How to Measure Length

What is length?

Length is the measurement of an object's longest side from end to end. For example, the length of a boat is the distance from the very tip of the bow (front) all the way to the rear. Length is measured in units, or small quantities of the same size that add up to the total length of an object. There are 2 major systems of measurement units: In the metric system, the standard unit of length is the meter. There are 10 millimeters in 1 centimeter and 100 centimeters in 1 meter. In the imperial system, the standard unit of length is the foot. There are 12 inches in a foot, and 3 feet make a yard. Most of the world uses the metric system. The United States, Myanmar, and Liberia use the imperial system.

Measuring Object Lengths

Use a measuring tool with the right units for your measuring system. Some measuring tools show markings for imperial and metric units, while some show only one system. Eyeball the length of the object or surface you’re measuring and grab a measurer that looks just as long or longer: Use a ruler for objects smaller than a piece of paper. Imperial rulers are 1 foot long and metric rulers are 30 centimeters long. Use a yardstick or meter stick for medium objects bigger than a ruler. Yardsticks are 3 feet long (36 inches) and meter sticks are 1 meter long (100 centimeters). Use a tape measure for long objects, like the length of a room. Imperial tape measures use feet and inches, and metric ones use meters and centimeters.

Line up one end of the object with the 0 (zero) mark on your measurer. When you use a ruler, look carefully for the 0 mark—it’s at the very end of some measuring sticks, but sometimes it’s away from the end. On tape measures, the 0 mark is the edge of the metal clip (the tang) at the end of the tape. Make sure the object and your measuring tool are lined up straight with each other. Otherwise, your measurement will be a diagonal instead of the straight length. Double check that your measuring tool reaches all the way along the object’s length. If it’s too short, grab a bigger measurer.

Look how far the object reaches on the measurer to read its length. Measuring tools use tick marks to help you see how many units long an object is. Read the tick mark closest to the other end of your object to see its length. Metric and imperial rulers use different sets of tick marks: Metric rulers: The biggest tick marks show centimeters. These tick marks have numbers by them to help you count centimeters easily. There are 9 smaller tick marks inside each centimeter. These smaller ticks are millimeters (there are 10 millimeters in 1 centimeter). Imperial rulers: The biggest tick marks with numbers are inches. Inches have different-sized ticks inside for different divisions: The largest ticks between inches are half inches. The mid-sized ticks between half inches are quarter inches. The second-smallest ticks between quarter inches are eighth inches. The smallest tick marks between eighth inches are sixteenth inches. Read measuring tapes the same way you’d read a ruler or measuring stick. The only difference is the extra markings that show feet or meters.

Choose the smaller whole number if the object ends between 2 units. Look at the end of the object—if it lands between 2 whole numbers (the big ticks for centimeters or inches), then choose the lower of the 2 numbers. The length of the object is that number of units, plus a fraction of another unit. For example, if the tip of a pencil lands between the 5-inch mark and the 6-inch mark, the pencil is 5 inches plus a fraction of another inch long. If the pencil tip landed exactly on the 5-inch mark, then it’s exactly 5 inches long.

Count the tick marks after your whole number. Each tick mark is a fraction of a unit. On metric rulers, every tick is 1 millimeter (1/10th of a centimeter). On imperial rulers, the smallest ticks are 1/16th of an inch. The longer ticks outline eighths, quarters, and halves of an inch for easier counting. If the pencil tip lands past the 5-inch mark, count tick marks from “5” to the tip. If you count 3 eighth-inch ticks, then the pencil is an extra ⅜ inch long.

Add your whole number and fraction together. The total of these measurements is the total length of your object. In our pencil example, add 5 inches to ⅜ inch. The total length of the pencil is 5 and ⅜ inches!

Round your measurement if the object ends between 2 tick marks. See which tick the end of the object is closest too. Round your answer to this tick mark since it’s the most accurate. For example, if something is closer to the 3rd millimeter tick mark than the 4th millimeter tick mark, call it 3 millimeters long. Consider why you're measuring when you round. If you’re measuring wood to cut, for example, round up since you can always cut off more.

Converting Metric and Imperial Units

Convert units to find length in a different measurement system. Since different countries use different systems, it’s helpful to know how to state an object’s length in both metric and imperial units. For example: Pretend you’re an American who wants to buy a shirt in France. You know your sleeve length in inches, but French stores measure clothes in centimeters. You’ll have to convert your sleeve length to centimeters to buy the right size. Some countries, like Canada, use a mix of both systems. In these places, knowing how to convert is very helpful! Some measuring tools have both metric and imperial tick marks. In this case, just read the ruler instead of converting units.

Look up how many metric units fit inside imperial units and vice versa. Say you’re converting feet to meters—you’ll need to know how many imperial feet there are in 1 metric meter to make an accurate conversion. Use a conversion chart for quick reference: Metric to imperial conversions: 1 millimeter = 0.039 inches 1 centimeter = 0.39 inches 1 meter = 1.1 yards 1 kilometer = 0.62 miles Imperial to metric conversions: 1 inch = 2.5 centimeters 1 foot = 0.3 meters 1 yard = 0.91 meters 1 mile = 1.6 kilometers

Multiply your measurement value by the corresponding conversion factor. Pretend you have a headboard that is 4 feet long, but need to know how many meters long it is. Multiply your measurement (4) by the number of meters per foot (0.30) to get the length in meters (1.2 meters). Alternatively, use an online conversion tool like Calculator.net for fast and accurate conversions.

Other Ways to Measure Length

Use a cloth tape measure to find lengths of clothing measurement. Cloth tape measures are shorter, more flexible, and less abrasive on your skin than regular tape measures. Shoppers and tailors take clothing measurements between specific points for accurate measurements: Sleeves: Measure from the point where your spine meets the collar of your shirt, over your shoulder, down to your wrist. Inseam: Measure from the lowest part of the crotch to your ankle (with your shoes off). Dress shirt length: On the front of the shirt, measure from the bottom of the collar down to the tail of the shirt. Dress length: Measure from the top of the straps (or hollow of your neck for strapless dresses) down to the hemline of the dress. Skirt length: Measure from your natural waist to the top of your kneecap along the side of your body.

Use an odometer to measure the length of a car trip. Odometers display the total distance a car has traveled. Note the odometer’s number before you start driving, then look again when you arrive at your destination. Subtract the original display number from the final display number to find the length of your trip. In imperial countries, cars measure distance in miles. In metric countries, distance is measured in kilometers.

Estimate length with nonstandard units. Nonstandard units are anything besides metric or imperial units. Try using parts of your body to make rough estimates—try your hand span for small objects, foot lengths for medium objects, and arm spans or steps for larger objects or surfaces. Alternatively, use any object you have on hand to make a rough measurement—try books, spoons, pens, or jump ropes. Measure your nonstandard unit with a ruler or tape measure later to get a length estimate in metric or imperial units.