How to Add or Subtract Days from a Date
Count forward to add, backward to subtract, rolling over month ends. Here's how, why month lengths and leap years trip you up, and when weekends count.
Lena Hoffmann · 3 min read
Guides & tutorials
Practical guides, how-tos and comparisons to get the most out of our free online tools.
July 25, 2025
What Is CAGR (and How to Use It)?
CAGR turns an investment's whole journey into one steady yearly rate. Here's what it means, the formula, why it's useful for comparing, and what it hides.
Camille Laurent · 3 min read
July 24, 2025
Cycling Power (Watts) Explained
Power is the objective measure of cycling effort. Here's what watts mean, why they beat speed and heart rate, what FTP is, and why watts per kilo matter.
Aisha Karim · 3 min read
July 23, 2025
Return Rate: The Number That Decides Whether Your Ecommerce Works
A returned order is not a cancelled sale, it is a sale that cost you money. Here is the return rate at which returns eat the entire contribution of the orders that stayed sold, for three product profiles and five margin levels — plus what bracketing does to your order count.
Camille Laurent · 14 min read
July 22, 2025
How to Measure Roof Pitch
Roof pitch is rise over run, like 6:12. Here's how to measure it with a level, how to convert it to an angle or percentage, and why it matters.
Marco Bianchi · 3 min read
What Is a Wilks Score?
The Wilks score compares powerlifting totals fairly across body weights. Here's the problem it solves, how it works, what the numbers mean, and its newer rivals.
Aisha Karim · 3 min read
July 21, 2025
The Caesar Cipher Explained: How Shift Ciphers and ROT13 Work
The Caesar cipher shifts every letter by a fixed amount. Learn how the shift works, why ROT13 is a special case, how to encode and decode by hand, and why the cipher offers no real security today.
Daniel Okonkwo · 4 min read
July 18, 2025
Bounce Rate Is Not What Most People Think It Is
A bounce was never a measure of dissatisfaction — it was a session that sent one hit. Google Analytics 4 replaced the definition entirely, and the new one is a configurable timer. Here is the same thousand sessions read five ways.
Camille Laurent · 13 min read
July 17, 2025
ARPU and the Averages That Hide Your Business
Average revenue per user is a mean over a distribution with no middle, divided by a denominator nobody defines. Here is the same month of revenue read five ways, and two opposite businesses landing on exactly the same ARPU.
Camille Laurent · 12 min read
Calories to Joules: The 4.184 Factor Explained
Convert calories to joules with the exact 4.184 factor, understand why food labels mix kcal and kJ, and read energy figures with confidence.
Daniel Okonkwo · 4 min read
July 16, 2025
Process Capability: Cp, Cpk and What Six Sigma Actually Claims
Cp compares the spec width to the process spread; Cpk penalises being off-centre. A process can have an excellent Cp and still make scrap — here is the case, with defect rates computed from the normal distribution rather than read off a table.
Camille Laurent · 14 min read
Interest Coverage and the Ratios a Lender Actually Tests
A loan agreement's covenants are the ratios that can put a solvent, profitable company into default. Interest coverage, times interest earned and DSCR are not three measures — and the one that adds principal repayment is the one that bites.
Camille Laurent · 11 min read