Geometric vs Arithmetic Mean: Which to Use
The arithmetic mean adds and divides; the geometric mean multiplies and takes a root. Learn which fits growth rates, ratios, and averages.
Lena Hoffmann · 3 min read
Guides & tutorials
Practical guides, how-tos and comparisons to get the most out of our free online tools.
October 29, 2025
How to Calculate Your Due Date
Naegele's rule adds 280 days to your last period — or subtract three months and add seven days. Here's how it works and why it's only an estimate.
Sofia Nunes · 3 min read
Heart Rate Zones Explained (and How to Find Yours)
Heart rate zones split your effort into five bands, from easy to all-out. Here's what each does, how to estimate your zones, and how to train with them.
Aisha Karim · 3 min read
October 28, 2025
Cooking Measurement Conversions
A few anchors — cups, tablespoons, grams — cover most recipes. Here's the handful worth memorising, why weight beats volume, and when to reach for a scale.
Marco Bianchi · 3 min read
What Size Air Conditioner Do I Need?
Around 20 BTU per square foot is the starting rule. Here's how to size a unit from your room's area, adjust for sun and use, and why bigger isn't better.
Marco Bianchi · 3 min read
How to Calculate Standard Deviation (Step by Step)
Standard deviation measures how spread out your data is. Here's the formula, a worked example, and the difference between population and sample.
Lena Hoffmann · 3 min read
October 27, 2025
How to Calculate Your GPA
GPA is a weighted average of your grades. Here's how to convert letter grades to points, weight by credits, and work out your overall GPA.
Lena Hoffmann · 3 min read
October 24, 2025
What Is Your FIRE Number? The 25× Rule Explained
Your FIRE number is the nest egg that funds financial independence. Learn the 25× rule, the 4% safe-withdrawal rate, and Coast, Lean and Fat FIRE variants.
Camille Laurent · 4 min read
How Much Sleep Do You Need?
Adults need 7–9 hours, and sleep runs in ~90-minute cycles. Here's how much by age, how to time your bedtime, and why quality counts too.
Sofia Nunes · 3 min read
What Are Macros (and How Do You Count Them)?
Macros are the protein, carbs and fat in your food. Here's what each does, how many calories they hold, and how to set your daily split.
Sofia Nunes · 2 min read
October 23, 2025
Markup vs Margin: What's the Difference?
Markup and margin both describe profit on a sale, but from different bases. Confusing them costs money — here's how each works and how to convert.
Camille Laurent · 2 min read
October 22, 2025
Ovulation and Your Fertile Window
Ovulation is usually 14 days before your next period, and the fertile window spans about six days. Here's how it works and how to estimate yours.
Sofia Nunes · 2 min read