How to measure for any material (and avoid over-ordering)
Two mistakes cost DIYers the most money: running out of material mid-project (which means a trip back to the store and a visible seam or color mismatch) and buying far more than needed (waste money and storage space). Both are avoidable with good measuring.
The golden rule: measure twice, calculate once
Before entering any number into a calculator, take three separate measurements of each dimension and use the average. For irregular spaces, break them into rectangles and add them up. Never estimate by eye - a wall that "looks like 12 feet" is often 11 or 13.
For area-based materials (paint, flooring, tile, mulch)
Measure length times width for each rectangular section. For L-shaped rooms, split into two rectangles. For circular areas (like round garden beds), use pi times radius squared (3.14 x r x r).
For linear materials (fence, baseboard, trim)
Walk the perimeter with a tape measure, or use a measuring wheel for long runs. Record every section including gates and openings (you will subtract them for pickets but add them back for the fence run itself).
For volume materials (concrete, mulch, gravel)
Volume = length x width x depth, all in the same units. Converting: 1 cubic foot = 12in x 12in x 12in. 1 cubic yard = 27 cubic feet. Use the calculators on this site to handle the math.
When in doubt, round up
Material that you have extra of can usually be returned, stored for touch-ups, or used for another project. A half-finished floor or wall because you ran short costs more to fix than extra material costs to buy.