Calculators / Guide
Guide
How to measure an odd-shaped area
Nearly every real garden bed is a shape with no name. Here is the method that gets you close enough to order confidently.
Split it, do not average it
The instinct with a curved bed is to pace out a rough length and width and multiply. That reliably under-measures, because the widest part of the bed becomes the whole bed in your head.
The better method is to draw the bed roughly on paper and cut it into shapes you can measure: rectangles, circles, half-circles and triangles. Three simple shapes measured properly beat one clever estimate every time. Every calculator on this site lets you add as many areas as you need and totals them.
The shapes you actually need
A rectangle is length times width. A circle is measured by diameter — walk the widest point across the middle, not around the edge. A triangle is the base times the height, halved, where the height is measured square to the base, not along a sloping side.
A ring, for a border around a patio or a bed around a tree, is the outer diameter minus the inner one. That one is worth using rather than approximating, because subtracting the hole makes a large difference on wide rings.
Curves: two workable approximations
For a bed with one curved edge, measure the rectangle that contains it and then subtract a triangle or two, or simply accept the rectangle and treat the difference as waste. On small beds the second approach costs a bag or two and saves twenty minutes.
For a kidney or crescent shape, split at the narrowest point and measure each lobe as its own rough rectangle. Errors on either side tend to cancel.
Measure depth twice
Area errors are annoying; depth errors are expensive, because depth scales the whole volume. Doubling the depth doubles the order.
Check the depth you actually want at the place it will be deepest and the place it will be shallowest, and use something closer to the average than the maximum. For beds against a house, remember you cannot pile material above the damp course or siding line.
A worked example
A front bed runs 22 feet along the house, is 6 feet deep at one end, tapers to 3 feet at the other, and has a 4-foot-diameter circle cut out for a tree.
Split it into a 22 by 3 foot rectangle (66 sq ft) and a triangle 22 feet long by 3 feet at the deep end (33 sq ft), giving 99 square feet. The tree circle is 12.6 square feet, so call it 86 square feet. At 3 inches of mulch that is 21.5 cubic feet, or 11 bags. Guessing '22 by 5' would have given you 110 square feet and three unnecessary bags.