Income tax is a tax a government charges on income. Which receipts are taxable, what reduces the taxable amount, the rates, when you must file a return, and whether tax is taken from your pay before you receive it are all set by the law of the jurisdiction and the tax year in question. The core idea is simple. The mechanics are not, and the same salary can produce very different tax bills in different countries.
The short answer
Income tax is a levy on income, defined by the tax authority of the place where you are taxed. To work out what you owe, you need three things: the jurisdiction, the tax year, and the type of income. Tax is not calculated on gross receipts. Allowances, deductions and reliefs come off first, and the rate is then applied to what remains, often in bands rather than as one flat rate.
What income is taxed, and what is not
Taxable categories are defined by statute and by the authority’s guidance, not by a general idea of what “income” means. Two official sources show how different the lists can look.
United Kingdom
HM Revenue & Customs says Income Tax is “a tax you pay on your income.” Its overview lists items that are taxed, including employment earnings, self-employment profits, some state benefits, most pensions, rental income, job benefits, trust income, and savings interest above the relevant allowance. HMRC, Income Tax overview The same guidance notes that some income may be tax-free, through certain allowances or exempt accounts. Whether a particular item qualifies depends on the income type and on your eligibility for the allowance or relief that applies to it.
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United States (federal)
Section 61 of Title 26 of the U.S. Code, as published in the 2022 edition, states: “Except as otherwise provided in this subtitle, gross income means all income from whatever source derived, including (but not limited to) the following items.” The starting point is therefore broad, and the exceptions elsewhere in the subtitle decide what is actually excluded. U.S. Code, Title 26, section 61 Readers working from this text should confirm the current version, because exclusions change.
How the tax amount is calculated
Most systems follow the same sequence: start with income, subtract what the law allows, then apply rates to the remainder. The steps differ in detail, so the following is a general pattern rather than a formula for any single country.
- Add up income from all taxable sources for the tax year.
- Subtract the allowances, deductions or reliefs that apply to you.
- Apply the rate or rates set for that year to the remaining taxable income.
- Compare the result with any tax already withheld or paid, to find what is still due or what may be refunded.
Allowances come off before any tax is charged
In the UK, the standard Personal Allowance is £12,570 for the 2026–27 tax year, which runs from 6 April 2026 to 5 April 2027. HMRC, Income Tax rates and Personal Allowances This is a figure for that UK system and that year only. It is not a global threshold, and it may not apply in full to everyone, since eligibility and the treatment of some income types can change the amount available to a given person.
Bands: why the top rate does not apply to all your income
In systems with bands, each slice of taxable income is taxed at the rate for the band it falls into. Crossing into a higher band does not raise the rate on the earlier slices. The following example uses hypothetical rates, not those of any real country, to show the mechanics:
- Taxable income: 50,000
- First 20,000 at a hypothetical 10% = 2,000
- Remaining 30,000 at a hypothetical 20% = 6,000
- Total tax: 8,000, an average (effective) rate of 16% on the 50,000 taxable income
The 20% figure is the marginal rate, meaning the rate on the next pound or dollar. The 16% figure is the effective rate. The two should not be confused when comparing countries.
How the tax is collected
Collection is where many readers first meet income tax, because it is taken from pay before they see the money. Other income is often not taxed at source, and then the person must report it.
Withholding through the employer (PAYE and similar systems)
In the UK, many employees and pensioners pay through Pay As You Earn (PAYE). The employer or pension provider uses your tax code to calculate the deduction each pay period. HMRC, How you pay Income Tax In the United States, the equivalent is withholding from employee pay. The IRS describes federal income tax as pay-as-you-go: tax is paid as income is earned or received. IRS Tax Tip 2022-94, published June 21, 2022, is the source for that description. This explanation covers U.S. federal tax only, not state or local taxes.
Returns and estimated payments
Self-employed people, people with complex affairs, and people with income that has not been taxed at source may need to file a return. HMRC says that people with more complex affairs, including some self-employed people or those with higher income, may use Self Assessment. The filing triggers are specific UK thresholds, so check the current HMRC page before relying on any of them. In the United States, estimated payments may be required when withholding does not cover the tax due. IRS Tax Tip 2022-94
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Headline rates alone are a poor basis for comparison. A fair comparison starts with the jurisdiction and tax year, then looks at what income is taxed, which allowances apply, how the rate bands work, and how tax is collected. The table below uses official sources for three jurisdictions. Where a source does not state a value, the cell says so.
| Feature | United Kingdom | United States (federal) | India |
|---|---|---|---|
| Starting point for taxable income | Listed income types, with exemptions and allowances (HMRC) | Gross income from all sources, unless excluded by the Code (26 U.S.C. §61) | Five heads: salary, house property, business or profession, capital gains, and residual income (Income Tax Department) |
| Main allowance cited | Personal Allowance £12,570 for 6 April 2026 to 5 April 2027 | Not stated in the source reviewed | Not stated in the source reviewed |
| Main collection method | PAYE through employer or pension provider; Self Assessment for some | Employer withholding, with estimated payments where withholding is insufficient | Not stated in the source reviewed |
| Scope of this row | UK, tax year stated | U.S. federal only, current as of the 2022 Code edition and IRS 2022 guidance | India, per the Income-tax Act, 1961 as amended by the Finance Act, 2026, as the department’s page states |
The India column shows how income is grouped, not how much is taxed. Its page is cited here for that classification only. Income Tax Department, Treatment of income from different sources
Quick Recap
How to check your own position
- Confirm your jurisdiction and the tax year. A rate or allowance from last year, or from another country, can produce a wrong figure.
- Find your income type on the tax authority’s own page, and note any allowance or relief that may apply to it.
- Check how tax is collected for you: through the employer, through a pension provider, or through a return you must file yourself.
- In the UK, HMRC says you can check your payments and estimate your tax online for the stated tax year. HMRC, Check you’re paying the right amount If you cannot use those services, HMRC directs you to contact it, or you may seek help from an accountant.
Mistakes that produce a wrong estimate
- Using gross pay as the taxable base. Allowances and exemptions usually reduce it first.
- Treating the marginal rate as the rate on all income. Only the slice in each band is taxed at that band’s rate.
- Assuming an exemption applies to everyone. Eligibility for allowances and tax-free accounts depends on the individual.
- Applying U.S. federal rules to state taxes, or UK rules to any other country.
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