Guides
Practical guides for finance, health, tax, and time calculations. Page 4 of 6.
How to Calculate Your Pregnancy Due Date
Learn how to calculate your pregnancy due date using Naegele's Rule and the LMP method, understand trimesters, and see how ultrasound can revise your estimate.
Tax & BusinessHow to Calculate Sales Tax on Any Purchase
Learn how to calculate sales tax forward and backward — the formula, worked examples, combined local rates, exempt items, and a free sales tax calculator.
FinanceHow Does a Mortgage Amortization Schedule Work?
Read principal, interest and annual balance rows, with tested mortgage examples and recurring extra-payment comparisons from the calculator engine.
FinanceHow Inflation Affects the Value of Money Over Time
Compare published U.S. annual CPI values from 1913-2025, reverse purchasing-power calculations and exact real-return arithmetic.
FinanceHow to Calculate Return on Investment (ROI) Step by Step
Learn how to calculate return on investment (ROI) with the exact formula, step-by-step examples for stocks and real estate, and what counts as a good ROI.
HealthWhat Is a Healthy Body Fat Percentage? (By Age & Sex)
See the circumference density equation used by DigiCalc, a reproducible example, invalid-input handling and limits of reference bands.
FinanceHow Much Car Can I Afford? The 20/4/10 Rule Explained
Find out how much car you can afford using the 20/4/10 rule, monthly payment math, and true ownership costs — with real examples for different incomes.
FinanceHow to Pay Off a Loan Faster (and How Much You Save)
Compare recurring extra payments and an up-front lump sum against your actual monthly payment, with tested examples and explicit timing assumptions.
HealthHow to Calculate Macros for Weight Loss: Step by Step
Convert illustrative calorie totals and fixed percentages into macro grams without double adjustments or a hidden safety guarantee.