The federal calculation starts with total income from every source, then applies a specific formula just for Social Security: up to 85% of benefits can be included in taxable income, depending on 'provisional income' (adjusted gross income plus tax-exempt interest plus half of Social Security benefits). Below $25,000 provisional income for a single filer ($32,000 married filing jointly), none of it is taxed; above $34,000 single ($44,000 MFJ), up to 85% is taxed. Everything in between is taxed on a sliding scale. This formula hasn't been adjusted for inflation since it was written decades ago, which is why more retirees owe federal tax on Social Security every year even though the thresholds haven't moved.

Ordinary income, pension payments, IRA and 401(k) withdrawals, wages, taxable Social Security, is taxed at the federal ordinary rates, which for 2026 run from 10% at the bottom to 37% at the top, with the brackets adjusted annually for inflation. Long-term capital gains and qualified dividends are taxed on a separate schedule, generally 0%, 15%, or 20% depending on total taxable income, which is why the mix between ordinary retirement account withdrawals and taxable brokerage account gains matters more than most retirees realize.

State tax calculations are built entirely differently, and vary enormously by state. States with no income tax, Nevada among them, skip this step entirely: there is no state calculation because there's no state income tax. States with an income tax typically start from federal adjusted gross income or federal taxable income and then apply their own additions and subtractions. Arizona subtracts out Social Security and military retirement pay in full, then applies a flat 2.5% rate to what's left. Colorado and New Mexico apply age- or income-based subtractions for pension and annuity income on top of similar Social Security treatment, then apply their own bracket structure.

Because the two systems are independent, a distribution strategy that's efficient federally isn't automatically efficient at the state level, and vice versa. A Roth conversion timed to stay under a federal capital gains threshold might still generate state tax in a state that doesn't distinguish between ordinary income and capital gains at all. Conversely, a withdrawal strategy built around an age-based state pension subtraction doesn't change anything about the federal Social Security inclusion calculation.

This is also where state residency and domicile timing becomes relevant. Which state's rules apply to a given year of income depends on where you were domiciled or resident during that tax year, not where you live now. A large IRA withdrawal or Roth conversion taken the year before a move to a no-tax state is still taxed by the state you were living in when you took it.

Withholding is another place the two systems diverge in practice, not just in calculation. Most IRA and 401(k) custodians default to withholding federal tax at a flat percentage on distributions unless you elect otherwise, and separately ask whether you want state tax withheld, a question that's meaningless in a no-income-tax state like Nevada but genuinely matters in Arizona, California, Colorado, or New Mexico. Retirees who don't actively manage withholding on both lines can end up underpaying one government while overpaying the other, generating a refund from one and a penalty notice from the other on the same distribution.

Estimated quarterly tax payments follow the same split-system logic. A retiree taking irregular distributions, a large one-time Roth conversion, an unusual capital gain, generally needs to consider federal estimated payments and, separately, state estimated payments if living in an income-tax state, since underpayment penalties are assessed independently by each government based on its own safe-harbor rules and deadlines.

Tax software and even some professional preparers sometimes blend the two calculations together in a way that obscures which government is actually driving a given result. If your total tax bill went up or down year over year, it's worth asking your preparer to break out exactly how much of the change came from the federal side, a bracket shift, a new deduction, more Social Security becoming taxable, versus the state side, a rate change, a new subtraction, a move. Treating the combined number as one undifferentiated figure makes it much harder to plan around either system deliberately.

Build your annual tax plan as two separate calculations, not one blended number. Model the federal ordinary and capital gains brackets, the Social Security inclusion formula, and your specific state's rules side by side, since a move that helps one can leave the other unchanged.

If you're comparing states, don't assume a lower headline income tax rate automatically translates to a lower total bill; the federal calculation is the same no matter which state you live in, so the actual comparison is only about the state layer on top of it.

• Treating 'my tax rate in retirement' as one number instead of two separate federal and state calculations.
• Forgetting that the Social Security inclusion formula (0%, 50%, or 85% taxable) is a federal calculation that most states either ignore entirely or apply completely differently.
• Assuming a state-level tax strategy, like an age-based pension subtraction, changes anything about the federal calculation, it doesn't.
• Not accounting for which state's rules apply based on domicile during the year income was actually recognized, not the state you live in when you file.