How to measure an irregular area for landscaping materials

Every material calculator wants one number: square feet. A plain rectangle is easy. A garden with a curved edge, a notch around the porch and a tree in the middle is not, and it is where most quantity mistakes happen. This guide covers four ways to get that number, from fastest to most precise, and how to check that the answer is sensible.

Before you measure

You need a tape measure of 25 feet or longer, something to mark points (flags, stakes or spray paint), a pencil and graph paper or a notes app. A second person makes long measurements much easier. Keep everything in feet. If you take a measurement in feet and inches, convert inches to decimals: 6 inches is 0.5 ft, 3 inches is 0.25 ft, 9 inches is 0.75 ft.

Sketch the shape first, even roughly, and label each measurement on the sketch as you take it.

Method 1: break it into simple shapes

This works for most beds, patios and lawns.

Rectangle: length x width.
Triangle: half the base x the height (the height is measured at a right angle to the base).
Circle: 3.14 x radius x radius. The radius is half the diameter.
Half circle: half of the circle's area.
Ellipse (oval): 3.14 x half the long axis x half the short axis.

If part of the shape is a hole, such as a tree well or a fire pit, work out its area and subtract it.

Worked example

A patio bed made of four pieces:

A main rectangle, 18 ft x 9 ft = 162 sq ft.
A triangular corner, with a base of 8 ft and a height of 6 ft = 0.5 x 8 x 6 = 24 sq ft.
A half circle bulging out at one end, radius 5 ft = 0.5 x 3.14 x 25 = 39.3 sq ft.
A round tree well to leave out, 4 ft in diameter (radius 2) = 3.14 x 4 = 12.6 sq ft.

Total: 162 + 24 + 39.3 - 12.6 = 212.7 sq ft.

At a mulch depth of 3 inches, that is 212.7 x 3 / 324 = 1.97 cubic yards. In metric, 19.8 square metres at 7.5 cm deep, or 1.5 cubic metres.

Method 2: measure the curve with offsets

For a gently curving bed, a long wavy border or a kidney shape, simple shapes are clumsy. Instead, run a straight baseline along one edge, then measure the distance from the baseline to the far edge at regular intervals, say every 4 feet. Add up the results with a trapezoid sum:

Area = spacing x (half the first width + all the middle widths + half the last width).

Example: a bed is 20 ft along its straight edge. The widths at 0, 4, 8, 12, 16 and 20 ft are 3, 5, 6.5, 6, 4.5 and 2 ft.

Area = 4 x (1.5 + 5 + 6.5 + 6 + 4.5 + 1) = 4 x 24.5 = 98 sq ft.

The closer the intervals, the better the answer. Use 2 ft intervals for a tight curve and 5 ft for a gentle one. If an edge has a sharp corner, put a measuring point at the corner.

Method 3: the grid on graph paper

Quikrete's patio guide suggests drawing a proportional outline of the shape on graph paper, each square standing for one square foot, then adding up the full squares and the three-quarter, half and quarter squares along the edge. It suits free-form patios and ponds, and it handles any shape.

If the shape is large, let each square stand for 2 ft by 2 ft instead, which is 4 square feet per square. A 20 ft by 15 ft shape then fits on a sheet 10 squares by 7.5 squares. Count the whole squares first, then estimate the partial ones around the edge to the nearest quarter, and multiply by the area each square represents.

Method 4: lay it out on the ground

For a shape you have not built yet, such as a new bed, lay a garden hose or rope on the ground to the shape you like. Then drive in stakes along the hose at the corners and every few feet around the curves, and measure between the stakes, either using Method 1 or Method 2. Adjust the hose until you like the look, and measure after the shape is final, not before.

Sanity checks

Three quick checks catch most errors.

The bounding box. Measure the longest length and the widest width of the whole shape. Your area must be less than their product. A shape that fills the whole box is a rectangle.
The circle ratio. A circle covers about 79 percent of the square around it. Quikrete's patio guide gives the example: a 20 ft square patio has 400 sq ft; a 20 ft circle only about 315.
The second pass. Measure the area a different way, with grid or shapes, and compare. Two answers within 5 percent are good enough for mulch and topsoil.

Slopes and uneven ground

Roseville Rock Supply says to measure along the slope surface rather than horizontally, because sloped ground has more surface than the plan view suggests. If you have a plan, the horizontal distance is shorter than the true surface. For steep banks, add a bit extra.

Depth varies too. For a hollow or a low spot, measure the depth at several points and use the average. Roseville Rock Supply recommends a buffer of 5 to 10 percent for settling and uneven ground.

From square feet to materials

Once you have your area in square feet:

Mulch, soil and gravel: area x depth in inches / 324 = cubic yards.
Concrete: area x thickness in inches / 12 / 27 = cubic yards.
Bags: area x depth in inches / 12 = cubic feet, then divide by the bag size.

For several areas with different depths, calculate each separately and add the volumes, instead of averaging depths. Our calculators for mulch, topsoil, gravel and concrete accept the area you have just worked out.