Every SIP instalment you invest creates a separate 'lot' of units, each with its own purchase date and price. When you redeem only part of your holding, the question of which lots get sold first is not up to you, it is decided by a specific accounting convention that directly determines whether your gain is short-term or long-term.
A Systematic Investment Plan results in a fresh purchase of units every month (or whatever frequency is chosen), each at the NAV prevailing on that date. Over several years, a single SIP can create dozens or even hundreds of distinct 'lots' of units, each with its own acquisition date and acquisition price (NAV at purchase). For capital gains purposes, each lot is treated as a separate acquisition.
Because FIFO determines which lots are deemed sold, it directly affects the holding period of the units being sold, which in turn determines whether the resulting gain is a short-term capital gain (STCG) or long-term capital gain (LTCG), each taxed differently for equity-oriented funds. If your earliest SIP instalments are now long-term (held beyond the equity threshold of one year) while your most recent instalments are still short-term, a partial redemption will, under FIFO, draw from the oldest (long-term) units first, until those are exhausted, before touching the more recent (short-term) units.
As an investor continues a long-running SIP and periodically makes partial redemptions, FIFO means that each redemption progressively consumes the oldest remaining lots first. Investors who redeem small amounts periodically (for example, for rebalancing or partial profit-booking) while continuing fresh SIP instalments will generally find that, for a long time, their redemptions draw from increasingly 'aged' (long-term) lots, while their newest instalments remain short-term until they age past the one-year threshold themselves.
Mutual fund Capital Gains Statements (provided by registrars like CAMS/KFintech, or available through fund house portals) typically show a lot-wise breakup of each redemption, listing the specific purchase date, purchase NAV, units redeemed from that lot, sale NAV, and the resulting gain/loss for each lot, already computed on a FIFO basis. This statement is the most reliable source for filling in the capital gains schedule of the ITR, rather than attempting to compute this manually.
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