, and the broker’s current execution steps.

Educational quantitative analysis based on published data. Not investment, tax, or legal advice. Consult a licensed professional before acting on any calculation. About TheFinSense.