There are three major measurements that every carpenter and contractor needs to be ready calculate in a moment’s notice. Not only do you need to know how to calculate board foot, square foot, and linear foot, but you also need to know when to use each.
If you swing by the lumberyard you’ll find different items sold in each of these formats and if you come in asking for 20 square feet of 2×4’s or 50 linear feet of plywood you’ll get laughed out of the store and picked on relentlessly. Yes, the lumberyard counter is still a bit like approaching the cool kids table in high school. You better know the right way to talk or you’ll be ridiculed.
In this post, I’ve got calculators for each in their respective sections so you can bookmark this page and save it future reference. In addition, I’ve got the formulas for each because in my opinion it’s best if you understand the concepts and formulas even if you don’t need a pencil and paper to do the math.
Board Foot Calculator
Calculating board feet is all about finding the volume of the wood. Typically rough lumber is sold by the board foot since it will be milled down into a specific product later. Just like finding the volume of water in a pool, calculating board feet of lumber encompasses three different measurements.
Here’s a breakdown of the variables:
- Thickness: The thickness of the wood in inches.
- Width: The width of the wood in inches.
- Length: The length of the wood in feet.
When calculating board feet, you typically use these dimensions to find the volume of the wood in cubic inches (thickness × width × length) and then divide by 144 (12 inches × 12 inches) to convert to board feet. This formula is commonly used in the lumber industry for estimating the amount of usable wood in a log or a piece of lumber.
Keep in mind that if you have multiple pieces of wood, you would calculate the board feet for each piece separately and then sum them up to get the total board feet for all pieces.
Square Foot Calculator
The formula for calculating square feet depends on the shape of the area you’re measuring. Typically in construction we’re focused on squares and rectangles, but that isn’t always the case. If you can round out your knowledge by learning to calculate the square footage of circles and triangles then you can figure out just about anything on a construction site.
Square foot calculations are used for finding the quantities of most things used in construction for things like wood flooring, sheathing, drywall, paint coverage, roofing materials, tile, glass, and more. Don’t forget that there will always be waste so once you have a square footage calculation be sure to add around 10-20% extra to account for off-cuts depending on how effective the layout is.
Square or Rectangle
Squares and rectangles are a piece of cake to calculate square feet. The formula is below. Unlike board feet you’re only calculating is 2 dimensions so the formula is simpler.
Area (in square feet) = Length (in feet) × Width (in feet)
Circles are a pain in construction, but they do exist and you need to be able to figure out the square footage of a circle or a half circle, or any other round-ish element. There are two options for figuring out the square footage of a circle below depending on which works best in your situation.
Here’s a breakdown of the variables:
- Diameter: The length from one side of the circle to the other
- Radius: The length from the center of the circle to one side (1/2 of the diameter)
- π (Pi): An irrational number you just have to memorize
Anybody who remembers their high school geometry will recognize that weird symbol at the beginning of each formula. It stands for “Pi” and it stands for 3.14 (the whole number goes on forever like 3.141592653589793238462643383279502884197…., but using 3.14 is sufficient for most scenarios unless you’re a mathematician).
Finding the square footage or area of a triangle is helpful because a lot of times you can break irregular shapes down into a rectangle and a triangle. The formula for finding the area of a triangle is below:
It’s important to know that for a triangle you are looking to find the height at the highest point which may not be the height of the tallest side, but rather the highest point on the whole shape.
Irregular Shape: For irregular shapes, you may need to break the area into smaller, more regular shapes and then sum their individual areas.
Remember to use consistent units (e.g., feet) when plugging in values into these formulas. If you have dimensions in inches, you may need to convert them to feet by dividing by 12 since there are 12 inches in a foot.
Linear Foot Calculator
Calculating the linear footage (sometimes called the lineal footage) of an object is simplest of all because it doesn’t involve math but rather counting. When it comes to linear footage you’ll likely be dealing with building elements like plumbing, electrical, siding, and trim.
Linear footage doesn’t account for the thickness or width of a particular item like board footage and square footage do. To calculate linear footage you are concerned only with the length of the item.
For example, if you need 10 pieces of 16’ clapboard then you need 160 linear feet of siding (10 X 16 = 160). Linear footage yields the least information about the item. You could have 4”, 6”, or 8” wide clapboards, but when looking at the linear footage you’d never know without further information.
So when dealing with linear footage you need to be specific in your request: 160 linear feet of 8” clapboard siding, 400 linear feet of 1/2” copper pipe, etc.
I hope this post has helped clarify some of the differences between these three items because I see a lot of confusion surrounding when to use which and what each one stands for. Keep this post handy and you’ll be ready to tackle any construction calculation.
Founder & Editor-in-Chief
I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.