Realising a loss to offset a gain is a standard technique in taxable accounts. What it achieves is often overstated, because the benefit is mostly timing.
The mechanism is straightforward
An asset held below its purchase price is sold, creating a realised loss that can be set against realised gains elsewhere in the same period.
The proceeds are typically reinvested in something similar, so the portfolio's overall exposure is maintained while the tax position improves.
The result is a lower tax bill for the year without a meaningful change to what the investor actually holds.
The saving is largely deferral
The replacement asset is purchased at the current lower price, which becomes its cost for tax purposes. A future sale will therefore show a larger gain.
The tax avoided now is broadly the tax paid later, with the difference being the value of holding the money in the meantime.
That deferral is genuinely worth something, particularly over long horizons, but it is much less than the headline saving in the year it occurs.
Rate differences can create real savings
Where a system taxes short-term and long-term gains differently, offsetting a highly taxed short-term gain with a loss produces a permanent benefit rather than a deferred one.
The same applies where the eventual gain is expected to fall into a lower band, for instance because income will be lower in retirement.
These outcomes depend on rate structures that differ by jurisdiction and are subject to change, so they cannot be assumed.
Repurchase rules constrain the technique
Most systems deny a loss where a substantially identical asset is bought back within a defined window around the sale.
The rules generally look through accounts, so repurchasing in a spouse's account or a tax-sheltered one can still trigger the restriction.
Practitioners work around this by buying a similar but not identical asset, which maintains exposure while remaining outside the definition, though the boundary is not always clear.
Where it does not apply at all
Tax-sheltered accounts produce no deductible losses, since gains within them are not taxed in the ordinary way. Harvesting is a taxable-account technique only.
Transaction costs, spreads and the risk of being out of the market during a reinvestment gap all reduce the benefit and can exceed it on small positions.
The technique also encourages selling assets an investor might otherwise hold, which is a reason to treat it as a secondary consideration rather than a driver of decisions.