Blog
Practical guides on the math, text and conversion tasks our tools solve, written by the people who build them.
What Grade Do I Need to Pass a College Class?
At most US colleges a D passes a class, though many majors, prerequisites and transfer credits need a C. Here is the real threshold, plus how to find the exact score you need on your final.
How to Read Roman Numerals
Seven letters, two rules. Learn how to read Roman numerals fast, with a 1 to 100 chart and worked examples like MCMXCIV for 1994 and MMXXVI for 2026.
How to Calculate Concrete (How Much You Need for a Slab)
The concrete you order is measured in cubic yards. Turning a slab's dimensions into that number takes one formula plus a 10 percent cushion.
How many words is a 5-minute speech? The words per minute math
A 5-minute speech runs 650 to 750 words at a natural speaking pace. The research behind words per minute, a chart for every talk length, and a 15-minute test that gives you your own rate.
SOS in Morse code and the other famous signals
The letters of SOS stand for nothing: delegates picked the rhythm in 1906. The story of the distress call, the Titanic night that made it famous, plus 73, 88 and how to spell I love you.
How to convert cm to inches: formula, mental math and chart
The cm to inches formula is a single division by 2.54. Here is why that number is exact, a mental shortcut that gets you within 1.6 percent, and a chart of the sizes you actually look up.
How to calculate percentage increase (formula and examples)
The percentage increase formula in one line, worked examples with real prices, and the two traps that catch almost everyone: asymmetry and percentage points.
How to calculate hours worked (with a full week example)
The exact method to calculate hours worked: end time minus start time minus breaks, the minutes-to-decimal conversion payroll expects, and the mistakes that shrink paychecks.
How much to tip in every situation: a practical US guide
The going rates for restaurants, bars, delivery, haircuts, taxis and hotels in the US, why tipping works the way it does, and how to split the bill without the awkward math.