Articles
Practical guides to the calculations behind everyday decisions: the formulas, worked examples, the assumptions each one makes on your behalf, and the mistakes that produce a confident-looking wrong answer.
Every article here starts from a question someone actually needs answered — what a pay rise is worth after inflation, whether the larger pack is really cheaper, how much a set of small subscriptions costs over a year, what a discount stacked on a discount actually comes to. Each one gives the formula, walks through a realistic example with numbers, and then does the part that matters most: explains where the calculation stops being reliable.
Where a calculator on this site covers the same ground, the article links to it, so you can read the reasoning first and then run your own figures. Results throughout are estimates for planning and educational purposes; rates, taxes, fees and personal circumstances change the outcome.
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Splitting the bill evenly feels easiest at the table, right up until someone who ordered a side salad and water is paying the same as someone who ordered steak and two cocktails.
Moving in on the 18th, upgrading a subscription mid-cycle, or starting a job mid-pay-period all raise the same question: what's the fair amount for a period that isn't a full one?
Promise an employee a $5,000 net bonus, pay them $5,000 gross, and taxes will quietly turn that promise into less money in their account than what was agreed.
Two accountants can depreciate the exact same $20,000 machine and report completely different expenses in year one — both correctly, just using different methods.
No one withholds tax from a freelance invoice. Estimating and setting aside the right amount every quarter is the only thing standing between a normal tax season and an unpleasant one.
Break-even point is easy to calculate for one product. Sell three products at three different margins, and the single-product formula quietly stops working.
Turnover rate is a percentage most managers can recite. Turnover cost is a dollar figure almost none of them have actually calculated — and it's usually larger than assumed.
The inventory count never quite matches what the books say it should be. Turning that gap into a percentage is the first step to knowing whether it's a rounding error or a real problem.
Basic ROI ignores time, which makes a 40% return over ten years look better than a 15% return over one. Here is the version that accounts for it.
A weighted average counts some values more than others. It is how course grades, GPAs and blended rates are calculated, and it is easy to get wrong by treating the weights as scores.
Inventory turnover connects the amount a business sells with the stock it keeps on hand. This guide shows how to calculate it and what the number can—and cannot—tell you.
Revenue recorded on a sale is not the same as cash collected. Accounts receivable days helps a business measure the gap between credit sales and customer payments.
Read the assumptions and limitations section before trusting any number for a real decision. Most of these calculations are short — a division, a multiplication, occasionally an exponent — and the arithmetic is rarely where things go wrong. What goes wrong is using a figure that does not match the situation: an advertised fuel-consumption number instead of your own, a total-spending figure where essential spending was needed, or a rate quoted on a different basis from the one you are comparing it against.
Several of the articles overlap on purpose. Percentage change, reverse percentages and stacked discounts are the same underlying idea applied to three different problems, and reading two of them together tends to make the third obvious.