A capital loss is not a dollar-for-dollar reduction of tax. It enters a netting process with capital gains, and only the resulting net amount affects taxable income under the applicable rules.
Short-term and long-term character matters
Capital transactions are separated by holding period. Net short-term gains are generally taxed at ordinary income rates, while net long-term gains may qualify for long-term capital-gain rates. The tax return nets categories in a prescribed sequence, so the same dollar loss can have different timing effects depending on the rest of the return.
The federal $3,000 limit
IRS Topic 409 states that when capital losses exceed capital gains, the deduction against other income is limited to the lesser of the net loss or $3,000 ($1,500 for married filing separately). This limit applies after gain-and-loss netting—not to losses used against capital gains.
Unused eligible losses generally carry forward
Losses that cannot be used in the current year generally retain their character and carry to later years. Carryovers should be reconciled to prior Schedule D information before planning new sales.
Why basis records matter
Gain or loss depends on adjusted basis and amount realized. Missing basis, corporate actions, transfers, inherited property, gifts, reinvested dividends, and prior wash sales can change the number. Tax-loss harvesting should begin with reliable tax-lot data.
A simplified example
If a taxpayer has $30,000 of capital gains and $18,000 of deductible capital losses, the loss may reduce net capital gain to $12,000 before other relevant items. It does not create an $18,000 tax refund. Rates, character, state treatment, and the rest of the return determine the actual effect.