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Plain-English guides to the concepts behind the calculators. No jargon for jargon's sake, no padding, and every formula worked out with a real example you can follow along with on the right calculator. Occasionally a little calculator history, too.
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Financial · Featured
How to Calculate Compound Interest (And Why It Matters)
The formula unpacked, a real 30-year example at 7%, why compounding frequency matters less than you think, and the Rule of 72 shortcut worth memorizing.
"You drop $10,000 into an index fund at age 35 and don't touch it until 65… with monthly compounding, that grows to about $81,165. The same $10,000 at simple interest would have grown to $31,000."
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Heritage · Featured
The Bowmar Brain: How the First American Pocket Calculator Changed Everything
In 1971 a Fort Wayne, Indiana company shipped a $240 four-function calculator and killed the slide rule. Five years later the company was bankrupt. The most important calculator nobody remembers — and the MBA case study hiding inside it.
"When you build a finished product on top of somebody else's critical component, the component supplier will eventually look at your margins and decide they want them."
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All articles
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Financial
How to Calculate Compound Annual Growth Rate (CAGR)
The CAGR formula explained in plain English, a $10,000 to $25,000 over 8 years worked example, why it beats simple averages for multi-year returns, and the mistake most people make.
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Scientific
Logarithms in 5 Minutes: The Inverse of Exponents
What logarithms are, how they undo exponents, the formulas for log base 10 and natural log, the three core properties, when you actually need them in real calculations, and why e is called the natural base.
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Basic
How to Calculate a Percentage Increase or Decrease
The formulas, the absolute vs relative distinction, why +20% then −20% is not zero, common shortcuts that actually work, markup vs margin, and a quick-reference table for reversing percentage changes.
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Gear Guide
The Best Calculators for the SAT and ACT: What's Actually Allowed (And What to Buy)
Not all graphing calculators are allowed on the SAT and ACT. The short list of what's permitted, the honest TI-84 vs TI-Nspire vs Casio comparison, and which one to buy if you're taking the test in the next six months.
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Scientific
Standard Deviation: What It Means and When to Use It
Standard deviation measures spread. The formula explained, a worked example on test scores from 65 to 95, why low SD means consistency, the 68-95-99.7 rule for normal distributions, and when variance matters more.
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How-to
How to Calculate a Tip (And Split the Bill Without Guessing)
The 15%, 18%, and 20% mental math shortcuts, a real $42.18 example with rounding rules, and the split-the-bill trap that shorts your server every time.
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Financial
How to Calculate a Car Loan Payment (With Trade-In, Down Payment, and a Worked Example)
The PMT formula applied to a $32,000 / 6 years / 7.5% auto loan, how trade-in and tax actually roll into the amount financed, the term-stretch trap (3 yr vs 7 yr is $5,400 more interest), and the 0%-vs-rebate math the dealer doesn't want you running.
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Financial
How to Calculate Debt-to-Income Ratio (DTI): The Number Lenders Care About Most
Your credit score gets you a quote. Your DTI decides whether the loan closes. The two formulas, the lender thresholds for conventional/FHA/VA, the three levers that actually lower it, and how to back into qualifying income.
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Gear guide
The Best Financial Calculators for CFA Candidates: HP-12C vs BA II Plus, Decided
The CFA Institute approves exactly two calculators for the exam. The honest comparison: who each is for, the catch with each, and the three-question decision tree for picking before Level I.
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Heritage
The HP-12C: The Financial Calculator That Refused to Die
Launched in 1981 at $150 with a weird input method nobody else used, and still made today in essentially its original form. The longest-shelf-life consumer electronics product on record — and why every CFA candidate still keeps one on their desk.
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Financial
What Is APR? The Real Cost of Borrowing, Explained
APR is supposed to tell you what a loan really costs — but a 24% credit-card APR is actually 26.82%, and a "lower APR" mortgage with discount points only pays off if you hold the loan past month 46. The math, the legal definition, and when APR misleads.
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Financial
How to Shop for a Mortgage Without Getting Outsmarted
$400k at 6.5% vs 6.0% is a $47,000 difference over 30 years. Seven steps to compare lenders, decode the Loan Estimate, and avoid the discount-points trick.
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Financial
Simple vs Compound Interest: How to Calculate Both
$10,000 at 6% for 20 years: simple interest pays you $22,000, compound pays $32,071. Both formulas worked side-by-side, a year-by-year gap table, and the daily-compounding trick credit cards quietly use against you.
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Heritage
The Abacus: The Original Calculator That Outlasted Every Replacement
4,000 years old, beat an electric calculator in a 1946 Tokyo speed contest, and still taught in Japanese elementary schools. The most stubborn computing device ever made — and why simple things outlast their replacements.
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Gear guide
The 5 Best-Selling Calculators on Amazon (Reviewed & Ranked, 2026)
Hands-on breakdown of the five calculators that dominate Amazon — TI-84 Plus CE, TI-30XIIS, Casio fx-9750GIII, BA II Plus Professional, and fx-300ES. Real prices, who each is for, and the honest cheaper alternative for each.
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Financial
APR vs APY: Why Banks Quote Them Differently
Same rate, two numbers, and the difference can cost you real money. The formula (APY = (1 + APR/n)ⁿ − 1), a worked 6% → 6.17% example, and why credit cards quietly charge more than they advertise.
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Financial
How to Calculate a Mortgage Payment (And What the Math Actually Does)
The PMT formula worked out on a $300k / 6.5% / 30-year loan, what amortization really means, why month 1 is 86% interest, and how one extra payment a year cuts 4–5 years off the loan.
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Scientific
Degrees vs Radians: When to Use Which
What a radian actually is, why calculus is allergic to degrees, and why sin(30) gives you completely different answers depending on one tiny mode indicator. Check your calculator before you trust the result.
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Basic
How to Calculate Percent Change Without Getting the Direction Wrong
100 → 150 is +50% but 150 → 100 is −33.3%, not −50%. A +50% / −50% round trip loses you 25%. The denominator trap, the percentage-point confusion, and a cheat sheet of common reversals.
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Graphing
Reading a Graph: Asymptotes, Holes, and What That Gap Means
f(x) = 1/(x−2) has a vertical asymptote. f(x) = (x²−1)/(x−1) has a hole. The graphing calculator may not show you the difference — but the algebra does. Three flavors of asymptote, worked.
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