Centimeter Ruler (cm)

A centimeter-first ruler with the metric scale on top. Calibrate, verify with a physical reference, then read cm and mm values from your screen.

Display settings

Not calibrated · estimate only

Calibration details

Resulting density: 96.0 PPI · Not calibrated · estimate only

Verify against a physical reference. Different displays can report identical parameters; display changes cannot always be detected.

Press and drag across the ruler to measure a distance in cm and inches.

Keyboard: focus the ruler and press Enter to start at zero, then Tab to − or + to adjust.

Measuring in centimeters

Diagram showing an object's straight edge aligned with the ruler's visible zero line — Place one straight edge exactly at 0, then read or drag to the far edge. This teaching diagram is not actual size.

Reading cm and mm

The centimeter scale follows the metric convention: a long mark every centimeter, a medium mark at each half centimeter, short marks for millimeters. Reading is additive — the last centimeter mark passed plus the millimeter marks beyond it. Because 1 cm is exactly 10 mm, you can switch to the millimeter ruler at any time without changing the geometry.

Common centimeter references

Objects with standardized sizes make good calibration checks. Lay one against the ruler; if the reading matches the table, the calibration is sound.

Object	Size
Bank card (ISO/IEC 7810 ID-1)	8.56 × 5.40 cm
A4 sheet (ISO 216)	21.0 × 29.7 cm
US Letter sheet	21.6 × 27.9 cm
AA battery (IEC LR6)	5.05 × 1.45 cm
CD or DVD	12.0 cm diameter

What centimeters are good for

Metric is the default for sewing patterns, paper and envelope sizes, furniture, packaging and most product listings outside the US. Craft and DIY dimensions between 1 and 30 cm are where a screen ruler is most practical: the object lies flat against the glass and the reading is immediate.

Converting to inches

Multiply centimeters by 0.3937 to get inches, or divide by 2.54. For woodworking and US hardware you usually want a fraction: 10 cm is 3 15/16 in. The unit converter shows the nearest 1/16 fraction, and the inch ruler guide explains how to read those marks.

How to use this ruler

Calibrate first if the badge says “estimated” — the ruler home page explains both methods.
Keep the cm tab selected. Numbered marks are centimeters; the short marks between them are millimeters.
Align one edge of the object with the 0 mark, read the centimeter just before its other edge, then count the millimeter marks past it.
Drag along the ruler for a displayed readout in cm, mm and inches; its decimals are not an accuracy guarantee.

Frequently asked questions

How do I read centimeters and millimeters on this ruler?

Every numbered mark is one centimeter; the nine short marks between two numbers are millimeters. An edge that lands three marks after “5” reads 5.3 cm, or 53 mm — the millimeter ruler shows the same scale with millimeter labels.

How many centimeters is my screen?

Once calibrated, the top ruler spans the full width of your browser window. A typical 13-inch laptop display is about 29 cm wide; a 6.1-inch phone about 7 cm in portrait.

What is 1 cm in inches?

1 cm is 0.3937 in, a little more than 3/8 in, and 2.54 cm make exactly one inch. Use the unit converter for other values, or switch to the inch ruler.

Can I measure something longer than the screen?

Not in one pass. Mark where the screen ends, slide the object and add the readings — or use the image measurement tool with a photo of the whole object next to a known reference.

Which everyday objects have handy centimeter sizes?

A bank card is 8.56 cm wide, an A4 sheet 21.0 × 29.7 cm, an AA battery about 5.05 cm long, a CD 12.0 cm across. They double as quick sanity checks of the calibration.

Measuring in centimeters

Place one straight edge exactly at 0, then read or drag to the far edge. This teaching diagram is not actual size.

Reading cm and mm

Common centimeter references

Objects with standardized sizes make good calibration checks. Lay one against the ruler; if the reading matches the table, the calibration is sound.

Object	Size
Bank card (ISO/IEC 7810 ID-1)	8.56 × 5.40 cm
A4 sheet (ISO 216)	21.0 × 29.7 cm
US Letter sheet	21.6 × 27.9 cm
AA battery (IEC LR6)	5.05 × 1.45 cm
CD or DVD	12.0 cm diameter

What centimeters are good for

Converting to inches

How to use this ruler

Calibrate first if the badge says “estimated” — the ruler home page explains both methods.

Keep the cm tab selected. Numbered marks are centimeters; the short marks between them are millimeters.

Align one edge of the object with the 0 mark, read the centimeter just before its other edge, then count the millimeter marks past it.

Drag along the ruler for a displayed readout in cm, mm and inches; its decimals are not an accuracy guarantee.

Frequently asked questions

How do I read centimeters and millimeters on this ruler?

How many centimeters is my screen?

Once calibrated, the top ruler spans the full width of your browser window. A typical 13-inch laptop display is about 29 cm wide; a 6.1-inch phone about 7 cm in portrait.

What is 1 cm in inches?

1 cm is 0.3937 in, a little more than 3/8 in, and 2.54 cm make exactly one inch. Use the unit converter for other values, or switch to the inch ruler.

Can I measure something longer than the screen?

Not in one pass. Mark where the screen ends, slide the object and add the readings — or use the image measurement tool with a photo of the whole object next to a known reference.

Which everyday objects have handy centimeter sizes?

A bank card is 8.56 cm wide, an A4 sheet 21.0 × 29.7 cm, an AA battery about 5.05 cm long, a CD 12.0 cm across. They double as quick sanity checks of the calibration.