Calcify Blog
Practical guides to the math behind our calculators, with worked examples you can check line by line.
Every article here starts from a question people actually run into: why the first years of a mortgage barely dent the balance, what a “2 point” rate increase means in percent, how a 10-business-day deadline lands on the calendar, or what an imported product really costs once duty and fees are added. We show the formula, work through real numbers, and point out the mistakes that most often lead to a wrong answer.
Tax and tariff figures are labelled with the year they apply to and come from official sources such as the IRS and US Customs and Border Protection. Each article lists when it was published and when it was last updated. For tax, tariff, medical, and other high-stakes decisions, check the linked primary source and a qualified professional before acting.
How Tariffs Affect Landed Cost: A Line-by-Line Walkthrough
· Updated · 9 min read · Related: Tariff Calculator
How US customs value, duty rates, processing fees, and freight combine into the true cost of an imported product, with worked examples and a margin check.
Read articlePercentage Change vs. Percentage Points: Why the Difference Matters
· 8 min read · Related: Percentage Calculator
A rate going from 4% to 5% is a 1 percentage point rise and a 25% increase. Learn when to use each, how to calculate them, and how headlines blur the two.
Read articleBusiness Days vs. Calendar Days: Getting Deadlines Right
· Updated · 8 min read · Related: Date Calculator
How to count business days and calendar days, why the first day usually is not counted, and how weekends, holidays, and month ends shift real deadlines.
Read articleHow to Calculate Age Precisely: Leap Years, Month Ends, and Other Edge Cases
· 8 min read · Related: Age Calculator
The borrow-and-carry method for exact age in years, months, and days, plus how leap-day birthdays, 31st-of-the-month dates, and time zones change the answer.
Read articleAmortization Schedules and Extra Payments: What an Extra $200 a Month Really Does
· 9 min read · Related: Mortgage Calculator
Read an amortization schedule line by line, see why early payments are mostly interest, and compare extra monthly payments, lump sums, and biweekly plans on a $320,000 loan.
Read articleUnderstanding US Tax Brackets in 2025
· 8 min read · Related: Tax Calculator
How 2025 U.S. federal marginal tax brackets work, with the standard deduction, a full worked example, and a clear distinction between marginal and effective rates.
Read articleHow to Calculate Mortgage Payments: A Complete Guide
· 10 min read · Related: Mortgage Calculator
The math behind monthly mortgage payments, how interest rates change your total cost, and how amortization works, with a full worked example.
Read articleHow Pregnancy Due Dates Are Calculated
· 8 min read · Related: Pregnancy Calculator
Naegele's rule, the 280-day standard, how ultrasound dating can change an estimate, and why most babies do not arrive on the exact due date.
Read articleTipping Etiquette: How Much Should You Tip?
· 9 min read · Related: Tip Calculator
Standard US tipping percentages for restaurants, delivery, travel, and services, international norms, and quick ways to work out a tip in your head.
Read articlePercentage Calculations You Use Every Day
· 8 min read · Related: Percentage Calculator
Discounts, sales tax, tips, grades, interest, and inflation: the three core percentage formulas and the mental shortcuts that make them fast.
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