Certified Formula Registry
Every calculator on this site runs on the formulas below ā each cited to a primary source (IRS, CFPB, HMRC, US Department of Labor, BLS, SEC) so you can verify the math yourself.
When a tax band, rate table, or statutory formula changes, we update the source data and record it in the changelog below.
Loan Amortization (EMI)
M = P Ć [r(1+r)^n] / [(1+r)^n ā 1]M = monthly payment Ā· P = principal Ā· r = monthly rate (annual Ć· 12) Ā· n = number of payments
Compound Interest
A = P(1 + r/n)^(nt)A = final amount Ā· P = principal Ā· r = annual rate (decimal) Ā· n = compounds per year Ā· t = years
Future Value of a Series (SIP / recurring investment)
FV = PMT Ć [(1+r)^n ā 1] / rFV = future value Ā· PMT = periodic investment Ā· r = periodic rate Ā· n = number of periods
Required Minimum Distribution
RMD = Account balance (Dec 31 prior year) Ć· IRS distribution periodDistribution period from the IRS Uniform Lifetime Table (26.5 years at age 73, decreasing with age)
Capital Gains Tax
Tax = (Sale price ā Cost basis) Ć applicable rateLong-term rates (held >1 year): 0% / 15% / 20% by taxable income. Short-term: ordinary income rates.
Overtime Pay
OT pay = hourly rate Ć 1.5 Ć overtime hours (federal minimum)FLSA requires ā„1.5Ć the regular rate for hours over 40/week for non-exempt employees
UK Stamp Duty (SDLT)
SDLT = Ī£ (portion of price in band Ć band rate)England/NI residential bands 2025/26: 0% to Ā£125k, 2% to Ā£250k, 5% to Ā£925k, 10% to Ā£1.5M, 12% above. +5% surcharge for additional properties.
PPF Maturity
Yearly: balance = (balance + deposit) Ć (1 + r)r = government-declared PPF rate (7.1% as of 2025) Ā· 15-year minimum term Ā· annual compounding
Gratuity (India)
Gratuity = (15 Ć last drawn salary Ć years of service) Ć· 26Salary = basic + DA Ā· applies to employees covered by the Payment of Gratuity Act, 1972 Ā· capped at ā¹20 lakh
Inflation / Purchasing Power
Future cost = Amount Ć (1 + i)^t Ā· Present value = Amount Ć· (1 + i)^ti = annual inflation rate (decimal) Ā· t = years
Net Present Value
NPV = Ī£ [CFā Ć· (1+r)^t] ā CāCFā = cash flow in year t Ā· r = discount rate Ā· Cā = initial investment
HSA Contribution Limits (2026)
Limit = $4,400 self-only / $8,750 family (+$1,000 catch-up at 55+) Ā· Tax savings = contribution Ć marginal rateHDHP required: min deductible $1,700/$3,400 Ā· max out-of-pocket $8,500/$17,000. Employer contributions count toward the limit.
FSA & Dependent Care FSA Limits (2026)
Health FSA = $3,400 (carryover $680) Ā· Dependent Care FSA = $7,500 ($3,750 married filing separately)DCFSA raised from $5,000 by the One Big Beautiful Bill Act, effective Jan 1 2026 ā first increase since 1986; not inflation-indexed.
HELOC Line & Payments
Max line = (home value Ć CLTV cap) ā mortgage balance Ā· Draw payment = balance Ć r/12 Ā· Repayment = amortization formulaCLTV = combined loan-to-value, typically capped at 80ā85%. Draw period is interest-only; repayment period fully amortizes.
4% Rule / Safe Withdrawal
Annual income = portfolio Ć withdrawal rate Ā· Monthly income = balance Ć (rate Ć· 100) Ć· 12The 4% guideline derives from the Trinity Study of historical 30-year retirement periods
Changelog
July 2026
Added 2026 HSA limits ($4,400/$8,750, Rev. Proc. 2025-19), 2026 FSA limits ($3,400/$680 carryover, Rev. Proc. 2025-32), the OBBBA dependent-care FSA increase to $7,500, and HELOC line/payment formulas.
July 2026
Added RMD (SECURE 2.0 ages), 2025 capital gains brackets, FLSA overtime, NPV/IRR, and SCF 2022 net worth percentile data.
June 2026
Added UK SDLT 2025/26 bands, PPF 7.1% rate, India gratuity formula (Act of 1972).
May 2026
Initial registry ā amortization, compound interest, future value of a series, inflation, 4% rule.
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Disclaimer: These formulas are provided for informational and educational purposes only and are not financial, tax, or legal advice. Tax bands, statutory rates, and contribution limits change; always verify against the cited primary source before relying on a result. See our Terms of Service.