Practical guides, how-tos and comparisons to get the most out of our free online tools.
The band and the cup are computed differently in the UK, the US, the metric EU, the French and Spanish markets and Italy. One body, its four labels, and the arithmetic behind each.
Sofia Nunes · 15 min read
Swap one cake pan for another by matching their area, then adjust batter depth and baking time so the cake rises and bakes evenly in the pan you actually own.
Marco Bianchi · 4 min read
The heat index combines air temperature and humidity into a single 'feels-like' number, and explains why a humid 90-degree day can be dangerous.
Marco Bianchi · 3 min read
A nap's effect depends on which sleep stage the alarm interrupts, and length is the only part of that you control. What 10, 20, 30 and 90 minutes actually do, with the arithmetic for the alarm.
Sofia Nunes · 14 min read
What password entropy really measures, why length beats complexity, and how bits of entropy translate into a realistic crack time.
Daniel Okonkwo · 4 min read
Work out a pay raise as a percentage and a new salary, then adjust for inflation to see your real raise. Formulas, worked examples and common mistakes.
Camille Laurent · 3 min read
Use the distance formula d = √((x₂−x₁)² + (y₂−y₁)²) to measure the straight-line distance between two points on a plane, with a worked example and the midpoint too.
Lena Hoffmann · 4 min read
Understand what hydration percentage means, how water-to-flour ratio shapes your crust, and which level to choose for pan, Neapolitan, or New York style pizza.
Marco Bianchi · 4 min read
How early you can test is the sum of three delays: when implantation happens, how fast hCG climbs from that moment, and what the test can detect. Each one is measured in the literature, so the chain can be computed end to end — and it explains exactly why an early negative is so often wrong.
Sofia Nunes · 12 min read
Doubling time is ln(2) times the interval divided by the natural log of the ratio. Here it is derived and worked on real pairs, with the two things most often got wrong online: the expected rate slows as the level rises, and a single value is meaningless because the reference range at one gestational week can span more than two hundredfold.
Sofia Nunes · 11 min read
A lender does not decide how much you can borrow. A ratio decides a monthly payment, and an interest rate turns that payment into a principal. On $7,500 of income the two steps give $1,805 and $285,571 — and clearing one $280 debt adds $44,299.
Camille Laurent · 14 min read
Ovulation is estimated far better by counting backwards from the next period than forwards from the last one, because the phase after ovulation is the stable one. Here is the inversion worked for every cycle length, the fertile window from the study that defined it, and the failure rates that make this unusable as contraception.
Sofia Nunes · 12 min read