How every number is computed

• India-calibrated, not generic. Every assumption — tax slabs, PPF rate, HRA Section 10(13A) rules, equity-MF historical ranges — is the Indian rulebook for FY 2025-26. Global templates that pretend India is "developed-market average" are wrong here.
• Open math.Every calculator’s state and formula lives in src/hooks/use-*.ts and src/lib/*-math.ts in the public repo. The source is the methodology.
• Educational only. Dhan Metrics is not a SEBI-registered investment adviser. Projections assume your inputs are accurate and that markets behave like their long-run averages — neither is guaranteed. Always consult a qualified adviser for material decisions.
• Defaults are honest. Where we pre-fill a return rate or contribution, we use a historical Indian average (e.g. 12% for equity MF, 7.1% for PPF at the time of writing), not a marketing number.

A Systematic Investment Plan compounds monthly contributions at the expected return rate. We use the standard annuity-due formula (contributions at the start of each month, not the end), which matches how most Indian MF SIPs auto-debit:

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

• P = monthly SIP amount
• r = monthly rate = annual return ÷ 12 ÷ 100
• n = number of months = years × 12
• E = existing portfolio (compounds alongside)
• FV = projected future value

The chart points are computed year-by-year using the same formula, which is why the "crossover year" callout you see on the SIP page is real math, not eyeballing — it’s the first year where compound growth exceeds total contributions.

Lumpsum uses the simpler FV = P × (1 + r)^n (monthly compounding, no contributions).

EMI uses the standard reducing-balance formula that every Indian retail loan follows — home, car, personal, education:

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

• P = loan principal
• r = monthly rate = annual rate ÷ 12 ÷ 100
• n = tenure in months

Total interest = (EMI × n) − P. The "1 extra EMI per year" what-if uses the well-known rule-of-thumb (~18% interest saved on a typical home loan) rather than a full prepayment simulation, so treat that tile as directional. Floating-rate loans, processing fees, insurance and prepayment charges are not included.

Loan Affordability uses net disposable × 30% (a conservative FOIR-style ceiling that protects savings + emergency headroom); Loan Eligibility uses a higher FOIR of 40–50% to match what banks actually approve.

PPF compounds annually with contributions at the start of each financial year (annuity-due, n = 15):

Maturity = A × ((1 + r)^n − 1) × (1 + r) / r

• A = annual contribution (capped at ₹1.5L for 80C)
• r = Government-declared annual rate
• n = years of contribution

NSC and KVP use simpler compound interest M = P × (1 + r)^n. Sukanya Samriddhi follows PPF math for the 15-year contribution window, then compounds (no further contributions) for 6 more years to the 21-year maturity.

Rate source. Defaults reflect the latest Government-declared quarterly rate at the time of writing (PPF 7.1%, NSC 7.7%, KVP 7.5%, SSY 8.2%). These revise every quarter — the calculator uses whatever you enter, so if rates change before you check again, edit and re-run.

EEE status. PPF, SSY and EPF are exempt-exempt-exempt — contribution under 80C, accrued interest tax-free, maturity tax-free. NSC accrual is taxable annually (though reinvested interest counts toward 80C in that year). KVP interest is fully taxable and not 80C-eligible.

The Income Tax calculator implements the FY 2025-26 New Regime slabs (Budget 2024-25, applicable AY 2026-27):

• 0 – ₹4L → 0%
• ₹4L – ₹8L → 5%
• ₹8L – ₹12L → 10%
• ₹12L – ₹16L → 15%
• ₹16L – ₹20L → 20%
• ₹20L – ₹24L → 25%
• Above ₹24L → 30%

Plus: ₹75K standard deduction, Section 87A rebate (up to ₹60K, NIL tax up to ₹12L taxable income), surcharge at 10/15/25%, 4% health & education cess. Old Regime Comparison adds 80C/80D/HRA/home-loan interest deductions over the (lower) old slabs with ₹50K standard deduction.

What we don’t include. Capital gains (LTCG/STCG), other-source income, agricultural rebate carry-forward, and specific industry exemptions are calculator-specific — each lives on its own page. Marginal relief on surcharge thresholds IS applied; capital gains separation is on the roadmap.

The exempt portion of your House Rent Allowance is the smallest of three rules:

1. Actual HRA received
2. Rent paid minus 10% of basic salary
3. 50% of basic salary (metro) or 40% (non-metro)

Whichever is smallest, that’s your exempt amount — the rest gets added back to taxable salary. Metros for this purpose are Mumbai, Delhi, Kolkata, Chennai (note: Bengaluru, Hyderabad, Pune are non-metro by tax law, even though they’re metros by every other measure). HRA exemption is available only under the Old Regime — choosing New forfeits it.

The Retirement Planner uses three steps:

1. Inflate expensesfrom today’s monthly figure to retirement-age value using the entered inflation rate.
2. Required corpus at retirement = corpus that can fund inflated annual expenses for the remaining years, at the post-retirement return rate. This is a real-rate annuity calculation, not a simple ×25 multiplier.
3. Monthly SIP needed is solved from the future-value formula given your years-to-retire, pre-retirement return and existing portfolio.

FIRE uses the 4% rule (or any withdrawal rate you set): FIRE number = annual expenses ÷ withdrawal rate. Years-to-FIRE is simulated month-by-month (savings + portfolio compounding) so the answer is exact, not a closed-form approximation.

Indian caveat.The 4% rule was derived from US data; Indian inflation has historically been higher (~5–7% vs US ~2–3%), so many planners use 3–3.5% for a safer SWR. The calculator lets you pick — but don’t blindly trust 4% for an Indian retirement.

All fund-analytics calculators (Past Returns, Rolling Returns, Drawdown, Risk Analysis, Sharpe, Sortino) pull real NAV history from AMFI — the industry source-of-truth for Indian mutual fund prices.

• CAGR over a window = (NAV_end / NAV_start)^(1/years) − 1.
• Rolling returnsapply CAGR across every possible start date in the dataset, then report min / median / max — the version of "average return" that isn’t cherry-picked.
• Drawdown tracks the running peak NAV; max drawdown = largest peak-to-trough fall as a percentage. Recovery time is months from trough back to the previous peak.
• Sharpe ratio = (annualised CAGR − risk-free rate) ÷ annualised volatility. We default risk-free to 7% (long-term G-Sec).
• Sortino ratiouses only downside deviation (returns below the risk-free rate) instead of total volatility — matches investor intuition that upside vol shouldn’t be penalised.

• Constant rates. Most projections assume your entered return / inflation rate holds for the full horizon. Real markets and inflation oscillate. Re-run every 1–2 years.
• Tax-on-redemption. Future values are pre-tax unless the instrument is EEE (PPF, SSY). For equity MFs, LTCG of 12.5% applies above ₹1.25L per FY (FY 2025-26).
• Sequence-of-returns risk. SWP and FIRE projections use a constant return, but real withdrawals in a down-market year deplete corpus faster. Monte Carlo and Stress Test calculators are built for this.
• Not advice. Dhan Metrics is not a SEBI-registered investment adviser. Every calculator is educational. Consult a qualified adviser for any material decision.