How to Calculate Pounds With Measuring Tape
You can determine the weight of an object, without the use of a scale, using a tape measure and basic mathematics. The weight of an object depends on its weight density and its volume. Materials with a larger weight density are more compact; for example, leather has a weight density of 60 lb. per cubic foot, while sawdust has a weight density of 13.1 lb. per cubic foot. The volume of an object is a direct measure of the space it occupies and is related to its physical dimensions.
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Instructions
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Rectangular Object
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1
Measure the length, width and height of the rectangular object in inches. For example, assume you have a block of iron that is 25 inches long, 10 inches wide and 5 inches high.
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2
Multiply the length by the width by the height to get the volume of the object in cubic inches. In the example from step 1, 25 inches times 10 inches times 5 inches gives a volume of 1,250 cubic inches.
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3
Divide the volume in cubic inches by 1,728 to convert to cubic feet, because 1,728 cubic inches equals 1 cubic foot. In the example, 1,250 cubic inches divided by 1,728 cubic inches equals a volume of 0.72 cubic feet.
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4
Look up the density of the material composing the object in kg per cubic meter on a density table. Convert the density in kg per cubic meter to a weight density in pounds. per cubic foot by dividing by 16.02. Continuing the example, the density of iron is 7,850 kg per cubic foot, so performing the conversion leads to 7,850 divided by 16.02, or a weight density of 490 lb. per cubic foot.
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5
Multiply the weight density of the object, in pounds per cubic foot, by the volume to arrive at its weight in pounds. The density of iron is 490 lb. per cubic foot. Completing the example, you have 490 lb. per cubic foot times 0.72 cubic feet, or a weight of 352.8 lb.
Cylindrical Object
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6
Measure the height and circumference of the cylinder in inches. Circumference measures the distance around the curved side of the cylinder. Assume the height and circumference of an iron cylinder are 40 inches and 10 inches, respectively.
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7
Divide the circumference by two times pi to get the radius in inches. Use 3.14 for pi. Radius measures the distance from the center of the circular end of the object to its edge. Continuing the example, you have a circumference of 10 inches divided by 6.28, or a radius of 1.6 inches.
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8
Multiply pi times the square of the radius times the height to obtain the volume of the cylinder in cubic inches. With the example, you have 3.14 times 1.6 inches times 1.6 inches times 40 inches, which equals a volume of 321.5 cubic inches.
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9
Divide the volume by 1,728 to change to cubic feet. In the example, 321.5 cubic inches divided by 1,728 cubic inches per cubic foot, or 0.19 cubic feet.
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10
Locate the density of the object, in kg per cubic foot, on a density table and then convert the value to a weight density in pounds per cubic foot. With iron at 7,850 kg per cubic foot, dividing by 16.02 leads to a weight density of 490 lb. per cubic foot of iron.
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11
Multiply the weight density of the object, in pounds per cubic foot, by the volume to arrive at the weight of the cylinder in pounds. Completing the example, you have 490 lb. per cubic foot times 0.19 cubic feet, which equals 93.1 lbs. for the iron cylinder.
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