How to read a ruler in cm, mm and inches.

How to read centimeters and millimeters

One centimeter (cm) contains 10 millimeters (mm). On a ruler with millimeter marks, each small interval is 1 mm, or 0.1 cm. The numbered long marks show whole centimeters; the halfway mark is 5 mm.

1. Align one edge of the object with the zero line, which may be inset from the ruler’s physical edge.
2. Find the last whole centimeter before the other edge.
3. Count the millimeter intervals after that line. Add them to the whole centimeters.

Count the spaces after the 3 cm line: the first small mark is 3.1 cm, not 3.0 cm. To express the reading entirely in millimeters, multiply the centimeters by 10: 3.7 × 10 = 37 mm.

If the object begins at zero, that endpoint reading is its length. Otherwise, subtract its starting reading. Keep the object parallel to the ruler so you measure its length rather than a diagonal.

How to read inches and fractions

Check how many equal intervals lie between two numbered inch lines. This online ruler has 16 intervals per inch, so every smallest step is 1/16 inch. A physical ruler may use eighths or thirty-seconds instead.

1. Find the last whole inch before the object’s edge.
2. Count the small intervals after it. On a sixteenth-inch ruler, six steps means 6/16 inch.
3. Simplify the fraction, then add it to the whole inches: 6/16 = 3/8.

Inch fractions to decimal inches

An edge six steps after the 2-inch line reads 2 3/8 inches. Divide the numerator by the denominator to convert the fraction: 3 ÷ 8 = 0.375. Add the whole inches to get 2.375 inches. The tool displays rounded decimal inches, so that reading appears as 2.38 in.

If an edge falls between lines, do not force it to the closest fraction. Record an estimate appropriate to the ruler’s marks, or use a finer measuring tool when you need more precision.

No physical ruler available? Choose a measuring method using what you have, or follow the one-inch screen measurement walkthrough to set the scale with a bank card or supported coin first.

Measure between two positions

Length is the distance from the first edge to the second. If a paper strip starts at 1.2 cm and ends at 6.7 cm, its length is 6.7 − 1.2 = 5.5 cm. Reading only the right endpoint would overstate it.

On the screen ruler, drag A to the first edge and B to the other edge. The large readout does this subtraction for you. A and B can move independently; neither edge has to sit at zero. Select a handle and use the arrow keys for small adjustments.

Snap to ticks settles an endpoint on a millimeter or 1/16-inch mark in the active unit. Turn it off when an edge falls between marks. Freeze locks the positions while you read or copy; choose Live to adjust them again.

Convert cm to inches without moving the span

One inch is exactly 2.54 cm, or 25.4 mm. To convert centimeters to inches, divide by 2.54. To convert inches to centimeters, multiply by 2.54. For example, 5 cm is approximately 1.97 inches, and 2 inches is exactly 5.08 cm.

Switching the tool between cm and in keeps the selected positions. It changes the labels and tick pattern, while the second readout shows the other unit. Converted positions may fall between the new marks; moving an endpoint with snapping enabled places it on that unit’s tick pattern.

Divide a length into equal parts

For equally spaced sections, divide the total span by the number of parts. A 6 cm span divided into three parts creates three 2 cm sections. The two internal boundaries are 2 cm and 4 cm from the first endpoint.

Choose a number under Divide into after placing A and B. Guides mark the internal boundaries, which may fall between ruler ticks. Divisions are calculated from the endpoints; the per-part text is rounded for display.

Before comparing any object with the screen, calibrate against a physical reference. Full screen gives you more room, but does not replace that check. If the object cannot fit beside the visible scale, use a physical ruler or tape measure.