Does this marginal-slab system apply to all types of income?

It typically applies to regular income like salary; some income types (like certain capital gains) are taxed under separate flat-rate rules rather than the marginal slab system, check your specific jurisdiction's rules for how each income type is classified.

Why do tax slabs usually include a tax-free threshold at the bottom?

The 0% bottom slab (a basic exemption limit) ensures a minimum amount of income isn't taxed at all, intended to leave low earners with more of their income for essential expenses before any tax applies.

Can deductions change which slab my income falls into?

Yes, most deductions (retirement contributions, certain insurance premiums, etc.) reduce your taxable income before slabs are applied, so a large enough deduction can genuinely move part of your income out of a higher slab, not just reduce the tax on income that's already there.

How Income Tax Slabs Actually Work (Your Whole Salary Isn't Taxed at One Rate)

A common misunderstanding is thinking that crossing into a higher tax slab means your entire income suddenly gets taxed at that higher rate. It doesn't, tax slabs are marginal: each slab's rate only applies to the portion of income that falls within that specific range, not to your whole income once you cross the threshold.

A worked example with three slabs

Say a simplified system taxes income 0-3 lakh at 0%, 3-6 lakh at 5%, and 6-9 lakh at 10%. Someone earning ₹7 lakh doesn't pay 10% on the full ₹7 lakh. Instead: the first ₹3 lakh is taxed at 0% (₹0), the next ₹3 lakh (from 3 to 6 lakh) is taxed at 5% (₹15,000), and only the remaining ₹1 lakh (from 6 to 7 lakh) is taxed at 10% (₹10,000). Total tax: ₹25,000, an effective rate of about 3.6% on the full ₹7 lakh, far below the 10% "bracket" rate.

This is exactly what the Income Tax Calculator computes automatically, applying each slab's rate only to the income within that slab and summing the results, so you get both the total tax and your true effective rate.

Marginal rate vs. effective rate

Your marginal rate is the rate applied to your next rupee of income, the rate of the highest slab you've reached. Your effective rate is your total tax divided by your total income, always lower than your marginal rate once more than one slab is involved. When deciding whether extra income (a bonus, freelance work, a raise) is "worth it" after tax, the marginal rate is the relevant number, since that's the rate applied to that specific additional income, not your effective rate on everything you already earn.

Why this matters for salary negotiations and bonuses

Because only the incremental income above a threshold is taxed at the higher rate, a raise or bonus that pushes you into a new slab never results in take-home pay lower than before the raise, a common (and mathematically impossible, under a marginal system) fear. It's worth running the numbers on the Income Tax Calculator before and after a raise to see the actual net increase in take-home pay, which is always positive even after the higher marginal rate applies to the new portion.