How to Measure an Area for Gravel, Mulch or Soil

Every quantity calculation starts with the area and the depth. Most mistakes come from measuring the area badly or mixing up units, and both are easy to avoid.

What you need

A tape measure, a pencil and paper, and a rough sketch of the area. Measure in feet if you are working in US units, and write each measurement on the sketch as you go.

Rectangles and squares

Area is length times width. A bed 12 feet long and 5 feet wide is 12 × 5 = 60 square feet. This is the easiest shape, and most areas can be broken into rectangles.

Circles

Measure the distance straight across the middle, called the diameter. Area is 3.14 × (diameter ÷ 2) × (diameter ÷ 2). A round bed 8 feet across is 3.14 × 4 × 4 = 50 square feet.

Triangles

Area is half the base times the height, where the height is the straight distance from the base to the far corner. A triangle with a 10 foot base and a 6 foot height is 0.5 × 10 × 6 = 30 square feet.

Irregular and curved areas

Split the area into shapes you can measure, work out each one, and add them up. A rough fit is fine, because you will add an allowance for waste anyway. For a curved bed, a common trick is to treat it as the nearest rectangle and adjust the ends.

The calculators on this site let you add several areas, each a rectangle, a circle or a known area. For a triangle, work out its area by hand and enter it under "I know the area".

Worked example: an L-shaped bed

An L-shaped bed made of one section 20 feet by 4 feet and another 10 feet by 4 feet, mulched 3 inches deep.

Area: 20 × 4 + 10 × 4 = 120 square feet
Volume: 120 × (3 ÷ 12) = 30 cubic feet
In cubic yards: 30 ÷ 27 = 1.11
With 5 percent extra: 1.17 cubic yards

Depth

Depth is a third measurement, and it is the one people most often get wrong. Measure the depth in inches, then divide by 12 to convert to feet before you multiply. Four inches is 4 ÷ 12 = 0.33 feet, not 4. Mixing inches and feet makes the answer twelve times too big or too small.

Converting to cubic yards

Cubic feet to cubic yards: divide by 27.
Square feet to square yards: divide by 9.
Square yards and cubic yards are different things. A cubic yard measures volume, and it is what mulch, soil and gravel are sold by.

Common mistakes

Mixing units. Keep length and width in feet and convert the depth from inches.
Measuring once. Measure twice, especially long runs.
Ignoring existing material. If there is already mulch or stone, measure it and add only the difference.
Forgetting the extra. Add an allowance for settling and waste. Five to ten percent is common.
Treating slopes as flat. Measure along the ground for a steep slope, or treat it as extra.

Try the same example in the mulch calculator by adding the two rectangles as separate areas.

Questions

How do I measure a sloped area?

For a gentle slope, measuring across the top of the ground is close enough. For a steep slope, measure along the surface, because the real area is larger than the flat footprint. Adding a little extra for the slope is a reasonable shortcut.

How do I convert inches to feet?

Divide by 12. Two inches is 0.17 feet, 3 inches is 0.25 feet, 4 inches is 0.33 feet and 6 inches is 0.5 feet.

How many square feet are in a square yard?

Nine, because a square yard is 3 feet by 3 feet. A cubic yard is 27 cubic feet, because it is 3 by 3 by 3. Square yards measure area and cubic yards measure volume, so they are not interchangeable.

Should I round my measurements?

For an estimate, rounding up to the next half foot is fine, because you are adding an allowance for waste anyway. For anything sold in fixed sizes, such as bags, rounding up avoids a second trip.