Money & Finance
15-Year vs 30-Year Mortgage: Payment, Interest and Trade-offs
A shorter mortgage costs far less in interest but asks for a much higher monthly payment. A side-by-side example shows the size of both effects.
Articles
Plain-English guides to the numbers in everyday life: how a calculation works, where people go wrong, and which tool to reach for.
Everyday Numbers
Acres, Hectares and Square Miles: Land Area Units Compared
Land listings and maps use acres in some places and hectares in others. See how the units relate, how to convert them exactly, and how to avoid the squared-factor mistake.
Health & Fitness
Activity Multipliers: How to Pick the Right One for Your TDEE
The activity factor you choose changes your calorie estimate far more than the BMR equation does. Here is what each level means and how to avoid the usual overestimate.
Money & Finance
APR vs APY: What the Two Rates Really Tell You
APR and APY are two ways to quote the same rate. One ignores compounding and one includes it, and mixing them up can make a worse deal look better.
Money & Finance
APR vs Interest Rate: What the Difference Means for a Loan
A loan with the lower interest rate is not always the cheaper loan. The APR folds in certain fees so offers can be compared, and a worked example shows how.
Health & Fitness
BMI for Children and Teens: Why Percentiles Replace Cutoffs
A child's BMI is calculated like an adult's, but it is judged against other children of the same age and sex. Here is how percentiles work and what the bands mean.
Health & Fitness
Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle for BMR
Different BMR equations give different answers for the same person. Compare Mifflin-St Jeor, Harris-Benedict and Katch-McArdle on one set of numbers and see why they differ.
Health & Fitness
Calorie Deficit Math: Setting a Target and Estimating a Timeline
A daily calorie deficit converts into a predicted rate of weight loss with simple division. Here is the arithmetic, why the plan slows down over time, and where the limits are.
Everyday Numbers
Celsius, Fahrenheit and Kelvin: Formulas and Oven Marks
Temperature scales do not share a zero, so conversion needs a multiplication and an addition. Here are the exact formulas, a quick estimate, and an oven chart.
Money & Finance
The Compound Interest Formula, Step by Step
Compound interest is interest that earns interest. Here is the formula, a full worked example, and how to rearrange it to find the interest, the time or the rate.
Money & Finance
Does Compounding Frequency Matter? Daily vs Monthly vs Yearly
Moving from yearly to monthly compounding adds a visible amount; moving from daily to continuous adds under a dollar. The numbers show where the gains run out.
Time & Dates
How to Convert Between Time Zones Using UTC Offsets
Skip the guesswork: convert the local time to UTC, then to the target zone. Includes half-hour offsets, date changes and a worked meeting-overlap example.
Math & Statistics
Correlation Coefficient: How to Calculate and Interpret r
Pearson's r measures how closely two variables follow a straight line. Here is the hand calculation, how to read the result, and where the number can fool you.
Time & Dates
Daylight Saving Time: Why Some Days Have 23 or 25 Hours
When clocks change, a local day is no longer 24 hours. Learn what shifts, how it alters durations and meeting times, and how to calculate safely.
Health & Fitness
Healthy Weight Range for Your Height: How It Is Calculated
A healthy weight range is a BMI band turned back into kilograms. Here is the arithmetic, a table for common heights, and why a range beats a single ideal number.
Health & Fitness
Heart Rate Zones: Percent of Max vs Heart Rate Reserve
Heart rate zones turn "how hard am I working?" into numbers. Learn the two common ways to calculate them and why the same percentage can mean two different heart rates.
Money & Finance
Hourly or Salaried? How to Compare Two Job Offers Fairly
A $24 hourly wage and a $52,000 salary are not comparable until you count hours and paid leave. Here is a simple method to line them up.
Math & Statistics
How Big a Sample Do You Need? Sample Size and Margin of Error
Sample size is a trade-off between precision and cost. Here is the survey formula, a worked example, and why bigger is not always better.
Money & Finance
How Loan Amortization Works: Where Each Payment Goes
Every payment on a fixed loan is the same size, but the split between interest and principal keeps changing. Here is how that schedule is built and read.
Money & Finance
How Overtime Pay Is Calculated: Time-and-a-Half Worked Through
Overtime math is simple once you know the regular rate and the threshold. Follow worked examples for hourly, bonus and salaried pay.
Money & Finance
How to Calculate a Monthly Loan Payment by Hand
A loan payment comes from one short formula. Work through it with real numbers, see how term and rate change the total cost, and learn to check a lender's quote.
Health & Fitness
How to Calculate BMI by Hand in Metric and Imperial Units
BMI is one division and one squared number. Here is the metric and imperial method, the common unit slips, and how to run the formula backwards to find a target weight.
Math & Statistics
How to Calculate Standard Deviation by Hand, Step by Step
Standard deviation tells you how spread out a set of numbers is. This walkthrough does it by hand on eight values and explains when to divide by n or n minus 1.
Health & Fitness
How to Estimate Body Fat With a Tape Measure (US Navy Method)
A soft tape and a log button are enough to estimate body fat. Here is the US Navy method step by step, what the percentage means, and how far to trust it.
Math & Statistics
How to Read a Confidence Interval (and What 95% Means)
A confidence interval is a range built from sample data. Learn how to calculate one for a mean and a proportion, and how to describe it without overclaiming.