FIFO vs Average Cost: Which Cost Basis Method Is Right for Your Investment Club?

What is cost basis?

Cost basis is the original purchase price of an investment, used to calculate the gain or loss when it's sold. For an investment club that has made multiple purchases of the same stock over time, you need a method to determine which shares you're selling when you exit part of a position.

The main methods

FIFO (First In, First Out)

You sell the oldest shares first. If you bought 100 shares in January and 100 more in June, and then sell 100 shares in October, you're treated as selling the January batch.

FIFO tends to produce larger long-term gains (because older shares have more chance of being held over 12 months) and is the default method for most US brokers if you don't specify otherwise.

Average Cost

All shares of a stock are pooled together. The cost per share used in any sale is the weighted average across all purchases.

Average cost smooths out the effect of buying at different prices. The gain is the same regardless of which batch you claim to be selling — because there's only one effective cost basis.

Specific Identification

You choose exactly which lot to sell at the time of the transaction. This gives maximum flexibility — sell your highest-basis lots to minimise gains, or your lowest-basis lots to maximise losses for tax-loss harvesting.

Specific identification requires lot-level record keeping and the ability to identify the specific shares at the time of sale. Most brokers support it but investment clubs need good software to use it effectively.

LIFO (Last In, First Out)

You sell the most recently purchased shares first. LIFO tends to produce short-term gains (because recent purchases are less likely to have been held over a year). For this reason, LIFO is rarely the right choice for most investment clubs.

The UK answer: you don't choose

For UK clubs, this isn't really a decision. HMRC requires Section 104 pooling — which is functionally equivalent to average cost. All shares of a company are pooled together and the gain is calculated against the weighted average cost.

You can't use FIFO or specific identification for UK CGT purposes. HMRC's matching rules (same-day, 30-day, then pool) are mandatory.

The US answer: it depends

For US clubs, the most common choices are FIFO or specific identification. FIFO is the default and requires the least record-keeping. Specific identification gives you the most control but requires lot-level tracking.

Average cost is commonly used for mutual funds but isn't the default for individual stocks.

How the holding period interacts with cost basis

In the US, the holding period (short-term vs long-term) is tied to the specific lot, not the average. Under FIFO, you're selling the oldest shares — likely long-term. Under specific identification, you choose, and that choice also determines whether the gain is short or long term.

This is why FIFO usually produces more LTCG on your K-1 (Box 9a) and less STCG — and why tax rates under FIFO are often lower than under specific identification of recent lots.

A comparison: same facts, three methods

Same shares, same sale price — but the taxable outcome ranges from $2,000 to $6,000 and the character changes from short-term to long-term. The after-tax difference can be significant.