Getting the inputs right
How to measure
Every calculator here is only as good as what you put into it. These are the measurements that matter and the ways people get them wrong.
Roofs: measure flat, then set the pitch
Take the building's footprint from the ground and split the roof into planes — one per continuous slope. A simple gable is two planes; a hip roof is four. Enter each plane's length along the eave and its depth from eave to ridge as it appears from above, then set the pitch and let the calculator do the rest.
Get the pitch from the attic: hold a level against a rafter, mark 12 inches along it, and measure straight down from the mark to the rafter. That drop in inches is your rise.
If you have already measured up the slope, do not also apply the pitch — that counts it twice and over-orders by 12 percent or more.
Decks and walls: rectangles are worth the trouble
Enter a deck as a rectangle even if it is an L — add a second area for the other leg. The reason is that joists are spaced along one side and span the other, so the calculator needs the two dimensions, not just their product.
For walls, measure the gross area including windows and doors, then subtract openings only if they are large. A single window in a wall is usually worth ignoring — it lands inside the offcut allowance, and running short of insulation is worse than having one batt spare.
Fences: one run per straight length
Walk the line and write down each straight length separately. Do not add them together first — the calculator needs them apart, because each run rounds up to a whole number of sections on its own, and two 50 foot runs do not need the same posts as one 100 foot run.
Then say how many ends of each run are free. A run standing alone has two free ends; a run meeting another at a corner has one; a run boxed in between two others has none. That is how corner posts get counted once instead of twice.
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.
Odd shapes
Break them into rectangles, triangles and circles and add the pieces. That is more accurate than any single clever measurement, and it is why the area calculators let you add as many shapes as you need.
For a curve you cannot resolve, measure the rectangle that contains it and accept over-ordering slightly. On materials that come in bags and bundles you were rounding up anyway.