Blog
Practical guides to the math behind the tools — learn the concept, then run the numbers.
Articles
Standard Deviation Explained: What It Is and How to Calculate It
What standard deviation measures, population vs sample, a step-by-step calculation for a 5-number dataset, the formula, real-world uses, and the 68-95-99.7 rule.
4 min read
How Strong Is My Password? (Entropy, Crack Time, and Best Practices)
What makes a password strong, how entropy and crack time are measured, why P@ssw0rd is weak, the 2024 NIST guidance, and how to create passwords you don't have to remember.
4 min read
Compound Interest Explained: Formula, Examples, and the Rule of 72
How compound interest works: simple vs compound, the A = P(1 + r/n)^(nt) formula, a worked $10,000 example, the Rule of 72, and why starting early beats investing more.
5 min read
How to Calculate BMI (Formula, Chart, and What It Means)
How to calculate BMI in metric and imperial units, with the formula, a worked example (70 kg, 175 cm = 22.9), the WHO category chart, and the limitations of BMI.
5 min read
What Pace Do You Need for a Sub-2-Hour Half Marathon?
To break 2 hours in a half marathon you need to run 5:41/km (9:09/mile). Here's the pace math, a full split table, and how to train for and hold that pace.
4 min read
How Extra Mortgage Payments Save You Thousands
See how extra mortgage payments cut total interest and shorten your loan, with a worked $400k example, the math behind it, three strategies, and when not to pay extra.
5 min read
KM to Miles Conversion Chart (+ How to Convert)
Convert kilometers to miles with an exact formula, a quick reference chart for 1–100 km, a mental-math shortcut, and the common distances runners and drivers actually use.
4 min read
How to Calculate a Weighted Grade (With Examples)
How to calculate a weighted grade step by step: the formula, a worked example with three assignments, how to find the score you need on your final, and a GPA table.
5 min read
How to Calculate a Discount (and Do Stacked Discounts Save More?)
How to work out a percent-off discount and the final price, a quick savings table, and the truth about stacked discounts — why 20% then 10% off isn't 30% off.
1 min read
Converting Fractions to Decimals (and When They Repeat)
How to convert any fraction to a decimal, a table of common conversions, the rule for when a decimal terminates vs repeats, and how to turn fractions into percentages.
1 min read
How to Calculate Your GPA (4.0 Scale Explained)
How GPA works on the 4.0 scale: the grade-to-points table, the credit-weighted formula, a worked example, and the difference between weighted and unweighted GPA.
1 min read
Understanding BMI: What It Measures and What It Misses
What BMI is, how it's calculated, the standard categories, and — importantly — what BMI doesn't tell you. A clear, honest explainer (not medical advice).
2 min read
How to Calculate VAT: Adding and Removing Tax
How to add VAT to a net price and remove VAT from a gross price — with the formulas, worked examples, and the common mistake of subtracting the percentage instead of dividing.
2 min read
Mean vs Median: Which "Average" Should You Use?
The mean and the median are both "averages" but can tell very different stories. Learn how to calculate each, when the mean misleads, and which to use for skewed data like incomes.
2 min read
Margin vs Markup: The Difference That Costs You Money
Margin and markup both measure profit but against different bases, and confusing them leads to underpricing. Here's the difference, a conversion table, and worked examples.
2 min read
How Much Should You Tip? A Practical Guide
A practical tipping guide: standard US percentages by service, whether to tip on the pre-tax or post-tax total, how to split a bill, and a quick tip table for common amounts.
2 min read
Percentage Increase vs Decrease: Why They Don't Cancel Out
A percentage increase and the same percentage decrease don't return you to where you started. Here's why, with the formulas, worked examples, and how to reverse a percentage change correctly.
2 min read
How to Calculate Percentages: A Complete Guide
Learn how to calculate percentages step by step: percent of a number, what percent one number is of another, and percentage change — with worked examples and a quick reference.
3 min read