NumberTally Guides & How-Tos
Plain-English explainers that show the formulas, walk through real examples, and link you straight to the calculator when you want to run the numbers yourself.
How to Calculate Overtime Pay
Time-and-a-half, double time, and a full worked week, step by step.
How to Calculate BMI
The BMI formula in imperial and metric units, with categories chart.
How Mortgage Payments Are Calculated
The lender formula, a full worked example, and why early payments are mostly interest.
Compound Interest Formula Explained
The A = P(1+r/n)^nt formula, a 20-year example, and the Rule of 72.
How to Calculate Percentage Increase
Pay raises, price hikes, and growth rates — plus percentage points vs percent.
How to Calculate Your Age in Days
The manual method with leap years, fully worked through.
Frequently asked questions
Are the guides free?
Yes — every guide is free, with no sign-up, just like the calculators.
Do the guides link to the calculators?
Yes. Each guide ends with a link to the matching calculator so you can apply the formula to your own numbers.
How often are guides updated?
Each guide shows its last-updated date at the top. Formulas are re-checked when tax rules or standards change.
Free to use. Your inputs stay in your browser — nothing is uploaded or stored.