Quick Answer: India’s income tax formula is a progressive slab-wise calculation: each portion of your taxable income falling within a slab is taxed at that slab’s own rate, and the amounts are summed to get your total tax before a 4% Health and Education Cess is added. For FY 2026-27, the New Regime has 7 slabs (nil to 30%) while the Old Regime has 4 slabs (nil to 30%) but permits deductions.
Key takeaways:
- Tax is calculated slab-by-slab, not by applying one flat rate to total income.
- “Marginal rate” is the rate on your last rupee earned; “effective rate” is your total tax divided by total income.
- The New Regime has more, narrower slabs; the Old Regime has fewer, wider slabs plus deductions.
- A 4% cess applies to the computed tax amount in both regimes.
- Surcharge applies additionally above certain high-income thresholds, further increasing effective tax rates.
Many Indian taxpayers misunderstand how the “slab” system actually works, assuming that crossing into a higher bracket means their entire income is taxed at that higher rate. This article breaks down the real formula step by step and compares how the New and Old Tax Regimes apply it differently for FY 2026-27.
Expert insight: The most common myth in Indian tax planning is “I’ll earn less so I don’t cross into the next slab.” In a progressive system, crossing a slab boundary only affects the incremental income above that boundary — never your entire income.
The Core Slab-Wise Tax Formula
Total Tax = Σ (income within each slab × that slab’s rate)
This summation formula is applied slab by slab, from the lowest to the highest bracket your income reaches, before adding cess and any applicable surcharge.
Marginal Rate vs Effective Rate
| Term | Meaning | Example |
|---|---|---|
| Marginal Rate | The tax rate applied to your next (last) rupee of income | A person earning ₹18 lakh has a marginal rate of 20% under the New Regime |
| Effective Rate | Total tax paid divided by total taxable income | The same person’s effective rate would be well below 20%, since lower slabs were taxed at lower rates |
New Regime Formula Walkthrough (FY 2026-27)
For a taxable income of ₹18,00,000 under the New Regime:
| Slab | Amount in Slab | Rate | Tax |
|---|---|---|---|
| 0 – 4,00,000 | 4,00,000 | Nil | 0 |
| 4,00,001 – 8,00,000 | 4,00,000 | 5% | 20,000 |
| 8,00,001 – 12,00,000 | 4,00,000 | 10% | 40,000 |
| 12,00,001 – 16,00,000 | 4,00,000 | 15% | 60,000 |
| 16,00,001 – 18,00,000 | 2,00,000 | 20% | 40,000 |
Base tax = ₹1,60,000. Adding the 4% cess: ₹1,60,000 × 1.04 = ₹1,66,400 total tax payable.
Effective rate = 1,66,400 ÷ 18,00,000 × 100 ≈ 9.24% — noticeably lower than the 20% marginal rate.
Old Regime Formula Walkthrough (FY 2026-27)
For the same ₹18,00,000, but assuming ₹3,00,000 of eligible deductions (80C, standard deduction, etc.) bring taxable income to ₹15,00,000:
| Slab | Amount in Slab | Rate | Tax |
|---|---|---|---|
| 0 – 2,50,000 | 2,50,000 | Nil | 0 |
| 2,50,001 – 5,00,000 | 2,50,000 | 5% | 12,500 |
| 5,00,001 – 10,00,000 | 5,00,000 | 20% | 1,00,000 |
| 10,00,001 – 15,00,000 | 5,00,000 | 30% | 1,50,000 |
Base tax = ₹2,62,500. Adding 4% cess: ₹2,73,000 total tax payable — higher than the New Regime result in this case, illustrating why regime choice depends heavily on the size of one’s deductions.
Where the Cess and Surcharge Fit In
The Health and Education Cess of 4% is applied uniformly to the computed tax (post any rebate), regardless of regime. A separate surcharge applies on top of tax for individuals with taxable income above ₹50 lakh, with rates increasing at higher thresholds (₹1 crore, ₹2 crore, and beyond), as notified by the Finance Act each year.
Common Mistakes When Applying the Formula
- Confusing marginal rate with effective rate when estimating take-home pay.
- Applying the Old Regime’s deductions while calculating tax under the New Regime (most deductions aren’t allowed there).
- Forgetting to add the 4% cess as the final step.
- Ignoring surcharge implications for very high incomes.
Best Practices for Applying the Slab Formula
- Always calculate slab-wise, never apply a single flat rate to total income.
- Compute tax under both regimes before deciding which to opt for.
- Remember cess applies after the rebate, not before.
- Check the current Finance Act/Budget notification each year, since slabs can change.
Try it yourself: Skip the manual slab-wise maths with DigiToolkit’s free Tax Bracket Calculator.
Related tools & guides on DigiToolkit
- Try the free Tax Bracket Calculator →
- How to Calculate Your Income Tax Bracket in India (FY 2026-27)
- What Is a Tax Bracket? A Simple Guide for Indian Taxpayers
- Tax Bracket Calculator: Free Online Tool + Guide
- Income Tax Slab Examples for Beginners (FY 2026-27)
- How to Calculate an IBAN: Step-by-Step Guide (India)
- How to Add Page Numbers to a PDF for Free (No Software)
- More Tax & Salary guides
Conclusion
India’s income tax formula is a straightforward slab-by-slab summation once you separate marginal rate from effective rate. The New Regime’s narrower slabs generally favour taxpayers with fewer deductions, while the Old Regime can still be more efficient for those claiming significant 80C, HRA, or home loan benefits.
FAQs
What is the difference between marginal and effective tax rate?
Marginal rate is the rate on your last rupee of income, while effective rate is your total tax divided by your total income — effective rate is always lower than or equal to the marginal rate in a progressive system.
Does the cess apply before or after the Section 87A rebate?
The cess is calculated on the tax amount after the rebate has been applied.
Which regime has more tax slabs?
The New Regime has seven slabs for FY 2026-27, while the Old Regime has four.
Does surcharge apply to everyone?
No, surcharge only applies to individuals with taxable income above ₹50 lakh, with higher rates at further thresholds.
Can I switch between regimes every year?
Salaried individuals without business income can generally switch regimes each financial year when filing their return.
