How to Measure Length
To measure the length of an object, choose the correct tool for measuring it, like a ruler for small, flat items, or a measuring tape for items with curves. Then, line up the zero mark on your measuring tool with one end of the object. Make sure to keep your measuring device perpendicular to the edge of the object so you get an accurate measurement. Finally, note the number on your tool that's closest to the other end of the object.
Choose the most appropriate tool. There are several tools you can use to measure length, but the right tool will depend on the unit system you plan to use and the size of the length you wish to measure. Rulers are stiff, straight edges with graduated markings. Typically, one side has markings for inches and the other side has markings for centimeters. These tools are good to use for relatively short lengths. Tape measures are flexible straight edges with graduated markings. Since most tape measures only measure one unit system (U.S. customary or metric), you'll need to find one that uses the unit system you need. Since these tools can bend, they're good to use when measuring the total length of an object that exists in more than one dimension (e.g., waist measurements, the circumference of a wood block, etc.). Meter sticks and yard sticks are similar in construction and length. Both are stiff straight edges with graduated markings. Meter sticks measure all lengths up to 1 meter (or 100 cm), and yard sticks measure all lengths up to 1 yard (or 3 feet). Odometers are tools that measure long lengths traveled by vehicles, like cars and bicycles. Pedometers measure long lengths traveled by a human or other living creature as it walks. These tools are good for measuring miles and kilometers, but they are calibrated by professionals and work without hands-on involvement from the user. Line up the "0" mark with one end. Find the zero (0) mark on one end of the measuring stick or measuring tape. Align this zero mark with the starting edge of the object to be measured. Note that the zero mark is not always located at the exact starting edge of the measuring tool. Look for the long line standing just above the labeled 0 —that line is the zero mark. Extend the measuring tool over the length. Lay the measuring stick or measuring tape flat over the surface of the object. Keep the entire tool perpendicular to the starting edge. Continue to extend the measuring tool over the entire length until you reach the other edge of the intended length. Identify the largest whole number. Go to the ending edge of the object to be measured and look for the largest whole number that shows just before that ending edge. Write down that whole number. Make sure that you write down the unit of measurement along with the whole number. When the edge lands between two whole numbers, use the lesser of the two values. For instance, if the edge of a measured length falls between 5 inches and 6 inches, use 5 inches for your measurement. Count the lines after that number. Count the total number of lines in between whole number sets, then count the exact number of lines existing between the largest whole number and the ending edge of the object to be measured. Mark these numbers down. The number of lines between whole number sets should be consistent for the entire tool. When using the metric system, there will usually be 9 lines (10 spaces) between whole numbers. When using the U.S. standard system, there will usually be 3 lines (4 spaces), 7 lines (8 spaces), or 15 lines (16 spaces). Count the number of lines exactly. If the edge falls between two lines, round up or down based on which line the edge lies closest to. For instance, if there are 7 lines (8 spaces) between sets on the ruler and your edge lands closer to the third line than the second line, use the third line. This would give you 3/8 inch (use the number of spaces for the denominator, not the number of lines). Add the measurements together. Add the number of lines, as a fraction, to the largest whole number. The sum of these two numbers should be the length of the object. Following the previous example: 5 inches + 3/8 inches = 5-3/8 inches Double-check your result. Since it's easy to make mistakes when taking length measurements, it's usually a good idea to take the measurement again by following through the same steps. Compare the results when finished. If the results differ from your first measurement, continue re-measuring until you find two matching measurements. Know the inch. The inch is the smallest unit of length you'll likely see when working within the U.S. standard. An inch is roughly the same length as the last joint of an average adult finger. This is only an estimate, though, and not an accurate way to measure inches. Understand feet. The foot is the second smallest unit you'll probably need, and 1 foot equals 12 inches. As its name suggests, the foot was originally named since it roughly matched the length of an adult male's foot. Since human feet vary so much in length, though, you cannot accurately measure U.S. standard feet using your own foot. Progress to yards. Yards are slightly larger than feet. To be more precise, 1 yard equals 3 feet. This also means that there are 36 inches in 1 yard. As an estimate, one yard is roughly the same length as the height of a standard acoustic guitar. Learn about miles. Miles are one of the largest units you'll see within this measuring system. There are 1,760 yards in 1 mile. This means that there are also 5,280 feet in 1 mile. Similarly, there are 63,360 inches in one mile. Learn about the meter. The meter is the basis of all length measurements within the metric system. One meter is roughly the same length as the height of a guitar. This is only an estimate, though, and not a reliable way to measure meters. Identify smaller measurements. Each smaller length unit gets smaller by a factor of 10. The ones you'll need to use most often are the decimeter, centimeter, and millimeter. In 1 meter, there are: 10 decimeters 100 centimeters 1000 millimeters Progress to larger measurements. Each larger length unit gets larger by a factor of 10. The ones you'll need to use most often are likely the decameter, hectometer, and kilometer. There are: 10 meters in 1 decameter 100 meters in 1 hectometer 1000 meters in 1 kilometer Know the relationship between both measurements. Since U.S. standard units and metric units do not follow the same scale, you'll need to know the mathematical relationship between the unit you have and the unit you need when converting one to the other. A few standard to metric conversions worth memorizing include: 1 inch = 2.54 centimeters 1 inch = 25.4 millimeters 1 foot = 30.48 centimeters 1 yd = 0.91 meters 1 mile = 1.6 kilometers A few metric to standard conversions worth memorizing include: 1 millimeter = 0.04 inches 1 centimeter = 0.39 inches 1 centimeter = 0.0325 feet 1 meter = 3.28 feet 1 meter = 1.09 yards 1 kilometer = 0.62 miles Convert most units with multiplication. When you know how many of the intended unit exists for one of your original units, you can multiply the original value by the conversion factor. Example 1: Convert 5.4 inches to centimeters. There are 2.54 centimeters in 1 inch, so: 5.4 * 2.54 = 13.72 centimeters Example 2: Convert 13.72 centimeters to inches. There are 0.39 inches in 1 centimeter, so: 13.72 * 0.39 = 5.4 inches Convert some units with division. If you only know how many of the original unit exists for one of the intended unit, you'll need to divide the original value by the conversion factor. Example 1: Convert 5.4 inches to centimeters. There are 0.39 inches in 1 centimeter, so: 5.4 / 0.39 = 13.8 centimeters Example 2: Convert 13.8 centimeters to inches. There are 2.54 centimeters in 1 inch, so: 13.8 / 2.54 = 5.4 inches 
Choose the most appropriate tool. Line up the "0" mark with one end. Extend the measuring tool over the length. Identify the largest whole number. Count the lines after that number. Add the measurements together. Double-check your result. Know the inch. Understand feet. Progress to yards. Learn about miles. Learn about the meter. Identify smaller measurements. Progress to larger measurements. Know the relationship between both measurements. Convert most units with multiplication. Convert some units with division. 