Cost Basis in Crypto: Average Cost vs FIFO (and Why Your P&L Looks Wrong)

If you've ever sold some BTC and thought “wait… why is my profit number so weird?”, you're not alone. In crypto, P&L isn't just price — it depends on which coins you sold. That choice is called cost basis.

Two people can make the exact same buys and sells, at the exact same prices, and still see different realized profit depending on whether their tracker uses Average Cost or FIFO.

What “cost basis” means (in plain English)

Every time you buy crypto, you create a lot: an amount you bought at a particular price (plus fees). When you later sell, the app must decide which lots were sold, so it can compute your realized profit.

This is why your P&L can look “wrong” when you compare two apps: they might both be correct — they just use different rules for choosing lots.

Average Cost vs FIFO: the core difference

Here are the two most common methods beginners run into:

Worked example (with numbers): one sell, two different profits

Let's use a simple scenario with one coin (ETH), ignoring taxes for the moment, and assuming fees are $0 to keep the math clean.

You bought 2 ETH total, then sold 1 ETH. The question is: which 1 ETH did you sell?

Method 1: FIFO (sell the oldest ETH first)

FIFO says the first ETH you bought (Jan 10 at $1,000) is the one you sold on Mar 20.

Your remaining holding is the Feb 05 lot: 1 ETH with cost basis $2,000.

Method 2: Average Cost (blend both buys)

Average Cost says: “You own 2 ETH at an average price.” The average cost per ETH is:

Same sell. Same market price. But realized profit is $800 vs $300 depending on the method. That's why your P&L can “look wrong” when you switch apps — the app didn't lose your data, it changed the accounting lens.

Why this matters for P&L (and not just taxes)

Even if you don't care about tax reporting yet, cost basis impacts the two numbers people look at most:

• Realized profit on a sale — what you “made” on that trade.
• Remaining cost basis — what your still-held coins are “carried at” for future sales.
• Performance interpretation — you might think you are a great trader (or a terrible one) based on realized P&L that is mainly a method difference.

Three common mistakes that make P&L look wrong

Most “wrong P&L” issues aren't bugs — they're missing context. Here are the three most common pitfalls.

Quick sanity checks when your profit number “feels off”

• Check the method: does the tool explicitly say FIFO, Average Cost, or something else?
• Pick one trade: compute it manually (like the example above) and see which method matches your app's result.
• Look for $0 cost basis: this often signals missing transfer history or missing buy lots.
• Verify fees: make sure fees are present and consistently included.

So which method should you use?

There's no universal “best” method. The right answer depends on your jurisdiction and your reporting needs. But for beginners tracking performance, the key is simpler:

If you're using a tracker mainly to learn from your decisions, consistency beats perfection. A consistent cost basis method makes trends visible and helps you avoid the psychological trap of celebrating (or panicking) over numbers that are mostly accounting artifacts.

A soft next step (Coinlio)

Coinlio is built to keep portfolio tracking practical and understandable. If you're trying to make sense of performance over time, pair the cost-basis basics here with a longer horizon view like Profit by Year so you can separate “method noise” from real strategy outcomes.

Educational content. Not financial or tax advice.