Online RulerHow to read between ruler marks
When an edge falls between two ruler marks, write down the range or label a single reading as an estimate. Use the two marks that border the edge. If you can clearly see that the edge sits near the middle or closer to one side, say so in your notes.
- The closest marks on either side give you a range you can check again.
- A halfway reading is an estimate unless the scale has a line at that point.
- A finer scale helps only if you can see both the marks and the object's edge clearly.
- Choose the scale and unit you need. Check how the short marks divide the space between two numbered marks.
- Align the near edge with zero. Put the scale as close to the object as possible and keep it parallel to the length you are reading.
- Look straight down at the far edge. Find the last mark before it and the first mark after it.
- Record both readings with the unit, or write a single reading with the word “about” if you can judge where the edge falls.
- If you need the nearest marked value, pick the closer line when you can tell which it is. Keep the original range if the choice is unclear.
Use the marks you have
On a fractional inch scale, look at how many equal spaces fit between neighboring inch numbers. Count the spaces from the lower whole-inch mark to the line you want. For an edge between lines, read each bordering line in the same way. A written range is enough when the lines are crowded or when the edge sits too close to the middle for you to pick a side.
Check your viewing angle
An edge can seem to slide along the scale as you move your head. This is easier to notice when the edge sits above the ruler surface. Bring the scale close to the edge and look straight down over the reading. Check the start from directly above it too. If a second look puts the edge in a different gap, reset the ruler and check where the movement came from.
Record what you can see
Use a range when the end is fuzzy, soft, or hard to place. If you must supply one value, use a marked value or a clearly judged estimate and say which you chose. Converting units changes how you write the length. Keep the original reading alongside a conversion so the range or estimate is still clear.
What if the line itself is wide?
Use the center of the printed line as its position. If the edge and line overlap in a way that makes the position hard to judge, call the reading approximate.
Can I report an exact halfway value?
If you judged the gap by eye, write “about” before the value. Use a finer scale if you need to check whether the edge lines up with a mark there.
Which unit should I write?
Record the unit on the scale you read. If a form asks for the other unit, use 1 in = 25.4 mm and carry over any range or estimate.