Methodology · Transparent by Design

Every calculator’s formula, assumption, and limit — in the open.

Every calculator on BillCraft ships with the exact formula it uses, the assumptions baked in, the known limitations, and the statutory section or circular it derives from. If you can’t reproduce our number with a calculator and a copy of the Bare Act, we’ve failed.

Last reviewed · 8 June 2026 Basis · Finance Act 2025 / Budget 2026 Editor · Prashant

01 — Income Tax Calculator

Old vs New Regime, with surcharge, cess, 87A rebate, and marginal relief.

Computes the total tax payable for an individual resident under both the Old Regime (with deductions under Chapter VI-A) and the New Regime under section 115BAC, applying slab tax, surcharge with marginal relief, and Health & Education Cess.

Formula (skeleton)

slab_tax       = sum_over_slabs(min(income, slab_top) - slab_bottom) * rate

rebate_87A     = min(slab_tax, ₹25,000 if income ≤ ₹7,00,000 (New) / ₹12,500 if ≤ ₹5,00,000 (Old))

surcharge      = surcharge_rate(income) * (slab_tax - rebate_87A)

marginal_relief = max(0, (tax_with_surcharge) - (tax_at_threshold + (income - threshold)))

cess           = 4% * (slab_tax - rebate_87A + surcharge - marginal_relief)

total_tax      = slab_tax - rebate_87A + surcharge - marginal_relief + cess

Assumes: Resident individual; not HUF, firm, or company. Surcharge thresholds ₹50L / ₹1Cr / ₹2Cr / ₹5Cr (capped at 25% under New Regime). 87A rebate as per Finance Act 2025.
Excludes: Special-rate income (LTCG/STCG taxed at 10/12.5/15/20%); these need the Capital Gains Calculator. Section 89 relief on arrears. AMT under section 115JC.
Sources: Income Tax Act 1961, sections 2(29C), 87A, 115BAC, First Schedule of Finance Act 2025. CBDT FAQs on 115BAC.

Known limitation: the calculator does not model Section 89 relief on salary arrears. If you received arrears for an earlier year, your actual tax may be lower — consult Form 10E.

02 — EMI Calculator

Standard amortising loan EMI, monthly compounding.

Computes the equated monthly instalment for a fully-amortising loan with a fixed interest rate, plus a month-by-month breakdown of principal vs interest and outstanding balance.

Formula

EMI = P × r × (1 + r)^n  /  ((1 + r)^n − 1)

where:
P = principal (₹)
r = annual_rate / 12 / 100   (monthly rate in decimal)
n = tenure_years × 12       (number of months)

Assumes: Fixed rate of interest for the whole tenure. Monthly compounding (standard for retail loans in India). No prepayment, no rate reset.
Excludes: Processing fees, GST on processing fees, insurance bundled with loan, MODT charges, statutory documentation charges. Floating-rate scenarios — these change EMI on every benchmark reset and are not modelled.
Sources: Standard amortisation formula; verified against RBI’s consumer EMI calculator and the published EMI tables on major bank websites for representative principal/rate/tenure combinations.

03 — FD Calculator

Quarterly compounding (Indian banks’ default), with TDS and senior citizen bonus.

Computes Fixed Deposit maturity, total interest, and TDS deducted, with an optional +0.5% senior citizen bonus on the rate.

Formula

maturity = P × (1 + r / n)^(n × t)

where:
P = principal
r = annual_rate / 100 (after senior bonus, if any)
n = compoundings per year (4 = quarterly, default)
t = tenure in years (fractional months supported)

Assumes: Quarterly compounding (the default at all major Indian banks). Senior citizen toggle adds +0.5 percentage points to the rate. TDS at 10% on interest exceeding ₹40,000 / FY (₹50,000 for seniors), aggregated across the bank.
Excludes: Premature withdrawal penalty (typically 0.5–1% reduction on the contracted rate). Bank-specific super-senior rates (80+) — user must enter the rate manually.
Sources: RBI Master Direction on Interest Rate on Deposits, 2016 (updated). Income Tax Act section 194A for TDS threshold. Output cross-checked against HDFC, SBI, ICICI, and Axis bank’s own FD calculators.

04 — RD Calculator

Recurring deposit with quarterly compounding.

Computes Recurring Deposit maturity for a fixed monthly deposit, with quarterly compounding as per Indian banking convention.

Formula

maturity = sum over months m of  P × (1 + r/4)^(4 × (T − m + 1)/12)

where:
P = monthly deposit
r = annual rate (decimal)
T = tenure in months
m = month of deposit (1 to T)

Assumes: Deposit made at the start of each month. Quarterly compounding. Interest accrues from the deposit date.
Excludes: Penalty for missed instalments. TDS computation (RDs were brought under TDS via Finance Act 2015; we show interest income but do not pre-deduct TDS in the maturity figure — user should subtract slab-rate tax separately).
Sources: RBI Master Direction on Interest Rate on Deposits, 2016. Income Tax Act section 194A. Verified against SBI’s own RD calculator.

05 — NPS Calculator

National Pension System corpus, pension, and 60% lumpsum split.

Computes NPS corpus at retirement based on monthly contribution, expected return, and tenure; then splits 60% as tax-free lumpsum and 40% mandatory annuity, with monthly pension at an assumed annuity rate.

Formula

corpus  = P × ((1 + r)^n − 1) / r × (1 + r)   (annuity-due, monthly)
lumpsum = 0.60 × corpus                                (tax-free under section 10(12A))
annuity = 0.40 × corpus
pension = annuity × annuity_rate / 12

Assumes: Contribution made at the start of each month (annuity-due). Constant expected return throughout accumulation. Default split 60% lumpsum / 40% annuity at exit (the regulatory minimum).
Excludes: NPS Tier-II withdrawals (taxable). Section 80CCD(1B) extra ₹50k deduction — that’s a tax-side benefit, not a corpus-side one. Variable returns across NPS Auto-choice glide-path; we assume single CAGR.
Sources: PFRDA regulations on exit and withdrawal. Income Tax Act section 10(12A) for 60% tax-free lumpsum. Section 80CCD for tax deduction (used in Income Tax Calculator, not here).

06 — PPF & SIP Calculator

Long-horizon disciplined savings with compound growth.

PPF mode computes a 15-year PPF maturity at the current government-notified rate (compounded annually). SIP mode computes mutual-fund SIP corpus given monthly investment, expected return, and tenure.

Formula — PPF (annual deposit at year start) maturity = sum over year y from 1 to 15 of A × (1 + r)^(15 − y + 1) Formula — SIP (monthly, end-of-month convention)

FV = P × ((1 + i)^n − 1) / i  × (1 + i)

where:
P = monthly investment, i = annual_rate/12/100, n = months

Assumes: PPF: annual deposit at the start of each financial year (best-case for the investor). Quarterly-notified rate held constant for the full 15 years (a simplification — the rate is reviewed each quarter).
Excludes: PPF: real interest computation uses monthly-minimum-balance method per PPF Rules 2019, not pure annual compounding. Our simplification slightly over-estimates maturity for investors who do not deposit by 5th of the month. This is disclosed in the calculator UI.
Sources: PPF Scheme Rules 2019 (Ministry of Finance). Latest quarterly interest-rate notification. SIP formula is the standard future-value-of-annuity, verified against AMFI’s SIP calculator.

07 — Capital Gains Calculator

STCG, LTCG, and the post-23-July-2024 rules.

Computes Short-Term and Long-Term Capital Gains tax across listed equity, equity mutual funds, debt mutual funds (post April 2023), gold, real estate, and unlisted shares, applying the rule-set in force on the transaction date.

Formula (listed equity, transaction on or after 23 July 2024)

gain    = sale_value − cost_of_acquisition − transfer_expenses
STCG    = gain × 20%        (section 111A, post 23-Jul-2024)
LTCG    = max(0, gain − ₹1,25,000) × 12.5%   (section 112A, post 23-Jul-2024)
cess    = 4% on the tax

Assumes: Listed equity / equity MF taxed under sections 111A / 112A. Real estate LTCG after 23-Jul-2024 at 12.5% without indexation OR (for property acquired before 23-Jul-2024) the new “lower of 12.5% no-indexation vs 20% with-indexation” rollback per Budget 2024 amendment.
Excludes: Section 54 / 54EC / 54F exemptions on re-investment — user must apply these manually after computing gross gain. Grandfathering for pre-31-Jan-2018 equity (we support this as a separate input, not an assumed default). Foreign-share / RSU taxation.
Sources: Income Tax Act sections 45, 48, 111A, 112, 112A. Finance (No. 2) Act 2024 amendments effective 23 July 2024. CBDT Circular No. 09/2024 on the new LTCG regime.

08 — Gratuity Calculator

Payment of Gratuity Act 1972, covered and non-covered formulas.

Computes gratuity payable on retirement / resignation / superannuation, applying the Payment of Gratuity Act 1972 formula for covered employees and the alternative formula for non-covered employees.

Formula — covered (Act-governed) employees

gratuity = (15 / 26) × last_drawn_salary × years_of_service

(last_drawn_salary = basic + DA, monthly)
(year fraction ≥ 6 months counted as full year)

Formula — non-covered employees gratuity = (15 / 30) × (avg_basic_DA_last_10_months) × completed_years

Assumes: Minimum 5 years of continuous service (else not eligible, except death/disablement). “Salary” = basic + DA only, as per the Act — not gross salary.
Excludes: Excess gratuity beyond the ₹20 lakh tax-free ceiling (per Income Tax section 10(10)(ii)) — we show the gross figure and flag the taxable portion separately.
Sources: Payment of Gratuity Act 1972, sections 2(s), 4, 4(2). Income Tax Act section 10(10). CBDT notification raising tax-free limit to ₹20 lakh effective 29 March 2018.

09 — GST ITC Calculator

Net GST liability after input tax credit set-off rules.

Computes net GST payable after applying the CGST / SGST / IGST set-off priority rules under sections 49, 49A, and 49B of the CGST Act, including Rule 88A’s clarification on IGST utilisation.

Set-off priority (post Rule 88A)

1. IGST output  ← IGST credit, then CGST credit, then SGST credit
2. CGST output  ← IGST credit (remaining), then CGST credit
3. SGST output  ← IGST credit (remaining), then SGST credit
4. No cross-utilisation between CGST and SGST
5. Remaining IGST credit must be fully used before any CGST/SGST credit, per Rule 88A

Assumes: Registered regular taxpayer (not composition). All ITC claimed is eligible (i.e., not blocked under section 17(5)). Inputs and outputs are for the same return period.
Excludes: Reverse-charge mechanism (RCM) computations — treated as separate output liability. Cess on luxury / sin goods. Refund of unutilised ITC (export / inverted-duty cases) — user must claim separately.
Sources: CGST Act 2017, sections 16, 17, 49, 49A, 49B. CGST Rules, Rule 88A inserted by Notification 16/2019-CT. CBIC Circular No. 98/17/2019-GST on order of utilisation.

10 — Stamp Duty Calculator

State-wise property stamp duty and registration fee.

Computes stamp duty and registration fee payable on a property purchase, applying the slab applicable in the chosen state and a women-buyer concession where the state offers one.

Formula (illustrative — Maharashtra)

stamp_duty   = max(consideration, ready_reckoner_value) × state_rate
(state_rate varies; Maharashtra: 5% urban / 6% metro / 4% rural; women −1% in some states)
registration = min(1% × value, state_cap)
(many states cap registration at ₹30,000)

Assumes: Sale-deed registration for a first-sale residential property. State rate-card as published on the state revenue department’s portal at the date of our last review.
Excludes: Gift-deed, lease-deed, mortgage-deed, partition-deed, family-settlement — each has its own rate-card. Stamp duty on share transfers / instruments outside immovable property.
Sources: Indian Stamp Act 1899 + the state-specific Stamp Act of the chosen jurisdiction. State revenue/IGR portal published rates. Updated when any state issues a fresh notification (verified annually).

11 — Why this matters

Methodology transparency = trustworthy results.

Most online finance calculators in India will not tell you their formula, their compounding convention, their TDS assumption, or which section of the Bare Act they derive from. That is fine when the calculator is right; it is a disaster when it is wrong and you can’t tell which.

We publish the formula so that:

You can audit us. If you have a copy of the Finance Act and a piece of paper, you can reproduce our number and tell us if we are wrong — and we will fix it per our 48-hour corrections policy.
You know what we are not computing. Many calculators silently leave out surcharge, marginal relief, 87A rebate, cess, or section 89 relief. We list these explicitly under “Excludes”.
You can use the result responsibly. An approximation is fine for a sanity check before filing — it is not a substitute for the official portal’s computation at submission time. Our calculators are second opinions, not the gospel.

If a calculator’s formula or assumption is wrong, write to us at supportbillcraft@gmail.com with the input set and the expected output. We will re-test, fix if needed, and add a dated correction note to the affected page.

For our full editorial workflow — sourcing hierarchy, verification protocol, correction timeline, AI disclosure — see Editorial Standards. For the author bio and credentials, see the Author page.