How income tax works
Income tax is a tax levied by governments on individual earnings. Most countries use a progressive tax system — the more you earn, the higher the rate on each additional dollar. Understanding this properly can save you thousands through smart use of deductions, contributions, and income timing.
The key concept most people misunderstand is marginal rate vs effective rate. Your marginal rate is the tax on your last dollar earned. Your effective rate is total tax divided by gross income — what you actually pay overall. If your marginal rate is 22%, you do not pay 22% on your entire salary.
United States income tax (2025)
The US has seven federal brackets from 10% to 37% for 2025. Before any brackets apply, you subtract the standard deduction — $15,000 for single filers, $30,000 for married filing jointly in 2025. On top of federal tax, employees pay FICA: 6.2% Social Security (on the first $176,100) and 1.45% Medicare. Self-employed workers pay the full 15.3%. Seven states have no income tax: Alaska, Florida, Nevada, South Dakota, Tennessee, Texas, and Wyoming.
Pre-tax contributions to a 401(k) (up to $23,500 in 2025) and Traditional IRA (up to $7,000) reduce your federal taxable income directly. At the 22% bracket, maxing out a 401(k) saves $5,170 in federal tax alone.
Example: A single filer in California earning $90,000 with a $10,000 401(k) contribution pays approximately $9,250 federal income tax, $5,790 FICA, and $3,850 California state tax — total $18,890. Take-home: $61,110.
United Kingdom income tax (2024/25)
UK income tax starts above the Personal Allowance of £12,570. Basic rate 20% to £50,270, higher rate 40% to £125,140, additional rate 45% above. The personal allowance tapers above £100,000 — creating an effective 60% marginal rate between £100,000 and £125,140. National Insurance adds 8% between £12,570 and £50,270, and 2% above. Scotland has separate bands from 19% to 48%.
Canada income tax (2024)
Canada uses federal plus provincial layers. Federal brackets run 15% to 33%. Ontario adds 5.05% to 13.16% on top. The Basic Personal Amount (federal: $15,705) shields your first earnings from tax. All workers contribute to CPP (5.95% up to $68,500) and pay EI premiums. RRSP contributions reduce taxable income dollar-for-dollar.
Australia income tax (2024/25)
Australia's Stage 3 tax cuts took effect 1 July 2024. Tax-free threshold: A$18,200. Then 19% to $45,000, 32.5% to $135,000, 37% to $190,000, 45% above. The LITO offset reduces tax by up to $700 for lower earners. The 2% Medicare Levy applies to most residents. Employer super at 11.5% is paid additionally by your employer.
How to legally reduce your tax bill
In the US, maxing your 401(k) at $23,500 saves $5,170 in federal tax at the 22% bracket. HSA contributions ($4,300 self-only) are triple tax-advantaged — deductible going in, grow tax-free, tax-free on qualifying withdrawals. Tax credits are even more powerful — a $2,000 Child Tax Credit reduces your bill by exactly $2,000 regardless of your bracket. In Canada, every RRSP dollar reduces taxable income directly. In Australia, salary sacrificing into super is taxed at just 15% inside the fund versus your marginal rate outside it.
Tax comparison: $75,000 salary across 4 countries
| Country | Income tax | Other levies | Take-home | Effective rate |
|---|---|---|---|---|
| 🇺🇸 US (single, no state) | $8,700 | $5,738 FICA | $60,562 | 19.5% |
| 🇬🇧 UK (England) | £8,432 | £3,262 NI | £63,306 | 15.6% |
| 🇨🇦 Canada (Ontario) | CA$16,710 | CA$5,300 CPP+EI | CA$53,010 | 29.7% |
| 🇦🇺 Australia | A$12,817 | A$1,500 Medicare | A$60,683 | 19.1% |
Approximate figures. US uses 2025 brackets and excludes state tax. Canada includes CPP and EI. Australia excludes employer super (11.5% paid additionally by employer).
Frequently asked questions
Does getting a raise push all my income into a higher bracket?
No — only the income above the new threshold is taxed at the higher rate. Everything below stays exactly the same. Never turn down a raise because of bracket fear.
What's the difference between a tax deduction and a tax credit?
A deduction reduces taxable income. A credit reduces your actual tax bill. A $1,000 deduction saves $220 at the 22% bracket. A $1,000 credit saves exactly $1,000 regardless of your bracket — credits are far more valuable dollar for dollar.
How much does a 401(k) contribution actually save in taxes?
At the 22% federal bracket, every $1,000 contributed to a 401(k) saves $220 in federal income tax immediately. Maxing it at $23,500 in 2025 saves $5,170 in federal tax alone — plus whatever your state charges. The investment then grows tax-deferred until retirement.
What is a Traditional IRA and who should use it?
A Traditional IRA allows up to $7,000 in pre-tax contributions per year ($8,000 if age 50+), reducing your taxable income just like a 401(k). The catch: if you or your spouse has a workplace retirement plan and your income exceeds certain limits, the deduction phases out. Best for those without a workplace plan or lower earners.
Why do UK taxes look different from US taxes?
National Insurance sits on top of income tax, but the NHS is funded through it — factor in what Americans separately pay for health insurance ($5,000–$20,000/year for many families). The true comparison is more balanced than it first appears.
How does Canada's RRSP reduce my taxes?
Every RRSP dollar reduces your taxable income. At a 33% combined marginal rate in Ontario, a CA$10,000 RRSP contribution saves approximately CA$3,300 immediately — and the investment grows tax-free until withdrawal.
What is the UK's 60% tax trap?
Above £100,000, the personal allowance tapers — losing £1 for every £2 earned over the threshold. This creates an effective 60% marginal rate between £100,000 and £125,140. Pension contributions are commonly used to bring income below this threshold.
How does Australia's Medicare Levy Surcharge work?
Earning above $93,000 without private hospital cover adds 1–1.5% MLS on your total income. For many at this level, basic private hospital cover ($1,000–1,500/year) is cheaper than paying the surcharge itself.