Guides
Short, plain-English explainers for the math behind our calculators — so you understand the result, not just the number.
How to Calculate Standard Deviation (Step by Step)
Standard deviation measures how spread out a set of numbers is. Learn the formula, the difference between population and sample, and a full worked example.
Read guide →Prime Factorization Explained (with Examples)
Prime factorization breaks a number into the prime numbers that multiply to make it. Learn the method, how to count a number's divisors, and worked examples.
Read guide →What Is a Set Partition? Stirling and Bell Numbers
Counting the ways to split a set of objects into groups uses Stirling numbers of the second kind and Bell numbers. Here's what they mean, with worked examples.
Read guide →Roman Numerals Explained: Chart, Rules and Examples
The seven Roman numeral symbols, how the additive and subtractive rules work, a quick chart, and step-by-step examples up to the year 2025.
Read guide →How Many GB in a TB? Data Storage Units Explained
1 TB is 1,000 GB in decimal units and 1,024 GB in binary units. Here's why there are two answers, the full ladder from bytes to terabytes, and why a new drive shows less space than the box says.
Read guide →How Mortgage Payments Work
Understand what makes up a mortgage payment, the amortization formula, and how to pay less interest over the life of the loan.
Read guide →Compound Interest, Explained Simply
How compound interest works, why starting early matters so much, and how regular contributions accelerate growth.
Read guide →Understanding Your BMI
What Body Mass Index measures, how to read the categories, and the limits of BMI as a health metric.
Read guide →How to Calculate Percentages
The three percentage questions that come up most — percent of a number, percentage change, and what percent one number is of another.
Read guide →Daily Calories and TDEE, Explained
The difference between BMR and TDEE, how activity multipliers work, and how to set calories for losing or gaining weight.
Read guide →How US Federal Income Tax Works
Marginal vs effective tax rates, the standard deduction, and why moving into a higher bracket doesn't tax all your income.
Read guide →How Much to Tip (and How to Split a Bill)
Typical tipping percentages in the US, whether to tip on the pre-tax total, and how to split a bill fairly.
Read guide →Converting Salary to Hourly (and Back)
How to turn an annual salary into an hourly rate, why 2,080 hours matters, and how to compare job offers fairly.
Read guide →How Currency Conversion Works
What an exchange rate is, why the rate you get differs from the mid-market rate, and how to avoid hidden fees.
Read guide →Understanding Debt-to-Income (DTI)
What DTI is, the front-end and back-end ratios, and why lenders care so much about it for a mortgage.
Read guide →The 50/30/20 Budget, Explained
A simple budgeting rule that splits your take-home pay into needs, wants and savings — and how to adapt it.
Read guide →Sleep Cycles and the Best Time to Wake Up
Why waking at the end of a 90-minute sleep cycle feels better, and how to plan your bedtime or wake-up time.
Read guide →Snowball vs Avalanche: The Fastest Way to Pay Off Debt
Two proven strategies for clearing multiple debts — one saves the most money, the other keeps you motivated. Here's how to choose.
Read guide →APR vs APY: What's the Difference?
APR and APY look similar but mean different things — one is what you pay, the other is what you earn. Understanding compounding is the key.
Read guide →How Much House Can You Afford?
Lenders use a few simple ratios to decide your budget. Learn the 28/36 rule, what counts toward it, and how the down payment changes everything.
Read guide →How Car Loans Work
From sales tax and trade-ins to APR and loan term — what really drives your monthly car payment, and how to pay less for the same car.
Read guide →Margin vs Markup: Pricing Without the Confusion
Margin and markup use the same two numbers but mean different things — and mixing them up quietly erodes your profit. Here's the clear version.
Read guide →How Big Should Your Emergency Fund Be?
The classic answer is 3–6 months of expenses — but the right number depends on your job, your dependents and your fixed costs. Here's how to size it.
Read guide →Understanding Your Take-Home Pay
Gross pay is the headline; take-home is what lands in your account. Learn what's deducted between the two and why your paycheck looks smaller.
Read guide →How to Calculate ROI
Return on investment turns a profit into a comparable percentage — but the simple formula hides two traps: time and total cost. Here's how to get it right.
Read guide →Roth vs Traditional IRA: Which One Should You Choose?
Roth or traditional? The whole decision comes down to one question: is your tax rate higher now or in retirement? Here's how to answer it, with a worked example.
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