How to Calculate Standard Deviation by Hand, Step by Step
Standard deviation tells you how spread out a set of numbers is. This walkthrough does it by hand on eight values and explains when to divide by n or n minus 1.
Articles, page 6
Plain-English guides to the numbers in everyday life: how a calculation works, where people go wrong, and which tool to reach for.
How to Count the Days Between Two Dates (and Avoid Off-by-One)
Counting days between dates is simple once you decide whether both end dates count. This guide covers the day-of-year method, year boundaries and leap days.
How to Count Working Days Between Two Dates
Working days are weekdays minus holidays. Learn the weeks-times-five shortcut, how to decide whether the start day counts, and how to add working days to a date.
How to Estimate Body Fat With a Tape Measure (US Navy Method)
A soft tape and a log button are enough to estimate body fat. Here is the US Navy method step by step, what the percentage means, and how far to trust it.
How to Estimate Roof Area From the Footprint and Pitch
You can estimate roof area from the ground: measure the footprint, multiply by a factor set by the pitch, and add an allowance for hips, valleys and waste.
How to Find the Area of an L-Shaped Room or an Irregular Yard
Almost any plan can be broken into simple shapes. Learn the four formulas you need, the add-or-subtract trick, and a method for curved edges.
How to Measure a Room and Order the Right Amount of Flooring
Flooring is sold in boxes or rolls, not exact areas. Measure the room in sections, add a waste allowance for the layout, and round up to whole boxes.
How to Pick a Winner Fairly: Random Draws Explained
A fair draw gives every entry the same chance. Learn the odds for one or several winners, a common dice trap, and a repeatable process for raffles and giveaways.
How to Read a Confidence Interval (and What 95% Means)
A confidence interval is a range built from sample data. Learn how to calculate one for a mean and a proportion, and how to describe it without overclaiming.
How to Scale a Recipe Up or Down Without Ruining It
Scaling a recipe is multiplication, but pan size, eggs and cooking time need extra care. Here is a step-by-step method with a full worked example.
How to Set an Hourly Rate From Your Annual Income Goal
Freelancers and contractors who divide a salary by a full-time year end up with a rate that is too low. Start from billable hours instead and the maths changes.
How to Split a Group Into Fair Random Teams
Dividing people into teams at random is simple once you shuffle and deal. The harder part is uneven numbers and mixed skill levels, which this guide explains with odds.