How Big Should Your Emergency Fund Be?
Three to six months of essential expenses is the usual rule. Here's how many months fit your situation, where to keep the money, and how to build 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 22, 2025
Fuel Cost for a Road Trip
Divide distance by fuel economy, then multiply by the price. Here's the calculation, metric vs mpg, what pushes it up, and how to split the cost.
Marco Bianchi · 3 min read
September 19, 2025
Title Tags, Meta Descriptions, and What Search Engines Do With Them
What Google's own documentation says about rewriting title links and about the meta description, rather than what SEO folklore says. Then the measurable part: titles are truncated by pixel width, so two titles of exactly sixty characters can render 204.55 pixels apart and only one of them survives.
Camille Laurent · 13 min read
What Is ROI and How Do You Calculate It?
ROI measures your gain relative to what you invested, as a percentage. Here's the formula, a worked example, and what ROI leaves out.
Camille Laurent · 3 min read
September 18, 2025
What Is the Debt-to-Income Ratio?
Your debt-to-income ratio compares your monthly debt payments to your income. Here's how to calculate it, why lenders use it, and how to improve it.
Camille Laurent · 2 min read
September 17, 2025
What Is Your Maximum Heart Rate?
A quick estimate is 220 minus your age, but it varies widely. Here's what max heart rate means, why it's only a guide, and how to use it for training.
Sofia Nunes · 3 min read
HEX vs RGB: How to Read and Convert Colours
HEX and RGB are two notations for the same colours. Here's how to read each, convert between them, and add transparency.
Daniel Okonkwo · 3 min read
September 16, 2025
Growing an Email List Is a Leaky Bucket
Constant additions against a proportional loss rate do not grow forever — they converge on a ceiling equal to additions divided by the loss rate. Here is that ceiling computed for six loss rates, why a list can grow while its engaged half shrinks, and the feedback loop that makes a bought list self-defeating.
Camille Laurent · 11 min read
September 15, 2025
Probability Basics, With Examples
Probability measures how likely something is, from 0 to 1. Here's the basic idea, how to combine events with AND and OR, and the complement.
Lena Hoffmann · 2 min read
September 12, 2025
The Lead-to-Customer Rate, and Why It Is Not One Number
The overall rate is the product of the stage rates, which makes a percentage gain anywhere worth the same percentage overall — and a ten-point gain worth wildly different amounts depending on where you put it. Plus the lag that makes a growing business look worse than it is.
Camille Laurent · 11 min read
What Is a Good Body Fat Percentage?
Body fat percentage is a better health signal than weight alone. Here are the healthy ranges by sex, how it's measured, and how to use it wisely.
Sofia Nunes · 2 min read
September 11, 2025
Impressions, Reach and Frequency Are Three Numbers, and You Need All Three
Impressions divided by reach is frequency — one identity that decides what a campaign actually is. The same impression budget split five ways, what a frequency cap really costs you, and why the three-exposure rule is a 1970s heuristic rather than a law.
Camille Laurent · 12 min read