Method 1: split the area into simple shapes
Sketch the area on paper, then draw lines that cut it into rectangles, triangles and circles or parts of circles. Measure each piece, work out its area and add them up. University extension guides use the same approach for lawns, and suggest keeping the sketch with its numbers for next time.[1]
For the bed above:
| Piece | Shape | Formula | Area |
|---|---|---|---|
| A | Rectangle | 16 × 5 | 80 sq ft |
| B | Rectangle | 6 × 4 | 24 sq ft |
| C | Half circle, 5 ft across | 3.1416 × 5² ÷ 8 | 9.8 sq ft |
| Total | 113.8 sq ft |
At a mulch depth of 3 inches that is 113.8 × 0.25 = 28.5 cubic feet, or 1.05 cubic yards. The calculator at the top of this page is preloaded with these three pieces so you can check it or swap in your own. The same shapes are available in the mulch, gravel and topsoil calculators.
Formulas for each piece
- Rectangle or square: length × width.
- Circle: 3.1416 × radius², which is the same as 3.1416 × diameter² ÷ 4. Measure the diameter straight across the widest point.[1]
- Half circle: half of the full circle, 3.1416 × diameter² ÷ 8.
- Triangle: base × height ÷ 2, where the height is measured at a right angle to the base.[1]
- Oval: length × width × 0.785 (π ÷ 4). Clemson's factsheet rounds this to 0.8.[1]
- Ring around a tree: outer circle minus inner circle.
Method 2: the offset method for free-form shapes
For kidney-shaped beds and lawns with no straight edges, extension services teach the offset method.[1]
- Lay a tape along the longest straight line across the area, from end A to end B.
- Divide that line into equal segments, for example every 8 ft.
- At the middle of each segment, measure the width of the area at a right angle to the tape.
- Add the widths and multiply by the segment length.
area ≈ (sum of widths) × segment length
A 40 ft line split into five 8 ft segments, with widths of 10, 14, 15, 12, 7 ft, gives (10 + 14 + 15 + 12 + 7) × 8 = 58 × 8 = 464 sq ft. Shorter segments follow the curves more closely and give a better estimate.[1] Enter the result as a Known area in any calculator on this site, or convert it with the square yard calculator.
Check the arithmeticAverage width × the full length of the line gives the same answer as the formula above. Multiplying the average width by the segment length alone gives an area that is far too small.
Metric measurements
Everything above works the same in meters: square meters × depth in meters = cubic meters, as in the cubic meter calculator. Every calculator here has a meters and centimeters switch. One foot is exactly 0.3048 m, so 1 sq ft = 0.0929 m².[2]
Subtract what you are not covering
Measure patios, paths, large boulders and planting holes that will not get material, and enter them as separate pieces you leave out, or subtract their area from your total before entering it as a known area. Once you have the area, the coverage chart shows how far each cubic yard goes.
Frequently asked questions
How do I measure a curved flower bed?
Use the offset method: run a tape along the longest line, measure the width every few feet at a right angle to it, then add the widths and multiply by the spacing.
How do I find the area of a triangle-shaped corner?
Multiply the base by the height and halve it. The height must be measured square to the base, not along the sloping edge.
Is it better to over- or under-measure?
Measure as accurately as you can, then add a small percentage for waste and uneven ground in the calculator rather than inflating the measurements.