FIFO is a cost-basis method that sells the oldest shares first when calculating capital gains. It's the default method in most jurisdictions and usually results in higher reported gains in rising markets.
Cost Basis = Purchase Price of Oldest Lot × Shares Sold
If you bought 50 shares at $40 in January and 50 at $60 in March, then sold 50 in June at $70: FIFO uses the $40 cost basis, resulting in a $30/share gain ($1,500 total).
AllInvestView uses FIFO as the default cost-basis method. Read our FIFO vs LIFO guide to understand which method minimises your capital gains tax.