How to Calculate Linear Feet

One of the most important parts of planning an upcoming construction or home improvement project is to determine how much material is needed. For many projects, this will mean finding the linear feet of material being used in the project because many common construction materials (like lumber and steel, for instance) are often measured in feet and sold by the foot. Additionally, with the right measurements, figures for linear feet can easily be extrapolated into figures for square feet and cubic (“board”) feet. Because of this, knowing how to find the linear feet of material needed for a project is an essential skill for any home improvement expert.

Finding Linear Feet of Material in a Project
Divide your project into different categories of materials. All construction projects (and the vast majority of home improvement projects) involve assembling separate raw materials into a complete whole. To be able to determine how many linear feet of each type of material your project will need, first, you’ll need to divide your materials by category, grouping identical building materials with each other.
As a running example, let’s pretend that we’re planning for a relatively easy project: building a bookshelf. Let’s say that the bookshelf’s sides are made from 2×4 boards and that the top, the bottom, and the three shelves in the middle are made from 1×12 boards. In this case, we would divide our building materials into two categories: 2×4 boards and ×12 boards.

Use a tape measure or ruler to measure each individual piece. Once you know what sorts of building materials you’ll be using in your project, you’ll want to measure the length of each individual piece. Because we’re dealing with linear feet (rather than square feet, for instance), we don’t need to worry about our building materials’ width or thickness. As you measure, take care not to measure the same pieces multiple times — it can be helpful to make a sketch of your project and label each piece with its length as you go. In our example, let’s say that the 2×4 boards we’re using for the sides of our bookshelf are both eight feet long and that the 1×12 boards we’re using for the top, bottom, and shelves are all six feet long.

Add up the lengths of different types of materials. Next, add up the lengths of individual pieces that are made from the same type of material to find a total length value for each material. This value represents the length of material you’d need if you were going to buy just one long piece for your project and cut it into smaller pieces as needed. If your project contains multiple pieces of the same material that have equal lengths, you can save time by multiplying the length of one of the pieces by the number of pieces. In our example, since we have two eight foot long side pieces made from2×4 boards and five pieces made from 1×12 boards (three shelves plus the top and bottom), we can find out totals by multiplying as follows:
2×4 boards: 8 × 2 = 16 feet

1×12 boards” 6 × 5 = 30 feet

Use your totals to determine the cost of your materials. When you know how much of each material you need for your project, you know how much you’ll hypothetically need to buy. Find the price of each type of material (per foot) and multiply by the total linear foot value obtained for the type of material to find the approximate cost of the material.
In our bookcase example, we need 16 feet of 2×4 board and 30 feet of 1×12 board. Let’s say that 2×4 board sells for $1.50 per foot and 1×12 board sells for $2.25 per foot. In this case, we would determine the costs of these materials by multiplying as follows:
2×4 boards: 1.50 × 16 = $24.00

1×12 boards: 2.25 × 30 = $67.50

Convert your foot value to other units of length if necessary. Not all building materials are sold in linear feet of material. Some are sold in different units of length, while others are sold in non-length units of measurement (like units of area, volume, etc.) If your materials aren’t sold in feet but are instead sold in another unit of length, convert your foot value to this new unit before calculating your prices. Usually, this is just a matter of simply multiplying or dividing by a constant. Below are instructions for converting feet to several other common units of length:[1] Feet to inches: Multiply by 12

Feet to yards: Divide by 3

Feet to centimeters: Multiply by 30.48

Feet to meters: Divide by 3.28

Be conservative with your purchases. When it comes to building projects, one of the most widely-circulated tips of the trade is to always bring slightly more material than you think you’ll need. Doing this gives you a little “wiggle room” to account for errors in your calculations or mistakes you make during the project. While this will increase the overall cost of your materials somewhat, it’s usually a wise idea in the long run because it eliminates the hassle of having to run back to the hardware store if you run out of materials halfway through your project (plus, extra materials can be stored for future projects).
In our example, we’ve calculate that we’ll need about 16 feet of 2×4 board and 30 feet of 1×12 board. To be safe, we may want to buy 20 feet and 35 feet, respectively. If we have any left over, we can always use it to put vertical dividers in some of the shelves.

Using Linear Feet to Find Other Values
Find square feet from length and width. Once you know the length of all the materials you need for your project, you can often use this information to help you make other calculations related to your project. For instance, since the two-dimensional area of a rectangular space is its length times its width, you can often use the length measurements for materials that form rectangles to find the area bounded by the materials. In this case, all you need to do is multiply the length measurements together. Note that, to obtain the values needed for a proper area calculation, some extra measuring may be required.
Let’s return to the example question above. Suppose that we want to cover the entire back of our bookshelf with particle board, which, for our purposes, is sold by the square foot (rather than the linear foot). In this case, since the sides of the bookcase are eight feet tall and the tops and bottoms are six feet long, it might seem like we need to multiply 8 — 6 to get our answer. However, this answer fails to account for the thickness of the 2×4 boards used as the sides of the bookcase, which make the entire bookshelf slightly more than six feet wide.

Let’s say that, after measuring, we find the 2×4 boards to be two inches thick. Since the bookcase has two side boards, it’s actually four inches (one third of a foot) wider than six feet. So, to find the area of particle board that we need, we’d multiply as follows:
8 × 6.33 = 50.64 square feet.

Know area equations for non-rectangular shapes. Not all projects will deal solely in rectangles — countless other shapes are possible. If you encounter a simple shape (like, for instance, a circle or triangle), you can usually simply plug a certain easy-to-obtain length value into a specific equation to get a value for the the shape’s area. As long as your measurements are all in feet, your answer will be in square feet. Below are area equations for a few common shapes:[2]
Circle: π(r)2 — r is the distance from the exact center of the circle to its edge (or its “radius”).

Triangle: (hb)/2 — b (“base”) is the length of one of the sides and h (“height”) is the length of the line from the opposite point that intersects the base at a right angle.

Square: s2 — s is the length of one of the sides.

Trapezoid: (1/2)(a + b)(h) — a and b are the lengths of the two parallel sides and h is the distance between them.

When possible, divide irregular shapes into smaller regular ones. Some projects will use two-dimensional shapes for which a simple area equation isn’t available. In these cases, try to break the irregular shapes into several smaller regular shapes with areas that can be calculated via simple equation. In some cases, this may require dividing the results of an equation to accommodate for the fact that only some fraction of a certain shape is being used.
Let’s return to our example problem from above. Suppose that, in additionto adding the particle board to the back of the bookcase, we want to add a three foot wide half-circle of particleboard on top of the bookcase so that we can mount a clock on it. There isn’t an easy equation for finding the area of a rectangular shape with a half circle extending out of the top. However, in this case, we can simply use the value we already have for the rectangular backing portion and add it to one half of the area of a three foot wide circle to determine our total as follows:
50.64 + (1/2)(π(1.5)2) = 50.64 + (1/2)(7.07) = 54.17 square feet

Find cubic feet from length, width, and height. Some projects will require you to find the volume of a three dimensional space. Because volume is length times width times depth, you can find the volume of box-shaped object or space by using the lengths of your materials to determine these dimensions and multiplying. As noted above, some extra measuring may be required. Let’s say that, in our example problem, we want to determine theapproximate three-dimensional volume of our bookcase. We know how tall and wide it is already, so we measure how deep the shelves are and get a measurement of 1.5 feet. With these three measurements, we can find the volume of the bookcase simply buy multiplying our dimensions as follows:

8 × 6.33 × 1.5 = 50.64 × 1.5 = 75.96 cubic feet.

Common Formulas for Determining Area
Rectangular or square shapes: Length x width

Non-equilateral Triangles: (Length x width)/2

Equilateral Triangles: square root of 3, divided by 4, multiplied by the length of a side squared.

Ellipse (circular shape): length radius x width radius x pi.

Vendors should have already determined the length, width and board feet of their material. Take note of the tags.

Remember that lumber is denoted by its rough size: the true dimensions of a typical 2×4, for instance, is actually closer to x .

The term “linear feet” is sometimes used synonymously with “lineal feet”. In fact, this usage is incorrect. The word “linear” is meant to denote measurement, while the word “lineal” is usually used in the context of genealogy or family history.

Related wikiHows
How to Calculate Meters to Feet

Sources and Citations

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