Guides
Plain-language explanations of the formulas behind our calculators.
How to read an amortization schedule, row by row
A plain-English walkthrough of a real 30-year amortization schedule: what each column means, when principal overtakes interest, and what extra payments change.
48 vs. 60 vs. 72-month car loan: what each term really costs
Monthly payment vs. total cost on 48, 60, 72 and 84-month car loans, the negative equity trap, and when a longer term can still make sense.
How much to invest each month to reach $1 million
The monthly amount needed to reach $1 million by age and return, what fees and inflation do to it, and how existing savings change the plan.
How to calculate the square footage of a room
Measure any room in square feet, including L-shaped and irregular spaces, convert to square meters, and work out how much flooring to order.
How to calculate a loan payment by hand, on a calculator or in Excel
The formula banks use to set your monthly loan payment, three ways to calculate it yourself, and how the term changes what you pay.
How much car can I afford? The 20/4/10 rule and the costs people forget
A practical way to set a car budget: the 20/4/10 rule, price ranges by income, and what longer loans really cost.
How much should I save each month? A goal-by-goal way to decide
Rules of thumb are a start. Here is how to work out a monthly savings amount for each goal, and how to fit them all into one plan.
How to calculate ovulation and find your fertile window
How to estimate when you ovulate from your cycle, which days are most fertile, and how to confirm it with tests and body signs.
Ideal weight for your height: what the formulas say, and what they don’t
Four well-known formulas give four different “ideal” weights. Here is where they come from, what they are good for and how to use them sensibly.
Percentage change vs. percentage difference: which one should you use?
They sound the same and give different answers. Here is when to use percentage change, when to use percentage difference, and why it matters.
Scientific notation explained: how to read, write and use it
What numbers like 10⁻⁴, 6.02 × 10²³ and 1.5E-6 mean, how to convert them, and how to calculate with them.
How to calculate ratios: simplify, scale and split amounts with confidence
How to simplify ratios, solve proportions and use ratios for recipes, shared costs and screen sizes, with worked examples.
How to convert kg to lbs (and lbs to kg), including a quick mental trick
The exact kilogram-to-pound factor, a quick way to do it in your head, and conversion charts for body weight, luggage and cooking.
How much storage do I need? MB, GB and TB explained with real file sizes
What MB, GB and TB mean in practice, how much space photos, videos and apps take, and how to choose the right storage size.
How much house can I afford? What lenders approve vs. what you can live with
Pre-approval tells you the maximum a bank will lend. Here is how to find the number you can actually live with.
How to improve your credit score: what moves the number and what doesn’t
Credit scores reward a few specific habits. Here is where to focus, how fast things change and what a better score is worth.
Should I refinance my mortgage? Do the break-even math first
Refinancing only pays off if you stay long enough to earn back the costs. Here is the simple calculation that tells you.
Simple vs. compound interest: which one is on your loan or savings?
Which type of interest applies to your car loan, credit card or savings account, and why it changes what you pay or earn.
How to build an emergency fund when money is already tight
How much to save, where to keep it and how to build an emergency fund even on a tight budget.
What is ROI? How to calculate return on investment without fooling yourself
ROI is easy to calculate and easy to misuse. Here is how to work it out properly, with stock, rental and marketing examples.
What is a healthy body fat percentage? Ranges, measurement and meaning
Two people can weigh the same and look nothing alike. Body fat percentage explains why. Here are the healthy ranges and how to measure yours.
How much water should you drink a day? A realistic guide by weight and activity
Eight glasses a day is easy to remember but not based on you. Here is how much water you need by weight, activity and climate.
How to calculate macros, and turn the numbers into actual meals
A step-by-step way to calculate protein, carbs and fat, plus how to turn those numbers into everyday meals.
What is BMR? Where your daily calories actually go
Most of the calories you burn each day go to simply keeping you alive. Here is what BMR is, how it is calculated and what changes it.
How to calculate standard deviation, and what the number is telling you
Two groups can share the same average and look nothing alike. Standard deviation captures the difference. Here is how to calculate and read it.
How to simplify fractions: three methods, from quick to foolproof
Three reliable ways to reduce a fraction to lowest terms, including a method that works on any numbers, however large.
How to calculate probability: the few rules that solve most problems
From dice and cards to "at least one" questions and expected value, here are the probability rules you actually need.
Celsius to Fahrenheit: the formula, a quick trick and a chart for everyday use
Convert Celsius and Fahrenheit exactly or in your head, with charts for weather, cooking, food safety and fevers.
GB vs. GiB: why your new drive shows less space than the box says
A 1 TB drive shows about 931 GB on many computers. Nothing is missing. Here is the difference between GB and GiB.
How is a mortgage payment calculated? The math behind your monthly bill
Your mortgage payment never changes, but what it pays for does. Here is the math, with a real example.
How to calculate BMI, and what your number actually tells you
The BMI formula in kilograms and pounds, a healthy weight chart by height, and what BMI can and can’t tell you.
Compound interest explained: why time matters more than the rate
Leave $10,000 alone for 30 years and it can grow more than sevenfold. Here is how compound interest does it.
How much do I need to retire? A practical way to find your number
There is no magic retirement number, but there is a reliable way to estimate yours. Here is the method, step by step.
How to convert units: one method that works for everything
A single step-by-step method that converts any unit, including area, volume, speed and temperature, plus quick checks to catch mistakes.
APR vs. interest rate: which number should you actually compare?
Two loans with the same rate can cost very different amounts. APR shows you why, but it has blind spots too.
How to pay off credit card debt faster (and keep it from coming back)
A $5,000 card balance can take 4.5 years or 12 months to clear. What makes the difference is simpler than you think.
15 vs. 30-year mortgage calculator: which term should you choose?
A 15-year mortgage saves a fortune in interest. A 30-year one buys flexibility. Here is how to choose with real numbers.
How does inflation affect your savings? What your money really buys over time
Your balance can grow while your buying power shrinks. Here is how inflation works on savings and how to stay ahead of it.
How many calories do I need a day? Estimate it, then check it against reality
Calculators give you a starting estimate. Here is how daily calorie needs are worked out, and how to find your real number.
Heart rate zones explained: train easier, get fitter
Most people train every session at the same medium-hard effort. Heart rate zones fix that. Here is how to calculate yours.
How is a due date calculated? From last period, conception or IVF
A due date feels like a deadline but is really the middle of a window. Here is exactly how it is worked out.
How to calculate percentages: 3 formulas that solve almost everything
Nearly every percentage question is one of three types. Learn them, plus the mental shortcuts and traps that catch people out.
Mean, median and mode: which "average" tells the truth?
The same numbers can give three very different averages. Here is how to calculate each one and when to trust which.
Metric vs. imperial: how the two systems differ and how to switch between them
Why most of the world uses metric, where imperial survives, and the reference points that make switching easy.