Compound Interest Calculator (2026)

Why compound growth matters for tax planning

Tax planning is not only about this April—it is about where money sits decades later. Compound interest math shows how IRA, 401(k), taxable brokerage, and HSA balances might grow given starting balance, periodic contributions, expected return, and years. The curve is nonlinear: year 30 adds more dollar growth than years 1–10 combined at reasonable rates.

This calculator is agnostic to account type until you interpret results. Roth growth is tax-free qualified; traditional growth is tax-deferred until distribution taxed as ordinary income; taxable accounts face annual drag from dividends and capital gains. Layer tax mentally or use retirement-specific tools on this site.

Inputs and formula

Future value combines initial principal compounded at (1 + r/n)^(nt) plus contribution annuity formulas for monthly adds. Enter annual nominal rate, compounding frequency, contribution size, and horizon years. The output chart (if shown) illustrates acceleration—the slope steepens with time.

Inflation reduces real purchasing power. A 7% nominal return with 3% inflation ≈ 4% real—pair with the inflation calculator so you do not confuse millionaire nominal with millionaire real.

2026 contribution context

401(k) elective deferral limit $24,500; IRA $7,500; HSA $4,400 self-only / $8,750 family (Rev. Proc. 2025-32). Maxing tax-advantaged space first changes after-tax wealth even if nominal compound curve looks identical pre-tax.

Employee FICA on wages (6.2% to $184,500, 1.45% Medicare) reduces cash available to contribute unless you prioritize auto-escalation from raises.

How to use the calculator

Step 1 — Starting balance

Current 401(k), IRA, or taxable investment total.

Step 2 — Monthly or annual contribution

Match what you actually invest, not wishful max unless testing a goal.

Step 3 — Rate and years

Run best/base/worst rate triads. Sequence-of-returns risk near retirement is not shown in basic compound math.

Worked example

$10,000 start, $500/month, 6% annual, 25 years. Ending balance lands near $345,000–$355,000 depending on compounding assumptions—roughly $150,000 from contributions and $190,000+ from growth. Delaying 5 years with same contributions might cut ending balance by 25%+ due to lost compounding window.

Rule of 72 shortcut

Divide 72 by rate to estimate years to double principal at fixed return. At 6%, ~12 years to double. Useful sanity check against calculator output.

Tax drag in taxable accounts

Annual tax on dividends and realized gains lowers effective compound rate—maybe 0.5–1.5% below headline market return depending on turnover and rates. Asset location (bonds in IRA, equities in taxable) is strategy this tool does not automate.

Debt is negative compound interest

Credit card balances compound against you. Payoff math mirrors growth with opposite sign—prioritize high APR debt before chasing 7% market assumptions in taxable accounts unless employer match demands otherwise.

Retirement readiness link

Project ending balance against 4% withdrawal heuristic for rough FIRE checks—$1M at 4% ≈ $40,000/year nominal before tax. Use the federal income tax calculator on expected retirement income mix (Social Security, traditional IRA, Roth).

Common mistakes

• Using nominal returns without inflation for retirement spending
• Assuming linear growth (forgetting compounding curvature)
• Ignoring fees and expense ratios shaving effective rate
• Contributing above IRS limits without correction plan
• Expecting historical equity returns as guaranteed forward rate

Related tools

401(k) calculator, inflation calculator, Roth vs traditional IRA calculator, contribution limits maximizer.

Additional planning notes for 2026

Tax software and professional preparers reconcile annual Form 1040 liability against W-2 withholding—not any single calculator snapshot. Use the interactive form above as a transparent starting point, then validate filing status, dependents, and pre-tax elections before you treat output as a filing estimate. Where this page references 2026 figures, they follow IRS Rev. Proc. 2025-32 and SSA wage base announcements unless noted otherwise.

Federal ordinary rates remain 10%, 12%, 22%, 24%, 32%, 35%, and 37%. Employee FICA is 6.2% on wages up to $184,500 plus 1.45% Medicare on all covered wages. Additional Medicare Tax at 0.9% applies above $200,000 single / $250,000 joint / $125,000 MFS. Standard deduction amounts are $16,100 single and married filing separately, $24,150 head of household, and $32,200 married filing jointly.

State residency, local income taxes, and voluntary payroll deductions can move cash results without changing the federal concepts modeled here. When a scenario spans multiple jobs, side income, or mid-year life changes, pair this page with the federal income tax calculator, paycheck calculator, and tax refund estimator so withholding, liability, and refund expectations stay aligned.

Documentation matters at filing time: retain 1098 and 1098-E forms, charitable acknowledgments, medical receipts, mileage logs, and employer plan confirmations that support positions you claim. Rounding in browser tools may differ slightly from payroll systems that round per pay period.

Recordkeeping and next steps

Keep source documents that support inputs you enter—pay stubs, Form W-2, 1098 series forms, receipts, and employer benefit summaries. Calculators on this site show math in the open for learning; they do not e-file returns or transmit data to the IRS. When results look close to decision thresholds (itemizing vs standard deduction, credit phaseouts, or installment affordability), rerun the scenario with conservative and aggressive assumptions to see sensitivity.

If your fact pattern spans deductions and credits, run this tool first, then validate withholding with the W-4 calculator. Cap and phaseout figures can change after publication—verify against the IRS page cited in the methodology notes.

Disclaimer

Educational compound math—not investment, tax, or fiduciary advice. Markets vary; past performance does not guarantee future results. Consult a financial planner for personalized allocation.