Tax computation: Slab rates, surcharge, cess, AMT, and the complete calculation

This lesson takes you through the actual mathematics of computing your Indian income tax — line by line, from Total Income through the final number you owe (or get refunded). Lessons 3, 4, and 5 established what goes into the computation; Lesson 6 shows you how the computation actually works.

Most filers rely on tax calculators or software that produce the final number without showing intermediate steps. That works fine until something goes wrong — a notice from the IT Department, a refund delay, a discrepancy with employer TDS — and then you need to understand the math yourself. This lesson is the reference for understanding what the calculator is doing.

We cover the standard slab calculation, surcharge with marginal relief at four different thresholds, the 4% cess, and the Alternative Minimum Tax (AMT) provisions that affect certain non-corporate filers. We also address special-rate incomes (lottery, online gaming, casual income) that are computed separately from slab-based income.

A reminder on terminology: the Income Tax Act 2025 (effective April 1, 2026) renumbers many sections — but for FY 2025-26 income (the current year), the 1961 Act sections apply. We use 1961 Act references throughout.

Every Indian tax computation follows the same sequence. The order matters because each step's output feeds into the next.

Why the order matters. Take a specific example: someone with ₹13 lakh salary and ₹2 lakh LTCG on equity.

If you incorrectly add the LTCG to slab-rate income before computing tax, you'd treat ₹15 lakh as slab-rate income. But Section 112A LTCG is computed at 12.5%, NOT slab rates. The correct sequence keeps them separate, applies the right rates to each, then sums up.

This is one of the most common errors filers make. Tax software handles this automatically, but understanding the manual sequence helps you verify the output.

Indian income tax uses a progressive slab system — different portions of your income are taxed at different rates, NOT all at one rate.

The critical distinction: marginal rate vs effective rate.

Marginal rate. The rate that applies to your NEXT rupee of income. If your taxable income is ₹15 lakh in the New Regime, your marginal rate is 15% (the rate in the ₹12-16 lakh slab).

Effective rate. Your total tax divided by your total income. Always lower than the marginal rate because earlier slabs were taxed at lower rates.

Common misconception. "I'm in the 30% tax bracket" doesn't mean you pay 30% of your total income as tax. It means the portion of your income above the 30% threshold is taxed at 30%, while earlier portions were taxed at progressively lower rates.

Marginal rate: 20% (next rupee taxed at 20%) Effective rate: ₹2,00,000 / ₹20,00,000 = 10%

The effective rate is exactly half the marginal rate in this case — a useful sanity check.

Marginal rate: 30% Effective rate: ₹4,12,500 / ₹20,00,000 = 20.6%

Note how Old Regime's higher rates at every level produce more than double the tax at ₹20 lakh income — explaining why deductions matter so much for Old Regime to be competitive.

Above ₹50 lakh income, surcharge adds to your tax bill. Surcharge is a percentage applied on top of the tax amount (not on income).

When income just barely crosses a surcharge threshold, marginal relief prevents disproportionate tax jumps. This relief is significant for high-income filers near the boundaries.

The principle. Tax + surcharge after crossing a threshold cannot exceed: (Tax at threshold income) + (Income above threshold).

In other words, the additional tax burden from crossing the threshold can never exceed the additional income earned beyond that threshold.

Health and Education Cess is a small but universal addition to the tax bill.

The rate. 4% on (Tax + Surcharge after marginal relief).

Applies to.

• Both Old and New Regimes
• All income levels (no exemption)
• All income types (slab-based and special-rate)

Computation example. If your tax + surcharge after all reliefs is ₹3,00,000: Cess = ₹3,00,000 × 4% = ₹12,000 Final tax = ₹3,12,000

What the cess funds. Officially, the cess revenue is earmarked for health and education programs. Though formally a "cess," it functions as a tax addition with no exemption.

Distinguishing cess from surcharge. Different things:

• Surcharge: Applies only above ₹50 lakh; rates vary by bracket
• Cess: Applies to everyone; flat 4%

Both are applied AFTER computing base tax (after rebate, marginal relief, etc.).

Several income categories are taxed at special flat rates that bypass the slab system entirely. These must be computed separately.

AMT under Section 115JC affects non-corporate taxpayers (individuals, HUFs, LLPs, partnership firms) who claim substantial deductions that reduce their regular tax below a minimum threshold.

Who does AMT apply to.

AMT applies if you are:

• A non-corporate taxpayer (individual, HUF, partnership, LLP)
• Claiming deductions under:
• Section 10AA (SEZ deduction)
• Section 80H to 80RRB (various business-specific deductions)
• Or have other deductions reducing tax below a certain threshold

Important exclusion. AMT does NOT apply to most individual salaried filers. It primarily affects:

• Businesses claiming SEZ benefits
• Partnerships/LLPs with substantial profession deductions
• Individuals with significant business income claiming heavy deductions

The threshold. AMT does not apply if your "adjusted total income" is ₹20 lakh or less.

How AMT works.

1. Step 1: Compute Regular Tax. Compute tax as usual under Old or New Regime.
2. Step 2: Compute Adjusted Total Income. Adjusted Total Income = Total Income + Section 10AA deduction + Section 80H to 80RRB deductions (except 80P) + Section 35AD deductions
3. Step 3: Compute AMT. AMT = 18.5% of Adjusted Total Income + Surcharge + Cess
4. Step 4: Compare. Pay the HIGHER of Regular Tax or AMT.

AMT credit. If AMT exceeds regular tax in a year, the excess is allowed as credit to be carried forward up to 15 years and offset against regular tax in future years when regular tax exceeds AMT.

When income is clubbed from family members (minor child, spouse with transferred assets, etc.), it's added to your taxable income for computation purposes. Special considerations:

Minor child income clubbed under Section 64(1A).

• Add to parent's income (the parent with higher income)
• Section 10(32) exemption: ₹1,500 per child per year (max 2 children)
• Net clubbed amount goes into the parent's regular slab calculation
• Doesn't separately attract higher rate

Tax computed on the combined figure under parent's regime and slab rates.

Spouse's income from transferred assets.

• Add gross income (no exemption equivalent to ₹1,500)
• Goes into your regular slab calculation
• Calculate using your regime

No special tax rate for clubbed income. It's treated as your income at your applicable slab/rate.

The Income Tax Act specifies rounding rules that produce the final number.

Section 288A — Rounding of total income. Total income is rounded off to the nearest multiple of ₹10. Amount of ₹5 or more is rounded up; less than ₹5 rounded down. Example: Total income of ₹15,67,847 is rounded to ₹15,67,850.

Section 288B — Rounding of tax payable. Tax amount is similarly rounded to the nearest multiple of ₹10. Example: Tax of ₹2,34,567 is rounded to ₹2,34,570.

Practical implication. These rules produce small variations between manual calculation and tax software output. Always round at the specified stages. Don't round intermediate amounts (paise should be tracked through the calculation; rounding happens at income and tax stages only).

For e-filing. The portal handles rounding automatically per these rules. Your figures may differ slightly from rough manual calculations — this is correct.

The official calculator. Available at incometax.gov.in (Calculators tab). Input your income figures, choose regime, get verified computation. Use this to cross-check any manual calculation or third-party software output before filing.