How we compute the numbers — and what we leave out.
Every result on this site is calculated from publicly cited tax tables. This page lists each tool’s method, every source we use, the dates we last verified them, and the things we deliberately don’t compute so you know where the math stops.
Three tools, each computed server-side from versioned, auditable code.
Every calculation runs on our server in TypeScript, not in the browser. The reasons: compliance (one auditable place for the math), trust (we can cite the exact source and verification date in every result), and integrity(the business-critical logic doesn’t ship in fork-able client-side JavaScript). Each tool exports a CALC_VERSION that gets stored with every result, so months later you can tell exactly which version of the math produced a given number.
The Paycheck Snapshot
CALC_VERSION 1.3.0Real take-home pay after federal income tax, FICA (Social Security + Medicare), state income tax, and state disability where applicable. Uses the IRS percentage method (Pub 15-T) for federal, and each state's official withholding schedule for state.
The Cash Flow Lens
CALC_VERSION 1.1.0Pure subtraction across user-entered income and expense categories. Surfaces a margin-of-comfort figure and where the largest single pressure sits. No tax math here — this tool consumes the Paycheck Snapshot's net figure rather than recomputing it.
The Debt Clock
CALC_VERSION 1.2.0Amortization schedule for a single fixed-rate debt at a given monthly payment, then a second schedule at payment + extra. Reports both payoff months and total interest paid. Uses standard monthly compounding (APR / 12).
Multi-Debt Avalanche
CALC_VERSION 0.1.0Month-by-month simulation across up to 5 debts. Every debt pays its minimum each month; the extra-payment budget is routed to the highest-APR debt first, then cascades to the next as each debt clears. A parallel snowball (smallest-balance-first) simulation runs on the same inputs to show the head-to-head difference. Mortgages are excluded — they're tax-advantaged and modeled separately in Refi Break-Even.
Emergency Fund Sizing
CALC_VERSION 0.1.0Months-of-fixed-costs target range based on convention from the CFPB Emergency Savings guidance and the FINRA Investor Education Foundation. Base of 3 months plus risk additions for variable income, single-earner households, and dependents. Lower bound is base + boosts; upper bound is the lower bound plus 3 months, capped at 12. Compares against the user's current balance and projects time-to-fill at their savings rate.
Savings Rate Reality
CALC_VERSION 0.1.0Closed-form years-to-FI projection. The user's savings rate (take-home minus spending, divided by take-home) becomes the input to a derivation of the future-value-of-annuity formula set equal to the 25× annual-spending target. Assumes a 5% real return on invested assets and a 4% safe withdrawal rate (Trinity Study + Bengen + MMM convention). Supports an optional existing-balance input and emits a sensitivity table at +5pp / +10pp / +15pp savings-rate bumps.
Refi Break-Even
CALC_VERSION 0.1.0Standard mortgage amortization on the new loan to derive the new monthly payment, then two break-even computations. Simple break-even is closing costs divided by monthly savings — the number every refi calculator surfaces. Total-cost crossover is a month-by-month walk of cumulative current-loan payments versus cumulative new-loan payments plus closing costs; the crossover month is when the new path becomes cheaper. Optional planned-holding-period drives a structural verdict (clear-win / marginal / underwater / refi-costs-more).
Three federal publications, re-verified per tax year.
The federal portion of every tax calculation comes from one of these three sources. We re-fetch and re-parse each one when the IRS or SSA publishes a new annual update (typically late October / early November), and again on a quarterly spot-check to catch mid-year errata.
All 50 states + the District of Columbia.
Each state’s source is the official Department of Revenue (or equivalent) withholding publication for tax year 2026. Italicized entries note states where the calculation has a known caveat — typically because the state uses a flat tax, no income tax, or a method we can’t fully replicate without information we don’t ask for.
Things we deliberately don’t include — so you know where the math stops.
Most paycheck calculators tell you what they cover. Few tell you what they don’t. These are the categories we know our calculation excludes — if any of them apply to your situation, your actual paycheck will diverge from our estimate. For decisions that depend on a precise figure, confirm with your payroll team or a licensed tax professional.
Local taxes
San Francisco, NYC, Portland, Cleveland, Detroit, Philadelphia, Birmingham, Louisville, Kansas City and others. State portion only.
Multiple jobs / spousal income
We treat each calculation as if it's a single W-2 source. The W-4 Multiple Jobs Worksheet adjustment is not applied.
Bonuses & supplemental wages
Bonus withholding (22% flat or aggregate method) and other supplemental wage rules are not modeled. Enter base wages only.
Equity compensation
RSU vesting, ISO/NSO exercises, ESPP purchases, and Section 83(b) elections are out of scope. These often dominate paycheck variance for tech employees.
Itemized deduction modeling
We assume the standard deduction. Mortgage interest, SALT, charitable contributions, and other itemized deductions aren't modeled in the paycheck-level withholding calc.
YTD wage progression
Each result treats the paycheck as if it's mid-year (steady state). Year-to-date wage caps like the Social Security cap aren't applied until you enter YTD wages, which we don't ask for.
Employer-specific deductions
Beyond 401(k)/HSA/health premiums you enter, we don't model garnishments, union dues, transit/parking, dependent-care FSA, or commuter benefits.
Personalized tax advice
Nothing on this site recommends a course of action for your specific situation. We compute figures from public tables — interpretation and decisions are yours.
When and how the tax tables get updated.
Tax tables change every year — usually announced October–December for the following tax year. Here’s the schedule we run on so the numbers stay current without quietly drifting:
Watch for IRS Pub 15-T draft
Auto-filed Linear ticket reminds us to begin tracking federal changes the moment the draft drops.
State DoR publications
Each state's publication is re-parsed into a versioned data/YYYY.ts file. The diff is reviewed in PR.
Run reference cases
Calc engine runs against published reference paychecks (IRS Pub 15-T examples, state DoR examples) to confirm parity.
New tax year live
New tables ship behind a date-gated switch. Old-year scenarios are still computable with their original calc_version.
What this site is, and what it isn’t.
Aplomia is not a financial advisor, planner, lender, broker, or tax professional. Every result is a planning estimate generated from publicly cited tax tables. Nothing on this site constitutes personalized tax or financial advice for your specific situation.
We don’t sell leads, don’t run display advertising, and don’t make money from your decision after a calculation — no incentive changes a tool’s result. Aplomia is free today; planned revenue comes from advisors who license our white-label embed, plus clearly-labeled affiliate or offer placements that never affect the math — see How we make money for the full picture.
If you believe a number on this site is wrong — different from the published source, or missing a relevant edge case — please report it at methodology-brendan@aplomia.com. We fix bugs in the calc engine within 5 business days of confirmation and post a changelog entry.