Online RulerHow to Read Millimeters Between Centimeters
Each centimeter on a metric ruler contains 10 millimeters. Find the numbered centimeter at or just before the endpoint, then count the small spaces past it. An endpoint three spaces past 4 cm reads 4.3 cm, or 43 mm. Each of those spaces adds 1 mm.
- Two neighboring centimeter lines bound 10 spaces. Nine millimeter lines sit inside that span.
- The middle line, often longer than its neighbors, is 5 mm past the preceding centimeter line.
- One inch equals 25.4 mm. Read the metric edge directly when the measurement calls for millimeters.
- Find the edge labeled cm or mm. Check the labels so you know whether the large numbers stand for centimeters or millimeters.
- Align the starting edge of the object with zero. Hold the object close to the markings and look straight down at the endpoint.
- Find the last whole centimeter at or before the end. Count each small space from that line to the endpoint, including the space that ends at the longer middle line.
- To write the result in millimeters, multiply the whole centimeters by 10 and add the small spaces. To write it in centimeters, put that extra count in the tenths place.
Count through the middle
Between 6 cm and 7 cm, the longer middle line marks 65 mm. The next two lines are 66 mm and 67 mm. If the endpoint is near the far boundary, count backward. The line just before 7 cm is 69 mm. It is one millimeter short of 70 mm.
Measure from another line
You can start from a clear numbered line when the ruler's end is chipped or covered. Read both ends in the same unit. If an object starts at 20 mm and ends at 54 mm, subtract 20 mm from 54 mm to get 34 mm. Align the starting edge carefully because the subtraction uses that starting position.
Check a printed scale
If you use the AR Ruler printable page, print at actual size and check its 10 cm test line against a physical ruler before measuring. Use the edge with centimeter labels for millimeter readings. Place the object close to the printed lines so you can see which division lines up with the endpoint.
Why are there nine small lines in one centimeter?
The small lines divide the span into ten spaces. The centimeter lines at both ends complete the first and last spaces.
How do I write a reading that lands on 8 cm?
Write 8 cm or 80 mm. The endpoint is on the centimeter line, so there are zero extra millimeters to add.
What if the edge lands between two millimeter lines?
If you need a reading to the nearest millimeter, use the closer line. If the edge appears centered between them, follow any rounding rule given for the task.