Guides that explain what the tool is doing
Each guide starts with a tool you already use, then shows what happens under the hood with real numbers.
Page 3 of 5HEX, RGB and HSL: three ways to write the same colour, and which to use
The indigo #6366f1 is also rgb(99, 102, 241) and hsl(239, 84%, 67%). This guide explains how each format works, how to convert between them by hand, and why HSL is the friendliest one when you want a lighter, darker or matching shade.
Half-Life Calculator (Radioactive Decay)Half-life: why things fade by half, again and again, and never quite reach zero
Half-life is the time it takes for half of something to disappear, and it applies to radioactive atoms, caffeine in your body and the age of ancient bones. This guide builds the formula from simple halving, with pictures and exact worked examples.
ROI CalculatorROI: why a 40% return can beat a 50% return, and what to count
Return on investment is the quickest way to judge whether money was well spent, and the easiest to misuse. This guide shows the formula, why time and costs change the answer, and the difference between ROI and the ad-spend ratio marketers quote.
Stock Average Calculator (Average Buy Price)Averaging down: how buying more at a lower price changes your average, and what it cannot fix
When a share you own falls, buying more lowers your average cost and cuts the rise needed to break even. This guide shows the exact arithmetic, why a fixed monthly amount buys at a lower average than the average price, and the risk that no formula removes.
BMR & Calorie CalculatorBMR and TDEE: how many calories your body burns before you lift a finger
Most of the energy you burn each day goes on staying alive, not on workouts. This guide explains BMR, activity multipliers and TDEE with a worked example, shows how the common formulas compare, and what a calorie target can and cannot tell you.
GCD & LCM CalculatorGCD and LCM: Euclid’s 2,300-year-old shortcut, and when two buses meet again
The greatest common divisor simplifies fractions, and the least common multiple tells you when repeating things line up again. This guide shows how to find both by hand, why Euclid’s method is so fast, and the one identity that links them.
Permutation & Combination CalculatorPermutations vs combinations: when order matters, and why a combination lock is misnamed
Counting arrangements and selections sounds like school maths, but it decides lottery odds, password strength and how many handshakes happen in a room. This guide builds both formulas from scratch and shows the numbers in pictures.
Dice Roller & Coin FlipTwo dice and a coin: why seven wins, and why a streak proves nothing
A fair die or coin is the simplest random machine there is, yet it fools people every day. This guide uses all 36 outcomes of two dice, the famous 17th-century gambling puzzle and a 5,000-flip experiment to show what chance really does.
Roman Numeral ConverterRoman numerals: how MCMXCIV adds up, and why 49 is XLIX, not IL
Roman numerals are seven letters and two simple rules, yet people get 49, 99 and 1994 wrong all the time. This guide shows the greedy method that never fails, the numbers with the longest and shortest forms, and why the system tops out at 3,999.
Morse Code TranslatorMorse code: why E is a single dot and Q takes four signs
Morse code looks like random dots and dashes, but it is a carefully designed compression scheme: common letters get short codes, rare letters get long ones. This guide shows the timing rules, the maths of why it works, and how fast operators really send.
Fibonacci Sequence GeneratorThe Fibonacci sequence: from rabbits to the golden ratio, and why slow code loves it
Add the last two numbers to get the next: 0, 1, 1, 2, 3, 5, 8, 13. That one rule hides a growth pattern, a famous ratio of about 1.618, a stair-climbing puzzle and the classic example of slow recursive code. This guide walks through all four with pictures and exact numbers.
Temperature ConverterCelsius, Fahrenheit and Kelvin: the conversion, a mental shortcut, and the one place they agree
35°C is a hot day, 95°F is the same hot day, and 308 K is the same day again. This guide gives you the exact formulas, a shortcut that is good enough for the weather, a cooking chart for oven settings, and the surprising temperature at which Celsius and Fahrenheit show the same number.
TOTP Code GeneratorHow the 6-digit code in your authenticator app works (TOTP explained)
Your phone shows a six-digit code that changes every 30 seconds and the website somehow knows it, with no internet connection between the two. The trick is a shared secret and a shared clock. This guide walks through exactly what happens, how safe it is, and where it still fails.
Data Storage ConverterWhy a 1 TB drive shows 931 GB: KB, KiB, MB and Mbps explained
You buy a 1 TB drive and your computer says 931 GB. Nothing is missing and nobody is cheating: two honest ways of counting meet in the same box. This guide shows where the 69 GB goes, why the gap grows with every size, and why your 100 Mbps plan downloads at 12 MB a second, with a picture for each step.
Normal Distribution Calculator (Probability)The bell curve: the 68-95-99.7 rule, z-scores and what “rare” really means
Heights, exam marks, delivery times and measurement errors all tend to pile up in the same bell shape. Once you know the mean and the spread, three numbers (68, 95 and 99.7 percent) tell you where almost everything lies, and a z-score tells you how unusual any single value is. Here is the whole idea, with pictures and worked examples.
Statistics CalculatorMean vs median vs mode: why “the average salary” is so often misleading
A company of nine people announces an average salary of ₹11.5 lakh. Eight of them earn between ₹4 and ₹7 lakh. Nobody is lying; the word “average” is. This guide explains the mean, median and mode, shows how one big number bends the mean, and gives a simple rule for picking the right one.
RD CalculatorRecurring deposit: ₹5,000 a month for 5 years, and how the maturity is worked out
A recurring deposit (RD) lets you save a fixed amount every month at a fixed rate. At 7% for five years, ₹5,000 a month becomes about ₹3.60 lakh. This guide shows where each rupee of that interest comes from, why the first deposit earns 70 times more than the last, and what tax and tenure do to the final number.
Prime Number CheckerPrime numbers: why they thin out, how to test one, and why your bank depends on them
There are 25 primes below 100 but only 168 below 1,000 and 78,498 below a million. They never run out, yet they get steadily rarer. This guide shows how to test a number by hand, why you only need to check up to its square root, and how a simple fact about primes protects online payments.
AI API Cost CalculatorAI tokens and API cost: why the bill depends on the answer length and the chat history
AI services charge by the token, not by the word or the message. Output tokens usually cost several times more than input, and a long conversation resends its whole history every turn. This guide explains tokens with simple rules of thumb, works through a monthly-bill example, and shows four ways to cut the bill without hurting quality.
Lumpsum Investment CalculatorLumpsum investment: what ₹1 lakh becomes in 10, 20 and 30 years
A lumpsum is money invested once and left alone. At 12% a year, ₹1 lakh becomes ₹3.11 lakh in 10 years, ₹9.65 lakh in 20 and ₹29.96 lakh in 30. At 8% the 30-year figure is only ₹10.06 lakh. Here is the formula, the effect of the rate, and when lumpsum beats a SIP.
Savings Goal CalculatorSaving for a goal: how much per month to reach ₹10 lakh
To have ₹10 lakh in 5 years you need about ₹16,667 a month if you earn nothing on it, ₹13,887 at 7% and ₹12,807 at 10%. The longer you give yourself, the more your money does the work. This guide shows the formula, the trade-off between time and monthly amount, and a simple plan.
Running Pace CalculatorRunning pace explained: from min/km to 5K, 10K and marathon times
Pace is the time you take for one kilometre. Hold 6:00 per km and you finish a 5K in 30:00, a 10K in 1:00:00, a half marathon in 2:06:35 and a marathon in 4:13:10. This guide shows the maths, a ready table for common paces and how to pick a pace you can actually keep.
Caesar Cipher & ROT13Caesar cipher: why only 25 keys means anyone can crack it
The Caesar cipher shifts every letter by a fixed number. With a shift of 3, HELLO becomes KHOOR. It is fun and historic, but there are only 25 useful shifts, so a computer (or a patient person) can try them all in seconds. Here is how it works and why it fails.
In-Hand Salary Calculator (CTC to Take-Home, India)CTC to in-hand salary: where a ₹12 lakh package actually goes
A ₹12 lakh CTC does not arrive as ₹1 lakh a month. Employer PF, gratuity, your own PF and professional tax come out first. In a typical split with a 40% basic, about ₹88,000 a month reaches your account before income tax. Here is the step-by-step walk from CTC to credit alert.
SWP CalculatorSWP: how long will ₹1 crore last when you withdraw every month?
A systematic withdrawal plan pays you a fixed amount each month from an invested corpus. At an 8% return, ₹50,000 a month from ₹1 crore never runs out if the amount stays flat, but if your withdrawals rise 6% a year with inflation, the money lasts about 21 years. The difference is the whole lesson.
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