How to calculate how much concrete, shingles, or siding is required for renovation or repair projects.
Nothing is more annoying than trying to figure out how much material you need to complete a project and you buy way too much or not enough and have to make a second trip to the lumber yard or home center. Hopefully these quick and easy formulas will help prevent these mistakes.
Ready-mix concrete, the stuff in the “cement” trucks is ordered by the yard. Many suppliers will help you figure out how much you need, but you may be paying for more material than you need or be shorted on your order if you don’t have an understanding of what is needed. Most concrete needs to be ordered by the yard and you should add 10% to your calculation for a safety factor. You don’t have to use any extra you ordered, but you will have to pay for it.
A yard of concrete is 27 cubic feet, or a cub 3 feet by 3 feet by 3 feet, or 46,656 cubic inches. If you are pouring a slab that is one foot thick, not likely, then a yard of concrete would cover 27 square feet, or a rectangle that is 3 feet by 9 feet. For a typical side walk which is 4 inches thick, then a yard will cover 81 square feet, or a 4 foot wide sidewalk 20 feet 3 inches long (20.25 feet). To calculate this, divide 46,656 by 4 and you get 11,664 square inches. Divide 11,664 by 144 to get 81 square feet.
46,656 cu. in. / 4 in. = 11,664 sq. in.
11,664 sq. in. ( 1 sq. ft. / 144 sq. in.) = 81 sq. ft.
If you have a pier for a deck post that is 12 inches in diameter, then you need to calculate how many square inches are in the circle multiplied by the height of the pier. The volume of a cylinder is V = 3.14 x radius squared x height. So a 12 inch pier 3 feet deep would be represented by the function:
V = 3.14 x 6^2 x 36 = 4,069.4 cubic inches. Divide 4,069 by 46,656 to get 0.087 yards. If you have 12 piers to pour, this would be 12 times 0.087 which is 1.044 yards. In this case you can order 1 yard and fill the bottom of the piers with a little extra gravel.
A trickier calculation that comes up is if you are pouring concrete around a fence post. Approximate the dimension of the hole and then subtract the dimension of the post. Assume a 10 inch diameter hole with a 4 x 4 post inside. The post will actually be about 3 ½ inches square, but you don’t need to be that exact. Use the volume for the cylinder for the hole and then subtract the area of the post times the depth of the hole. Example: A 10 in. diameter hole 24 inches deep would result in 1,884 cubic inches. The post would remove 384 cubic inches from the cylinder resulting in 1,500 cubic inches or 0.032 yards per hole. You could fill about 31 holes in this case, 1 yard divided by 0.032 yards equals 31.25.
Shingles and Siding
Shingles and siding are measured in squares. A square equals 100 square feet, or a square that measures 10 feet by 10 feet. The shingles are usually packed 33 square feet per bundle, but could be less for architectural shingles which are thicker. Roof area is calculated by dividing the total roof area by 100 to determine how many squares are needed. Roofers generally add another 10 percent for simple roofs, and 15 percent for roofs with hips and valleys. It takes about 80 shingles will cover one square. You can determine the square footage of your roof by going into your attic and measuring the length of your home and then from the front eave along the rafter to the roof ridge, the highest point, usually in the center. Example: Your home is a simple gable roof, 40 feet long and the rafter is 18 feet long. This makes half of the roof area 720 square feet, so the total roof area would be 2 times this or 1440 square feet. This would be 1584 when you add the 10 percent, (1.1 times 1440 = 1584) or 16 square.
If you’d like to go up on your roof and measure it more accurately, be careful. Also many homes are not rectangles so you may have to use the formula for the area of a triangle to get the total square footage closer. The area of a triangle is A = base x height divided by 2.
Siding is also measured in squares, but you have to be careful when using individual clapboards and setting the reveal. Vinyl siding will be sold in squares, but if you have to repair or install wood clapboards with a reveal, you need to check the wide of the siding and the reveal. Eight inch clapboard may have a 1 ½ inch lap, leaving only 6 ½ inches showing. Different suppliers will help with ordering, most wood or cement siding boards are sold be the linear foot so they will have a conversion to change the number of square feet into linear feet based on the reveal you select if it is different from the recommended reveal.
Typical Siding Conversion Factors
Measuring the area of the walls is fairly straightforward and unless you have a lot of windows and doors, it is best not to subtract the area for these. You will also need the area of a triangle again to calculate the siding for the gable areas.
Gable Siding Calculation
In this figure the height would be the segment DB and the base would be the segment AC. Example: For a gable that is 24 feet (AC) wide and the perpendicular height is 8 feet (DB), then Area = ½(24 x 8) = 96 square feet, or about one square.
Whether you are planning on doing your own repairs or projects or contracting it out, it is always a good idea to know exactly how much material you will need or are paying for.