How to Measure Irregular Areas for Mulch, Concrete and Pavers

The rule: split it into simple shapes

Any flat area can be divided into rectangles, triangles and circles (or parts of circles). Work out each piece and add them. Where a shape has a cut-out, such as a tree ring inside a bed, subtract the cut-out.

Sketch the area on paper first, mark each piece, and write measurements on the sketch as you go. It prevents measuring the same strip twice.

Formulas

The radius is half the diameter. If you can only measure around a circle (its circumference), divide it by 6.28 to get the radius.

L-shapes and notched patios

Split an L into two rectangles. For an L that is 20 feet along one side and 14 feet along the other, with arms 8 feet wide: one rectangle is 20 × 8 = 160 square feet; the other is (14 − 8) × 8 = 48 square feet. The total is 208 square feet. Note the subtraction: the corner square belongs to only one of the two rectangles.

Curved beds: the offset method

For flowing curves, lay a straight tape down the long axis of the bed. Every 2 or 3 feet along it, measure the width of the bed at right angles to the tape. Add all the widths, divide by the number of measurements to get the average, and multiply by the length.

Example: a bed 18 feet long with widths of 3, 4.5, 5, 6, 5.5, 4 and 2.5 feet. The average is 30.5 ÷ 7 = 4.36 feet; × 18 = 78.4 square feet.

Closer intervals give a more accurate figure. For a bed with sharp changes in width, measure every foot near those changes.

Converting units

• Inches to feet: divide by 12. 4 inches = 0.333 feet.
• Square inches to square feet: divide by 144.
• Square yards to square feet: multiply by 9.
• Cubic feet to cubic yards: divide by 27.
• Square meters to square feet: multiply by about 10.76 (NIST).

Mixing units is the most common mistake in material estimates: multiplying feet by inches without converting gives an answer 12 times too large.

Depth and thickness

For volume materials, area is only half the job. Measure depth in several places. Existing mulch beds often still have an inch or two of old material, and excavations for slabs and patios are rarely an even depth. Use the average, and for concrete, err slightly deep, since running short is worse than a leftover bag. Our cubic yards, feet, bags and tons guide covers turning area and depth into an order.

Tools that help

A long tape (100 feet is handy for yards), stakes or landscape flags to mark corners, and string to make straight reference lines. For long edges, a measuring wheel is faster than a tape. A phone's map app can give a rough area for very large spaces, but measure on the ground before ordering.

Before you dig

If the project involves digging, such as post holes, a patio base or edging, call 811 a few days ahead to have underground utility lines marked. The service is free in the United States.

Common questions

How do I measure a kidney-shaped bed?

Run a tape down the longest length, measure the width across it every 2 or 3 feet, average the widths and multiply by the length. This offset method handles curves well.

How do I convert feet and inches to decimal feet?

Divide the inches by 12 and add them to the feet. 10 feet 6 inches is 10.5 feet; 10 feet 3 inches is 10.25 feet.

How accurate does my measurement need to be?

For mulch, soil and gravel, within a few percent is fine because you add a margin anyway. For pavers and concrete, measure more carefully, especially thickness, because small errors multiply across the whole area.

What's the easiest tool for big areas?

A 100 foot tape or a measuring wheel. A wheel is quick for long, straight edges; a tape is better for widths and short distances.