This section is arithmetic and geometry on their own terms, without a currency symbol or a body measurement attached — the same formulas that show up throughout the finance and health sections, worked directly rather than applied to a specific problem. Three areas make up most of what's here: percentages, descriptive statistics (mean, median, quartiles, variance and the like), and geometry (area, volume, and the Pythagorean relationships behind a right triangle).
Calculators in this section
A percentage change and a percentage point are not the same thing
This is the single most common source of error in the whole section, and it shows up constantly outside it too, in reporting about interest rates and survey results. If a rate moves from 5% to 10%, that is a 5 percentage point increase, but it is a 100% percentage increase — the rate has doubled. Both descriptions are correct; they answer different questions. "Percentage point" measures the raw arithmetic difference between two percentages, while "percentage increase" or "percentage decrease" measures that difference relative to the starting value. Mixing the two up makes a change sound far smaller or far larger than it actually is, which is exactly why this section keeps a dedicated percentage increase and decrease calculator separate from a plain difference calculator, rather than assuming one figure answers both questions.
Mean, median and why they disagree
The mean is every value added up and divided by how many there are; the median is the middle value once they're sorted. They agree closely on a roughly symmetric dataset and diverge sharply once outliers or skew are involved — average household income, for instance, is reliably pulled upward by a small number of very high incomes, which is exactly why income statistics are almost always reported as medians rather than means. Quartiles, variance and the z-score calculators in this section extend the same idea: they describe how spread out a dataset is and where a single value sits relative to the rest of it, which a mean or median alone can't tell you.
Geometry: area, volume, and the triangle formula underneath both
The geometry calculators cover two and three dimensions with the same handful of underlying formulas repeated across different shapes. Area calculations for triangles and quadrilaterals, and the hypotenuse calculator built on the Pythagorean relationship a² + b² = c², cover flat shapes. Volume calculations for cylinders and cuboids extend the same reasoning into three dimensions by adding a third measurement. The hypotenuse formula in particular shows up well beyond triangles on a page — it's the same relationship used to work out a diagonal measurement, a direct distance between two points, or a cable run across a rectangular space, which is why it's one of the more generally useful calculators in this section even outside a maths classroom.
A handful of one-off calculations
Not everything here fits neatly into one of the three groups above. Weighted average shows up whenever different values carry different importance — a grade point average weighting courses by credit hours, or a portfolio return weighting holdings by size — and it differs from a plain mean precisely because it refuses to treat every input as equally significant. X-intercept and similar algebra calculators solve a single equation for the point where a line crosses an axis, a narrower and more mechanical task than the statistics calculators nearby, but one that still comes up constantly once you're plotting anything by hand rather than in a spreadsheet.