Methodology

Every calculator here makes a claim about your money. This page is where you check it. For each tool you get the formula it runs, the assumptions built into that formula, where the underlying data comes from, and what the tool does not account for.

The last section of each entry is the one worth reading. Assumptions are where financial calculators quietly go wrong, and a tool that hides them is asking for trust it has not earned.

Written and maintained by Roland Sta. Romana. If you find a mistake, say so on the contact page and it gets fixed.

All 12 tools

Investing

Inflation Calculator

Converts an amount of money from one year into another year's purchasing power.

The formula

value in target year = amount × (CPI of target year ÷ CPI of start year)

Worked example

The 1990 annual CPI was 130.7 and the 2025 CPI was 318.3. So $100 in 1990 is 100 × (318.3 ÷ 130.7) = about $244 in 2025 money.

Assumptions

  • ·Uses annual average CPI, not month-specific figures, so results are year to year rather than date to date.
  • ·CPI measures a national urban basket. Your personal inflation rate differs if your spending is weighted toward housing, healthcare, or tuition, which have all outrun the headline index.
  • ·Forward projections assume a fixed annual rate you choose. Nobody knows future inflation.

Sources

What it does not do

CPI is a contested measure. Methodology changes over the last 40 years mean very long comparisons, say 1913 to today, are directionally right but should not be read to the dollar.

Open the Inflation Calculator →

Investing

Compound Interest Calculator

Projects the future value of a starting balance plus regular contributions.

The formula

A = P(1 + r/n)^(nt) + PMT × [ ((1 + r/n)^(nt) − 1) ÷ (r/n) ]

P = starting principal, r = annual rate as a decimal, n = compounds per year, t = years, PMT = contribution per compounding period

Doubling estimate uses the Rule of 72: years ≈ 72 ÷ annual rate

Worked example

$10,000 at 7% compounded monthly for 20 years, with no contributions, grows to 10,000 × (1 + 0.07/12)^240 = about $40,275.

Assumptions

  • ·The return rate is constant. Real markets are not, and the order of good and bad years changes your outcome even when the average is identical.
  • ·Contributions are made at the end of each compounding period.
  • ·No taxes, fees, or fund expense ratios are deducted.
  • ·The Rule of 72 is an approximation. It is accurate to within a few months for rates between roughly 6% and 10%, and drifts outside that range.

Sources

  • · Standard compound interest and future value of an annuity formulas

What it does not do

This is a growth projection, not a forecast. Treat the output as what happens if the assumption holds, not as what will happen.

Open the Compound Interest Calculator →

Investing

DCA Simulator

Backtests dollar-cost averaging against a lump sum using real historical prices.

The formula

shares bought on each date = contribution ÷ price on that date

DCA value = total shares accumulated × latest price

lump sum comparison = (total invested ÷ price on the first date) × latest price

Assumptions

  • ·Fractional shares are allowed, so no purchase is rounded down.
  • ·No commissions, spreads, or taxes.
  • ·Dividends are not reinvested, because the underlying FRED price series are price indices rather than total return indices. For dividend-paying assets this understates both strategies.
  • ·A contribution scheduled on a non-trading day uses the next available price.

Sources

What it does not do

Past price history is the only thing being tested here. A strategy that won over one window frequently loses over the next one, which is the actual lesson of the tool.

Open the DCA Simulator →

Retirement

FIRE Number Calculator

Calculates the portfolio size that would support your spending indefinitely, and projects when you reach it.

The formula

FIRE number = annual expenses ÷ withdrawal rate

at the default 4% rate this is the same as annual expenses × 25

Coast FIRE number = FIRE number ÷ (1 + r)^(65 − current age)

projected monthly income in retirement = (FIRE number × withdrawal rate) ÷ 12

Worked example

$50,000 of annual spending at a 4% withdrawal rate gives 50,000 ÷ 0.04 = a $1,250,000 FIRE number.

Assumptions

  • ·The 4% default comes from the 1998 Trinity Study, which tested 30-year retirements on US stock and bond history. A retirement starting at 40 may run 50 years or more, which is outside what that study tested.
  • ·Returns are applied as a smooth monthly rate. This hides sequence-of-returns risk, the single largest threat to an early retiree.
  • ·Expenses are assumed flat in real terms. Healthcare and long-term care usually are not.
  • ·No Social Security, pension, or inheritance is counted, so the number is deliberately conservative on that front.
  • ·The accumulation projection stops at 100 years.

Sources

  • · Cooley, Hubbard and Walz, "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable" (the Trinity Study), 1998

What it does not do

A single withdrawal rate is a rule of thumb, not a guarantee. Later research has argued both for lower rates in high-valuation markets and for higher rates with flexible spending.

Open the FIRE Number Calculator →

Budgeting

Net Worth Calculator

Totals what you own, subtracts what you owe, and compares the result to US medians for your age.

The formula

net worth = total assets − total liabilities

Assumptions

  • ·Asset values are whatever you enter. Property and vehicle estimates are usually the least reliable input.
  • ·Comparison figures are medians, not averages. Median is the right choice here because a small number of very large fortunes drags the average far above what a typical household holds.

Sources

What it does not do

The benchmark is US household data. If you live elsewhere the comparison is not meaningful, though the arithmetic still is. Benchmarks are also per household, so a couple entering combined figures is comparing like with like, while an individual is not.

Open the Net Worth Calculator →

Budgeting

Emergency Fund Calculator

Sets a savings target in months of expenses and works out how long it takes to get there.

The formula

target = monthly essential expenses × months of cover

months to goal = ⌈ (target − current balance) ÷ monthly contribution ⌉

Worked example

$3,000 of monthly expenses with a 6-month goal gives an $18,000 target. From $4,000 saved, adding $500 a month, that is ⌈14,000 ÷ 500⌉ = 28 months.

Assumptions

  • ·Interest earned on the balance is not counted, so a high-yield savings account gets you there slightly faster than shown.
  • ·Expenses are treated as flat.
  • ·The 3 to 6 month convention is a norm, not a law. One unstable income usually warrants more, two stable incomes often less.

Sources

  • · Standard months-of-expenses convention used across consumer finance guidance

What it does not do

The calculator has no view on whether your listed expenses are genuinely essential. That judgment stays with you.

Open the Emergency Fund Calculator →

Debt

Debt Payoff Planner

Simulates paying off multiple debts month by month under avalanche, snowball, or proportional strategies.

The formula

each month, for each debt: interest = balance × (APR ÷ 12)

balance = balance + interest − payment

any payment freed by clearing a debt rolls onto the next target debt

avalanche targets the highest APR first, snowball the smallest balance first

Assumptions

  • ·Interest is applied monthly on the full balance. Credit cards in reality use average daily balance, so a real statement will differ by a small amount.
  • ·APR is fixed for the life of the payoff. Variable card rates move.
  • ·Minimum payments are the fixed figures you enter. Card issuers usually recalculate them downward as the balance falls, which is why real payoff is slower than a plan like this suggests.
  • ·No new spending is added to any account.
  • ·No late fees, penalty APRs, or promotional 0% expiry.

Sources

  • · Standard amortization arithmetic applied per debt, per month

What it does not do

Avalanche always wins on arithmetic. Snowball sometimes wins in practice because clearing a whole account early keeps people going. The tool shows both because the cheaper plan is worthless if it gets abandoned.

Open the Debt Payoff Planner →

Debt

Student Loan Payoff Planner

Models multiple student loans and shows what a fixed extra monthly payment saves in time and interest.

The formula

each month, per loan: interest = balance × (rate ÷ 12)

balance = balance + interest − payment

extra payment is applied by avalanche, snowball, or proportionally across loans

savings = (total interest on the standard plan) − (total interest with the extra payment)

Assumptions

  • ·Fixed interest rates. Federal loans are fixed; private variable-rate loans are not.
  • ·Payments are applied to interest first, then principal, which is the standard federal allocation order.
  • ·No capitalization events, deferment, or forbearance.
  • ·Simulation is capped at 600 months.

Sources

  • · Standard loan amortization; federal payment allocation order

What it does not do

This models straight repayment only. It does not model income-driven repayment, forgiveness programs, or Public Service Loan Forgiveness, and paying extra can actively work against you if you are pursuing forgiveness. If that is your situation, the answer this tool gives is the wrong question.

Open the Student Loan Payoff Planner →

Real Estate

Rent vs. Buy Calculator

Compares 30 years of renting against 30 years of owning, including the opportunity cost of the down payment.

The formula

monthly mortgage payment: M = L × [ i(1 + i)^n ] ÷ [ (1 + i)^n − 1 ]

L = loan amount, i = monthly rate, n = 360 payments

net cost of buying = cumulative costs − equity built − tax savings + opportunity cost of the down payment

opportunity cost = down payment × [ (1 + investment return)^years − 1 ]

Assumptions

  • ·A 30-year fixed mortgage. No ARMs, no refinancing.
  • ·Homeowner's insurance estimated at 0.5% of home value per year, maintenance at 1% per year.
  • ·Rent rises at the annual rate you set, compounding.
  • ·The down payment is assumed to be invested at your chosen return if you rent instead. This is the line item most rent vs. buy calculators leave out, and it is often what decides the answer.
  • ·HOA dues, PMI, utilities, and closing costs on the sale are not modeled.

Sources

  • · Standard fixed-rate mortgage amortization formula

What it does not do

Both appreciation and investment return are guesses you supply. Change either by two points and the break-even year moves substantially. Run it a few ways rather than trusting one result.

Open the Rent vs. Buy Calculator →

Taxes

Freelancer Tax Estimator

Estimates self-employment tax, federal income tax, and quarterly payments for 1099 income in tax year 2026.

The formula

SE tax base = net self-employment income × 0.9235

SE tax = 12.4% Social Security on that base up to the $184,500 wage base, plus 2.9% Medicare with no cap (15.3% combined)

half of SE tax is deducted from income before federal income tax

federal income tax = progressive 2026 brackets applied to (income − half SE tax − QBI deduction − standard deduction)

each quarterly payment = total estimated tax ÷ 4

Worked example

On $80,000 of net freelance income, the SE tax base is 80,000 × 0.9235 = $73,880, and SE tax is 73,880 × 0.153 = about $11,304. Half of that, $5,652, comes off before income tax is figured.

Assumptions

  • ·Tax year 2026 figures: $16,100 standard deduction for single filers, $32,200 married filing jointly, $24,150 head of household. Social Security wage base $184,500. Section 199A QBI limits begin phasing in at $201,775 single and $403,500 joint.
  • ·The 92.35% factor exists because you may not pay SE tax on the portion that represents the employer half. It is in the law, and most quick estimators skip it.
  • ·State tax uses an approximate blended effective rate per state, not that state's actual bracket table. Nine states levy no income tax and are handled as zero.
  • ·Assumes standard deduction, not itemizing.
  • ·No credits are applied: no child tax credit, no EITC, no education credits.

Sources

What it does not do

This is an estimate for planning, not a return. State handling in particular is approximate. If your income is uneven across the year, the annualized income installment method may cut your penalty exposure, and this tool does not model it. Talk to a CPA before you file.

Open the Freelancer Tax Estimator →

Taxes

Salary Take-Home Calculator

Turns a gross salary into estimated take-home pay after federal tax, state tax, and FICA for tax year 2026.

The formula

federal taxable income = gross − 401(k) contribution − standard deduction

federal tax = progressive 2026 brackets applied to that figure

Social Security = 6.2% of gross, capped at the $184,500 wage base

Medicare = 1.45% of gross, uncapped, plus 0.9% on gross above $200,000 single or $250,000 joint

take-home = gross − federal − state − FICA − 401(k)

Worked example

On a $70,000 salary, Social Security is 70,000 × 0.062 = $4,340 and Medicare is 70,000 × 0.0145 = $1,015, for $5,355 of FICA before any income tax.

Assumptions

  • ·Tax year 2026 brackets and standard deductions per IRS Revenue Procedure 2025-32.
  • ·401(k) contributions reduce federal taxable income but not the FICA base. This trips people up constantly and many calculators get it wrong.
  • ·The additional Medicare thresholds of $200,000 and $250,000 are set in statute and are not indexed to inflation, so they do not change year to year.
  • ·State tax is modeled as no tax, a flat rate, or a full bracket table depending on the state.
  • ·Health premiums, HSA and FSA contributions, and other pre-tax deductions are not modeled.
  • ·Assumes standard deduction and no credits.

Sources

What it does not do

Your actual paycheck depends on your W-4, your employer's withholding method, and local city or county taxes that this tool does not model. Expect the estimate to land close, not exact.

Open the Salary Take-Home Calculator →

Budgeting

Financial Literacy Quiz

Twenty questions across five areas of personal finance, scored with an explanation for every answer.

The formula

score = correct answers ÷ 20

per-category score = correct answers in that category ÷ questions in that category

Assumptions

  • ·Questions cover US rules and US products. Some answers do not transfer to other countries.
  • ·Every question has one defensible answer. Genuinely contested topics were left out rather than scored as right or wrong.
  • ·Each explanation states why the correct answer is correct, so a wrong answer still teaches something.

Sources

  • · FICO scoring weights: payment history 35%, amounts owed 30%
  • · IRS contribution limits and general consumer finance conventions

What it does not do

A score here measures familiarity with concepts, not how well you manage money. Plenty of people who would score badly are doing fine, and the reverse is also true.

Open the Financial Literacy Quiz →

A note on tax figures

Tax constants change every year. The IRS publishes the following year's inflation adjustments each October, and the Social Security Administration announces its wage base at the same time. Both tax tools are reviewed against those releases when they land, and the figures in use are listed above so you can check them against the source yourself.

DuckDollar is educational and is not tax or financial advice. Read the editorial policy for how these tools get built and reviewed.