Educational Guides & How-To Articles

Loan & Mortgage Guides

EMI, compound interest, mortgage payments — the formulas and the honest limits of the math.

Tax & Salary Guides

Income tax brackets, reverse GST, zakat, and the gross-to-net salary journey.

Health & Fitness Guides

BMI and what it ignores, BMR/TDEE math, calorie deficits, and body metrics beyond the scale.

Math & Statistics Guides

Percentage formulas, statistics basics, and how random number generators actually work.

Business Guides

Break-even with a café example, margin vs markup, and target-profit back-solving.

Construction Guides

Concrete volume and bag counts, the 10% wastage rule, and exact unit-conversion factors.

Science Guides

Ohm's law, density, and force — physics formulas with worked numbers and unit rules.

Engineering Guides

Voltage drop, gear ratios, and torque — the assumptions behind the formulas.

Auto & Vehicle Guides

Fuel cost per km, car EMI, and the EV-vs-petrol decision done honestly.

Career & Freelance Guides

Hourly-to-annual conversion, freelance rates that survive tax, overtime pay.

Home & Garden Guides

Room area, paint coverage, wallpaper rolls, and AC sizing with waste allowances.

Family & Parenting Guides

Family budgets, baby growth percentiles, and college-savings monthly math.

Food & Nutrition Guides

Daily calorie needs, macro splits, protein targets, and recipe scaling.

Lifestyle Guides

Moving costs, rental deposits, and the true annual cost of daily habits.

Regional Finance Guides (India)

FD maturity, PPF conventions, SIP futures, and GST math with regional rules.

Everyday Guides

Exact age, days between dates, and electricity bills decoded line by line.

Utilities Guides

Password entropy, UUID guarantees, and the concepts behind the tools.

How to Calculate Percentage — Formula, Examples & Common Mistakes

What a percentage really means, the one formula behind every percentage problem, worked examples (increase, decrease, reverse), and the mistakes that trip people up.

How to Calculate Loan EMI — Formula & Step-by-Step Example

How banks work out your monthly loan payment, the EMI formula explained in plain words, a full worked example, and how interest rate and tenure change the payment.

How to Calculate Your Exact Age in Years, Months & Days

The difference between exact age and simple year subtraction, how to count years, months and days by hand, and how leap years affect the total.

How to Calculate BMI — Formula, Ranges & What the Number Means

The BMI formula for metric and imperial units, the official weight ranges, worked examples, and the honest limitations of BMI as a health measure.

How to Calculate Zakat — Nisab, Rates & Worked Examples

The 2.5% zakat formula step by step, what counts as zakatable wealth, gold and silver nisab thresholds, and a full worked example.

How Income Tax Is Calculated — Brackets & Marginal Rates

Why a raise never pushes all your income into a higher bracket, marginal vs effective rate, and worked examples with country-specific calculators.

How Currency Conversion Works — Rates, Spreads & Hidden Fees

What mid-market rate really means, how spreads and fixed fees change what you receive, and why DCC at foreign terminals costs you 3–8%.

How to Calculate a Discount — Percent Off & Stacked Discounts

The basic percent-off formula, the reverse calculation from sale price to original, and the stacked-discount trap (30% + 20% is not 50%).

How to Calculate a Tip — Rates, Splitting & Tipping Abroad

Mental-math shortcuts for 10/15/20% tips, standard rates by service, how to split a bill fairly, and how norms change outside the US.

GST & Sales Tax Explained — Adding, Removing & Reverse Tax Math

Add tax to a price, remove tax from a tax-inclusive receipt, and why the reverse formula divides instead of subtracting — with country rates.

How Compound Interest Works — Formula & Growth Examples

Why compounding beats simple interest, the formula behind it, and how rate and time multiply your savings growth.

How Mortgage Payments Work — Principal, Interest & Amortization

What the bank actually charges you each month, how amortization shifts your payment from interest to principal, and what LTV means for your rate.

Calculator & Finance Glossary — 36 Terms Explained

Plain-English definitions of every term our calculators use: APR, EMI, CAGR, BMI, TDEE, LTV, standard deviation and more, grouped by field.

How Inflation Is Calculated — CPI & Purchasing Power

What the inflation rate measures, the cross-year money formula, and what $100 becomes at 2%, 5% and 8% over 20 years.

Gross Salary vs In-Hand Salary — Take-Home Pay Explained

Why CTC ≠ gross ≠ net, the gross-to-net calculation step by step, and which CTC components never reach your bank account.

How Much You Need to Retire — The 25x Rule Explained

Your retirement number from annual expenses, where the 4% guideline breaks down, and the levers that move the target most.

How to Calculate Calories — BMR, TDEE & Deficit Math

The Mifflin-St Jeor formula, activity multipliers, realistic deficit targets, and how to read the scale without fooling yourself.

Debt Snowball vs Avalanche — Which Pays Off Faster?

Both strategies compared on one debt list: the math of interest saved and the psychology of quick wins, plus the hybrid option.

How Random Number Generators Work — PRNG vs True Random

Seeds, algorithms and the difference between Math.random and crypto-grade randomness — plus when each is the right tool.

How to Calculate Concrete — Bags, Yards & the 10% Rule

Volume formulas for slabs, footings and columns, mix ratios, and how many 60/80-lb bags a pour actually takes.

Metric to Imperial Conversion — Exact Factors & Formulas

Exact conversion factors for length, weight, volume, temperature and data, and why precision beats memorized approximations.

How GPA Is Calculated — Weighted, Unweighted & Cumulative

The quality-points formula, credit-hour weighting, the 5.0 scale question, and how semester GPA rolls into cumulative.

Break-Even Analysis — Formula & Margin of Safety

Fixed vs variable costs, contribution margin, and a full café example extended to a target-profit calculation.

Password Strength — Entropy, Crack Time & Passphrases

The bits = log2(pool) × length formula, why length beats symbol swaps, and passphrases that beat 16-character chaos.

Baby Cost — First-Year Budget Formula & Worked Example

One-time gear vs monthly essentials vs childcare, a computed $4,500–$20,000 example, and why the childcare line dominates every published average.

Ideal Weight — Devine Formula vs the BMI Range

Why the single-number formula and the healthy-BMI band disagree, and which question each one actually answers.

BMR — Mifflin-St Jeor, Line by Line

The resting-burn formula with a worked example, TDEE multipliers, and why two reputable equations disagree by 74 kcal.

Profit Margin — and the Markup Trap

Gross vs net margin formulas, and why a 40% margin is a 66.7% markup — the mix-up that underprices businesses.

Ohm's Law — V = IR With Real Numbers

Three forms, series vs parallel worked circuits, power dissipation, and where the law stops being linear.

Gear Ratio — Speed for Torque

The driven÷driver formula, a 1,500-rpm worked example, and how multi-stage gearboxes multiply ratios.

Fuel Cost — the Three-Variable Formula

Distance ÷ efficiency × price, a 15,000 km worked year, and what an efficiency jump is actually worth.

Freelance Rate — Income ÷ Billable Hours

Why dividing your old salary by 2,080 underprices you, and the 60% utilization rule that fixes it.

Paint Coverage — Area × Coats ÷ Spread Rate

Wall area minus openings, the two-coat rule, and the adjustments (texture, sprayers, color change) that change the buy.

Macros — the 4-4-9 Rule, Worked

Calories to grams with a checked 2,400 kcal example, common splits compared, and sanity bands before trusting any split.

SIP Math — Where ₹5,000/Month Goes

The annuity FV formula, computed 5- and 10-year outcomes, and why the last years contribute most of the growth.

FD vs PPF vs SIP — One Sum, Three Machines

₹1 lakh computed three ways with tax treatment included, and the goal-date logic that picks the right instrument.

EV vs Petrol — the Per-KM Math

1.20/km vs 6.67/km computed, the 82,000/year example, and the five factors the simple math leaves out.

Wedding Budget — Split the Total, Then Guard It

A worked category split with a real buffer line, and why guest count is the lever that moves everything.

The Rate Card — Hourly Floor, Project Layer, Retainer Top

Three pricing modes computed from one floor rate, with the risk factor that keeps scope drift profitable.

Screen Time — the Compounding Cost of Daily Hours

3 h/day = 45.6 full days a year: the table, the wake-day framing, and using numbers without the guilt spiral.

Electricity Bill, Decoded — Watts × Hours × Rate

The kWh formula on a fridge vs an AC, tiered slabs, and why chargers are folklore rather than the problem.

Business Days — the Math of Deadlines

Why a calendar month is only ~22 workdays, how deadline math drifts, and where naive counts break.

What Do I Need on the Final? — the Weighted Formula

The exact formula, why heavier finals need less over-performance, and reading the impossible answers honestly.

Room Area — Shapes and the Waste Rule

Rectangles, L-shapes and circles, plus the 10% waste allowance that keeps material orders from ending one box short.

AC Sizing — BTU Math and the Oversizing Trap

BTU per sq ft with climate adjustment, tonnage conversion, and why bigger units cool worse.

Protein Need — g/kg Bands, Then Real Food

0.8–2.0 g/kg by activity, a 112 g worked day with food equivalents, and why distribution beats totals.

CAGR — the One Number That Smooths a Bumpy Investment

The compound annual growth rate formula, why it beats averaging yearly returns, and the four situations where it misleads.

NPV vs IRR — Which Investment Metric to Trust

Both metrics on one worked project, the reinvestment assumption that makes IRR overpromise, and the decision rule when they disagree.

Net Worth — One Subtraction, Whole Balance Sheet

Assets minus liabilities with a full worked balance sheet, what to include and exclude, and why the quarterly trend beats the snapshot.

Debt-to-Income Ratio — the 36/43 Rule, Worked

Front-end vs back-end DTI on a real monthly budget, the thresholds lenders actually apply, and what a good ratio hides.

Credit Card Minimum Payments — the Trap in Actual Numbers

How the minimum is engineered, ₹80,000 at 42% APR worked three ways, and why paying slightly more than the minimum changes everything.

Rent vs Buy — Every Cost Line, Then the Verdict

The full both-sides ledger including the invested down payment, the breakeven idea, and the situations where renting wins on purpose.

Body Fat Percentage — the Navy Tape-Measure Method

The circumference formula worked step by step, body fat vs BMI on the same body, and the honest ±3–4% error band.

Daily Water Intake — the Formula, the Myth, Your Sweat

The 30–35 ml/kg baseline with exercise and heat adjustments, where "8 glasses" came from, and when thirst is the better guide.

Heart Rate Zones — Math for Training, Error Bars Shown

Max-HR formulas compared, the five zones computed on a worked example, and why a ±10 bpm formula error can shift you a full zone.

Sleep Cycles — Where the 90-Minute Rule Bends

Bedtime math worked backwards from your alarm, how many cycles adults need, and the honest limits of the 90-minute average.