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Guides
Concept-first explanations that support the calculator and stand on their own. Each guide links back to official definitions where they matter.
How progressive income tax works
Why your first dollars are taxed less than your last, and how the band walk produces the total tax bill.
Marginal vs effective tax rate
The single most important distinction in any tax calculator — and the one most often misunderstood.
Allowances vs deductions vs credits
Three different ways the tax system lowers your bill — and why mixing them up changes the math.
Bracket creep and inflation
When frozen thresholds meet rising wages, real income gets taxed more. How the US and UK differ on indexation.
How to read your payslip
Every line on a US and UK payslip explained — gross, deductions, withholdings, net pay, and what to verify.
When a raise does not help
Phase-outs, tapers, and the 60% UK Personal Allowance cliff — why effective marginal rates often beat the headline.
Tax residency vs domicile
Two concepts that sound like one and are not. Who taxes your income this year, who taxes your estate, and why treaties matter.
US state taxes: the gap
Why the federal calculator stops there, and what 0-13% of state tax on top does to the same gross paycheck.
Self-employment tax basics
SE tax on the US, Self Assessment and Class 2 / Class 4 NI in the UK — what the calculator does not yet cover.
Tax refunds explained
Why some people get paid back and others owe — withholding vs actual liability, US W-4 and UK PAYE mechanics.
Retirement accounts explained
401(k), IRA, Roth, HSA on the US side; workplace pension, SIPP, ISA on the UK side — contribution limits and tax treatment.
Tax year and filing deadlines
US calendar year vs UK personal year, all the key dates, and the penalties for getting them wrong.
Common tax myths — and the corrections
Myths that recur every year, debunked. Higher-rate at £40k? Refund = bonus? 60p of every £1 over £100k?
Why the calculator differs from your tax software
Same brackets, different output. How credits, deductions, and pre-tax elections explain the gap.
US Form W-4 explained
Every step, every line of the 2020+ W-4 — for single filers, married couples, multiple jobs, and dependents.
UK tax codes explained
1257L, BR, K, D0, D1, S, C — what every code means and how it changes your PAYE.
Common tax mistakes
The mistakes that recur every year — wrong W-4, missed NI category, forgotten state tax — and the fix for each.
Moving between states or countries
US state-to-state, US outbound, UK outbound, treaty tie-breakers, and the pre-move checklist.
The 401(k) mega-backdoor
Up to $72k/year into Roth wrappers via after-tax contributions and in-plan conversions. Eligibility and traps.
Marriage Allowance UK
The £252 transfer, eligibility, how to claim, and the 4-year back-claim for missed savings.
Itemize vs standard deduction
When itemizing wins, what counts toward Schedule A, the SALT cap, and the break-even points.
Capital gains basics
Short-term vs long-term, US 0/15/20% rates, UK 18/24% rates, NIIT, losses, and tax wrappers.
Equity compensation
RSUs, NQSOs, ISOs, ESPP — how each works and the tax treatment of each.
What triggers an IRS audit
DIF scores, information matching, Schedule C red flags, and how to reduce your audit risk.
Working from home deductions
US Form 8829, the $5/sq ft simplified method, and UK P87 flat-rate expenses.
LLC vs S-Corp vs sole proprietor
When S-corp election saves tax, the QBI deduction, and reasonable compensation rules.
Tax-efficient withdrawal order in retirement
Which accounts to draw from first, Roth conversions, RMDs, and ACA subsidies.
How to compare take-home pay between countries
Normalization, social contributions, exchange rates, healthcare, pensions, and what to deliberately ignore.