Guides: how to calculate
Direct explanations, with formulas and examples.
How to calculate compound interest
The compound interest formula with examples and why time makes money grow.
How to calculate percentage
Learn how to calculate percentages: X% of Y, increase, discount and change.
BMI: what it is and how to calculate
Understand Body Mass Index, the formula, the WHO ranges and its limitations.
Simple vs compound interest
The difference between simple and compound interest, with formulas and a side-by-side example.
How to calculate the average (mean)
The arithmetic and weighted mean, with examples, and how it differs from the median.
How to calculate a tip
How to calculate a tip, how much to give and how to split the bill.
How to calculate your daily calories
Calculate your BMR and TDEE (daily burn) and use them to lose or gain weight.
How to calculate a discount
How much you pay and save with a discount — and why stacked discounts don’t add up.
How to use the rule of three
Direct and inverse rule of three, step by step, with everyday examples.
What is ROI and how to calculate it
Return on investment (ROI), the formula, an example and why the time frame matters.
How to convert a fraction to a decimal
Turn any fraction into a decimal number (and a percentage) with a simple division.
Pythagorean theorem: formula and examples
The a² + b² = c² formula, how to find the hypotenuse or a leg, and practical uses.
How to create a strong password
What makes a password truly secure: length, variety and the mistakes to avoid.
How to calculate calories burned
Use each activity's MET to estimate how many calories you burn exercising.
What is the rule of 72
The mental shortcut for how long money takes to double with compound interest.
How to calculate the area of a circle
The π·r² formula, with examples, and how to find the area from the diameter.
How to convert interest rates (monthly ↔ annual)
Why 1%/month isn't 12%/year, and how to convert equivalent rates correctly.
How to convert a fraction to a percentage
Turn any fraction into a percentage in two simple steps, with examples.
Arithmetic vs geometric sequences
The difference between arithmetic and geometric sequences, with term and sum formulas.
Kinetic and potential energy: the formulas
The difference between energy of motion and of position, with formulas and how one becomes the other.
How to calculate free fall
Fall time and impact speed from height — and why mass doesn't matter.
What is the golden ratio
The number φ ≈ 1.618, how to calculate it and why it appears so much in art and design.
What is acceleration and how to calculate it
The acceleration formula, what m/s² means and how to find distance in accelerated motion.
How to find the original price before a discount
Reverse percentage: why you divide (not add) to get back to the original value.
Arithmetic, geometric and harmonic mean
The three most common means, when to use each and how they relate.
The 50/30/20 budget rule
How to split income into needs, wants and savings — and what to do when it doesn't fit.
What is dividend yield
How to calculate a stock's dividend yield and why a high DY isn't always good.
What is a factorial (n!)
What n! means, how to calculate it, why 0! = 1 and where it appears.
Calorie deficit: how to lose weight safely
What a calorie deficit is, how many calories to cut and why going slow works better.
How to calculate square footage
How to find a room's area, L-shaped spaces and converting between m² and ft².
Waist-to-height ratio: the simple health indicator
Why the waist-to-height ratio can beat BMI and how to read the number.
Heart rate training zones
What the 5 zones are, the Karvonen method and how to train in each.
How to calculate molar mass
Add up atomic masses to find any formula's molar mass, step by step.
What is the heat index
Why humidity makes it feel hotter and how to read the heat index.
How to size a staircase (Blondel formula)
How many steps, riser height and tread depth for a comfortable, safe staircase.
How to know if a year is a leap year
The calendar rule, why 2000 was a leap year and 1900 wasn't, and what Feb 29 is for.
How many steps in 1 km (and per day)
How many steps make 1 km, how height changes the math and what the 10,000-step goal means.
Color contrast and accessibility (WCAG)
What contrast ratio is, WCAG's AA and AAA levels and how to pick legible colors.
How much house can I afford (the 28/36 rule)
The 28/36 rule lenders use to figure out how much mortgage payment fits your income.
How many cups in a gallon (and other kitchen measures)
How many cups in a gallon, a quart and a liter — the full kitchen-measure table.
How much water should I drink a day
The practical body-weight math, the "8 glasses" myth and when to drink more.
The 30% rent rule
How much income can go to rent, where the 30% rule comes from and when to bend it.