Guides
Plain-English explainers for the things our tools calculate — how the math works, what the numbers mean, and how to use them. Every guide is paired with a free tool so you can run your own numbers.
How Mortgage Amortization Works
Why early payments are mostly interest — with a real schedule.
FinanceCompound Interest, Explained
How money grows on itself, with worked examples.
FinanceHow Big Should Your Emergency Fund Be?
A practical way to size your safety net.
FinanceBMI: What It Measures and Its Limits
What BMI tells you — and what it misses.
HealthTDEE: Calories You Burn in a Day
How daily calorie needs are estimated.
HealthHow to Estimate Paint for a Room
Measure once, buy the right amount.
HomeHow Much Concrete Do You Need?
Volume, bags and waste, made simple.
HomeMD5 vs SHA-256: Which to Use
Speed, security and the right job for each.
DevHow to Verify a File's Checksum
Confirm a download on Windows, Mac & Linux.
DevCooking Conversions: Cups, Grams & Ounces
Why a cup of flour isn't a cup of sugar.
EverydayMetric vs Imperial: A Quick Guide
The conversions worth memorizing.
EverydayHow to Calculate Days Between Dates
Counting days, weeks and business days right.
EverydayHow Much House Can You Afford?
The 28/36 rule and what really fits your budget.
FinanceDebt Snowball vs Avalanche
Which payoff method clears debt faster.
FinanceAPR vs APY: What's the Difference?
Why the same rate can mean two numbers.
FinanceHow Car Loans Work
Term, APR and how to pay less interest.
FinanceHow to Calculate Your Macros
Protein, carbs and fat from your calories.
HealthBody Fat Percentage Explained
Healthy ranges and how it's measured.
HealthHow Much Water Should You Drink?
Beyond the 8×8 rule — what the science says.
HealthHow Much Flooring Do You Need?
Square footage, waste factor and boxes.
HomeMulch, Soil & Gravel: Estimating Volume
Cubic yards, depths and how many bags.
HomeHow JSON Web Tokens (JWT) Work
Header, payload, signature — and the gotchas.
DevBase64 Encoding, Explained
What it is, how it works, and what it isn't.
DevHow to Calculate Percentages
The three formulas and quick mental math.
EverydayScientific Notation, Explained
Writing huge and tiny numbers as m × 10ⁿ.
MathStandard Deviation, Explained
Measuring spread — with a worked example.
MathProbability Basics
Calculating the odds, the right way.
MathRatios and Proportions
Cross-multiply, scale and compare.
MathRegex for Beginners
The pattern language, demystified.
DevCron Jobs Explained
Read and write cron schedules confidently.
DevWhat Is a UUID?
128-bit unique IDs and when to use them.
DevHow Passwords Should Be Stored
Hashing, salting and slow KDFs.
DevWhat Size Heater or AC? (BTU)
Size cooling and heating to the room.
HomeRoof Pitch Explained
Rise over run, angle and roof area.
HomeHow GPA Works
Credit-weighted grade point average.
EverydayTime Zones & Scheduling
Plan across zones without the headaches.
Everyday