Guides & how-tos
Practical answers with the math shown — every guide pairs with a free calculator. Written by Mohamed Zakrya.
Health & Fitness
Why a Taller Person Gets a Higher BMI for the Same Build
Three numbers in the formula were chosen rather than measured: the exponent, the constant, and the line at 25. One of them is worth nine grams. Another is worth five index units.
August 19, 2026 · 10 min read
Finance & Math
Why the Minimum Payment Shrinks as Fast as the Balance
The first minimum on a 5,000 balance at 22% is 101.83, and 10.17 of it touches the debt. Hold that same figure instead of paying it and the same money clears the card 690 months sooner.
August 19, 2026 · 10 min read
Math
Why the Quadratic Formula Loses Accuracy on One of Its Two Roots
Completing the square takes four lines and produces the formula. It also shows which of the two roots is safe to trust, and the calculator on this site was corrected to use the other route.
August 19, 2026 · 10 min read
Math
Geometry Backwards: Finding the Radius from the Area
Every geometry formula is printed in the direction you need least. Running it backwards is not the same operation reversed; it is a root, and roots can refuse.
August 17, 2026 · 10 min read
Math
When Cross Multiplication Gives You the Wrong Answer
Cross multiplication answers every question you put to it, including the ones it should refuse. The test is one line: if the input were zero, would the output be zero?
August 17, 2026 · 10 min read
Math
How to Add Fractions With Different Denominators — and Why the Rule Works
1/2 + 1/3 done the wrong way gives 2/5 — smaller than the 1/2 you started with. A sum must beat both addends, and that check is free.
August 16, 2026 · 10 min read
Math
Percent Grade to Degrees: Why a 100% Grade Is 45 Degrees, Not Vertical
A 100% grade is 45 degrees. Percent grade has no ceiling; degrees stop below 90, and that mismatch is where every conversion error starts.
August 16, 2026 · 9 min read
Math
Scale Factor: Why Doubling the Size Gives Four Times the Area
Twice as big means four times the paint and eight times the water. Lengths take k, areas take k², volumes take k³, and every scaling error is the wrong exponent.
August 16, 2026 · 9 min read
Unit Converters
Converting Temperature Differences: Why 10 °C of Change Is 18 °F, Not 50
A day that warms from 10 °C to 20 °C warms by 18 °F, not 50. The 32 belongs to temperatures — a difference never sees it.
August 16, 2026 · 9 min read
Utilities
How Long Should a Password Be?
Eight characters with every symbol you can find: 5 hours. Twelve lowercase letters and nothing else: 6 days.
August 15, 2026 · 10 min read
Dates & Time
How Time Card Rounding Changes Your Overtime Pay
The same 70 minutes of rounding is worth $25.67 in one week and $38.50 in the next. Nothing changed but the total.
August 15, 2026 · 10 min read
Math
Percentage Difference vs Percentage Change: Which One Do You Want?
40 and 60 are 40% apart, 50% more, and 33.3% less. All three are correct. The denominator is the decision.
August 15, 2026 · 9 min read
Electricity & Energy
What Size Solar System Do I Need to Go Off Grid?
7 panels cover a 10 kWh day with 0.08 kWh to spare. Refilling the battery on that surplus takes 125 days.
August 15, 2026 · 10 min read
Electricity & Energy
How Long Will a Battery Run My House? The Two Tests It Has to Pass
A 100 Ah battery is 1,200 Wh, 600 Wh of it usable — and none of that says whether your fridge will start.
August 14, 2026 · 9 min read
Electricity & Energy
What Is a Demand Charge? Load Factor and Your Real Rate per kWh
Two businesses on one tariff can pay 22.27 cents and 14.57 cents a kilowatt-hour, decided by nothing but how evenly they drew power.
August 14, 2026 · 9 min read
Electricity & Energy
What Is Power Factor? The Current Your Watts Do Not Show
Watts describe the result. Volt-amps describe the burden — and conductor heating follows the square of the difference.
August 14, 2026 · 9 min read
Finance & Math
How to Calculate a Discount: Sale Prices, Stacked Deals, and Tax
The percent-off formula, the sequential math behind stacked deals, and the discount-then-tax order the register actually uses.
August 14, 2026 · 13 min read
Home Improvement
How Much Paint Do I Need? The Complete Room-by-Room Guide
A gallon covers about 350–400 sq ft — but the real number depends on coats, surface, and sheen. Here's how to get it right, with a room-size cheat sheet and the calculator to do the math for you.
August 14, 2026 · 7 min read
Cooking
Laphing 101: Everything You Need to Know About This Tibetan Delicacy
Laphing is Tibet's spicy cold-noodle street food — slippery mung-bean jelly noodles in fiery chili sauce. Learn its origins, how it's made, and how to make it at home.
August 14, 2026 · 17 min read
Home Improvement
How Many Deck Boards Do I Need? Rows, Gaps and the Fencepost
A six-inch board is three different widths, and the gap between boards is part of what they cover.
August 13, 2026 · 9 min read