How Income Tax Is Calculated: Brackets, Deductions and Withholding

Progressive means marginal

Almost every developed country taxes income progressively, which means the rate rises in bands as income rises. But only the income inside each band is taxed at that band's rate. This is the single most misunderstood idea in personal finance, and the misunderstanding is always in the same direction: people believe a pay rise into a higher bracket taxes everything at the higher rate.

It does not, and there is no income at which earning more leaves you with less after income tax. What can create that effect is means-tested benefits withdrawing as income rises, or a student loan repayment threshold. Which are separate systems layered on top of the tax scale.

The consequence worth remembering is that your marginal rate, the rate on your next dollar, is always higher than your average rate. Decisions at the margin, like whether to take extra work or claim a deduction, are priced at the marginal rate. Your overall burden is the average one.

Taxable income is not your salary

Tax is calculated on taxable income, which is your assessable income minus allowable deductions. Deductions are expenses incurred in earning that income: tools, work-related travel between sites, professional memberships, self-education tied to your current role, and a share of home office costs.

A deduction is worth your marginal rate, not its face value. A $1,000 deduction saves $300 at a 30% marginal rate and $450 at 45%. It is a reduction in taxed income, not a refund of the expense. Spending money purely to get a deduction always leaves you worse off in cash terms.

Offsets work differently. They reduce the tax itself rather than the income, so a $700 offset is worth $700 to anyone who qualifies regardless of their rate. Some offsets are refundable and can produce a payment; most are not and can only reduce tax to zero.

Withholding, and why refunds exist

Employers withhold tax from each pay using schedules published by the tax authority. Those schedules estimate your annual liability by assuming your current pay continues all year, which is an approximation rather than a calculation.

At the end of the year the return works out the actual liability, and the difference between it and what was withheld is your refund or your bill. A refund is therefore not a bonus. It is a return of money withheld in excess during the year, typically because you had deductions the schedules could not know about, or worked part of the year.

Bills usually have a specific cause: a second job where the tax-free threshold was claimed twice, income with no withholding such as interest or a side business, a capital gain, or a study loan the employer was not told about. Each is fixable once identified, which is why reconciling the numbers is worth the hour it takes.

Payroll taxes are a separate layer

Income tax is rarely the only deduction. In the United States, Social Security and Medicare together take 7.65% of wages from the employee, with Social Security stopping at a wage base, $184,500 for 2026, while Medicare continues without limit and adds a further 0.9% above high income thresholds.

In Australia, the 2% Medicare levy sits on top of income tax, with a surcharge for higher earners without private hospital cover. Superannuation is not a tax at all: it is a compulsory contribution paid by the employer on top of wages, and it does not reduce your take-home pay.

Confusing these layers is why gross-to-net calculations done in your head are usually wrong. A calculator that models each layer separately, and says which country's rules it follows, is the only reliable way to compare two offers.

Questions people ask

Will a pay rise push me into a higher tax bracket and cost me money?

No. Only the income above each threshold is taxed at the higher rate, so earning more always leaves you with more after income tax. What can reduce your net gain is a student loan repayment starting, or a means-tested benefit withdrawing. Both separate systems from the tax scale itself.

What is the difference between a deduction and an offset?

A deduction reduces the income you are taxed on, so it is worth your marginal rate. $1,000 saves $300 at a 30% rate. An offset reduces the tax owed directly, so it is worth its full face value to anyone who qualifies. Refundable offsets can produce a payment; non-refundable ones can only reduce tax to zero.

Why did I get a tax bill instead of a refund?

Your withholding was less than your actual liability. The usual causes are a second job with the tax-free threshold claimed twice, untaxed income such as interest or dividends, a capital gain, or a study loan your employer did not withhold for. Comparing your total liability against the total withheld on your income statement shows the gap immediately.

Is a big tax refund a good thing?

It is your own money coming back after being withheld in excess all year, so financially you would have been better off holding it. Many people prefer the forced saving, which is a legitimate preference. Just not a financial gain. If the refund is consistently large, adjusting your withholding gives you the money as you earn it.

Can I deduct working from home costs?

Usually yes, for the portion genuinely related to work: a share of electricity, internet, phone and the decline in value of equipment. Most tax authorities offer both a fixed rate per hour and an actual-cost method, and the fixed rate is simpler but often smaller. Occupancy costs like rent or mortgage interest are generally not deductible for employees.

Sources

• IRS Rev. Proc. 2025-32: inflation adjustments for tax year 2026. United States federal brackets and standard deduction
• ATO: Tax rates for Australian residents. Australian resident income tax rates