The Rule of 72, Explained
The rule of 72 estimates how long money takes to double: divide 72 by the interest rate. Here's why it works, when it's accurate, and how to flip it.
Camille Laurent · 2 min read
Guides & tutorials
Practical guides, how-tos and comparisons to get the most out of our free online tools.
September 1, 2025
August 29, 2025
Word Count: Reading Time and Limits
A word count is words separated by spaces. Here's how it's counted, why limits exist, how it maps to pages and reading time, and when characters count instead.
Daniel Okonkwo · 3 min read
August 28, 2025
What Is Aspect Ratio? 16:9, 4:3 and How to Compute It
Aspect ratio is the proportion of width to height. Learn what 16:9 and 4:3 mean, how to compute a ratio from pixel dimensions, and where each ratio is used.
Lena Hoffmann · 3 min read
How Much Gravel Do I Need?
Multiply area by depth for a volume, then convert to weight — gravel is about 1.5 tonnes per cubic metre. Here's the method, depths by job, and how to order.
Marco Bianchi · 3 min read
August 27, 2025
What Is a UUID (and When to Use It)?
A UUID is a 128-bit identifier that's unique without any central authority. Here's what it looks like, why it's useful, the versions, and when to use one.
Daniel Okonkwo · 2 min read
August 26, 2025
Frame Rates, Timecode, and the Drop-Frame That Drops Nothing
Frames = duration × rate is trivial until the rate is 29.97. Non-drop timecode then drifts exactly 3.6 seconds an hour. Drop-frame corrects it by skipping 108 frame numbers — the same 108 — but the correction is not perfect: counting every frame shows a residual of 3.6 milliseconds per hour.
Daniel Okonkwo · 18 min read
How to Convert a Fraction to a Decimal
Divide the top number by the bottom. Here's the rule, why some fractions repeat, how to round them, and how to turn the result into a percentage.
Lena Hoffmann · 2 min read
August 25, 2025
What Is a Percentile? The 90th Percentile Explained
A percentile tells you where a value ranks within a group. Learn what the 90th percentile really means and why a percentile is not the same as a percentage.
Lena Hoffmann · 3 min read
August 22, 2025
How Big Is That Audio File?
Uncompressed audio size is exact arithmetic: sample rate × bit depth × channels × seconds ÷ 8. CD audio is 176,400 bytes per second. A 320 kbps MP3, by contrast, is bitrate × duration and does not depend on the sample rate at all — 2,400,000 bytes a minute whether it came from 44.1 or 48 kHz.
Daniel Okonkwo · 15 min read
How to Calculate Your Net Worth
Net worth is what you own minus what you owe. Here's how to total your assets and debts, what to include, and why the trend matters more than the number.
Camille Laurent · 3 min read
August 21, 2025
Gradients, Banding, and Why the Middle Looks Muddy
Interpolating in sRGB averages gamma-encoded numbers, so the midpoint of red to green is #808000 when the half-light answer is #bcbc00 — 57.2% too little light. Banding is a separate arithmetic problem: 8 bits give 256 steps, and a dark gradient may have only 28 of them. Here is both, computed.
Daniel Okonkwo · 17 min read
Pregnancy: Weeks to Months
Pregnancy is counted in weeks, but months aren't exactly four weeks. Here's why it's 40 weeks, how weeks map to months and trimesters, and a quick reference.
Sofia Nunes · 2 min read