Numbers that
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Finance, investment, and planning calculations — explained and demystified.
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Your Savings Goal Has Four Variables and You Pick Two
Each RD Instalment Is Its Own Deposit — and That Changes the Math
Each RD instalment earns interest only from the day it lands, so the effective return on your total deposit is roughly half the headline rate. The arithmetic explained.
Down 50%, Up 50%, Still Down 25%: Percentage Asymmetry
Down 50% then up 50% leaves you down 25%. Percentages multiply rather than add, and losing track of the base causes errors in pricing, VAT and risk reporting alike.
The Ten Days That Never Happened: October 1582 and the Limits of Date Calculators
Subtract days across October 1582 and the calendar swallows ten of them. Why calendar reform broke history — and how to interpret date calculations across the Julian–Gregorian divide.
71% of Your First Payment Is Interest: Reading the Schedule
In month one of a 25-year mortgage, 71% of your payment is interest. The crossover point and the shape of that curve explain every overpayment and refinancing decision.
Real vs Nominal: Plan in Today's Money, Not Future Money
A €600,000 pension in 25 years is worth about €324,000 today. Why real terms beats nominal for planning — and the cases where the nominal number is what matters.
Flat Rate vs Reducing Balance: The Same 7% Costs Double
A "7% flat rate" loan costs nearly double a 7% reducing-balance loan. Here's how to spot flat-rate quoting, what APR actually includes, and how to compare offers.
Why 31 January Plus One Month Has No Right Answer
Adding one month to 31 January has no correct answer, and "add one day" isn't "add 24 hours". The date arithmetic edge cases that break calendars and billing systems.
Your Fixed Deposit Is Only Guaranteed Up to a Point
Deposit protection is per banking licence, not per brand — and it includes accrued interest. Two ways a large fixed deposit ends up partly unprotected by accident.
Why a 25% Average Return Can Leave You With Nothing
An investment can average 25% a year and return nothing. Here's the gap between arithmetic and geometric means, why volatility drags compound growth, and sequence risk.
Why Salary Sacrifice Beats a Regular Pension Contribution — NI Savings, Employer Passthrough, and the £100,000 Taper
Salary sacrifice reduces gross salary before both income tax and National Insurance are calculated — unlike relief-at-source pensions which only save income tax. An employee on £40,000 sacrificing £4,000 saves £320 in NI on top of income tax relief, and their employer saves £552 in employer NI which many pass on to the employee's pension. Here's the full calculation, the personal allowance taper at £100,000, and what salary sacrifice reduces (statutory pay, mortgageable income) versus what it doesn't.
Why the Entire Money Market Uses Simple Interest — T-Bills, Day Count Conventions, and the Rule of 78s
The entire money market — Treasury bills, commercial paper, repos, and the Fed Funds Rate — uses simple interest, not because it's simpler but because the compounding period would need to be fractions of a day for instruments measured in weeks. Here's how day count conventions (Act/360, Act/365, 30/360) affect interest calculations by enough to matter in the inter-bank market, the discount rate vs investment yield distinction that makes T-bill comparisons tricky, and why the Rule of 78s front-loads interest at the expense of early repayers.
A ₹10 NAV Fund Isn't Cheaper Than a ₹500 NAV Fund — The NAV Misconception and What Actually Predicts Fund Quality
A fund with NAV ₹10 is not cheaper or better value than a fund with NAV ₹500 — NAV is total assets divided by units outstanding, reflecting fund age and accumulated history, not future return potential. Here's the worked example showing identical percentage returns regardless of NAV level, the metrics that actually predict fund quality (expense ratio, manager tenure, AUM appropriateness), and why direct plans' lower expense ratio compounds to a meaningful difference over a 20-30 year SIP horizon.
