Guides & how-tos
Practical answers with the math shown — every guide pairs with a free calculator. Written by Mohamed Zakrya.
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 Much Should You Tip? Rates by Situation, With the Math
The customary rates by setting, the three-line formula, and the split order that keeps the tip at the rate you chose.
August 14, 2026 · 12 min read
Travel
How Much Will Gas Cost for My Trip? Road-Trip Fuel Math
Three numbers set the fuel bill: miles driven, the MPG you actually get, and today's price. Most bad estimates get the middle one wrong.
August 14, 2026 · 12 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
Home Improvement
How Many Gallons Is My Pool? Average Depth and the Three-Part Floor
The mean of the shallow and deep ends is exact for one shape of floor, and most pools are not that shape.
August 13, 2026 · 9 min read
Home Improvement
How Many Pavers Do I Need? Pavers, Base, Bedding and Joint Sand
Four quantities come out of one measurement, and the paver count is the easy one — the base is where a patio is usually ordered short.
August 13, 2026 · 9 min read
Home Improvement
How Many Stairs Do I Need? Risers, Treads and the Code Edge
You do not choose the riser height. You choose how many, and the height is whatever the division hands back.
August 13, 2026 · 9 min read