Online centimeter and millimeter ruler
Choose cm for centimeter labels or mm for millimeter labels. Both metric modes have 1 mm divisions; ten millimeters equal one centimeter.
Place a ruler on any edge.
Choose an edge from the toolbar, or select a shortcut below.
Built for real measurements
Accurate by design.
Browser pixels are not physical units. OG Ruler accounts for your display so the scale on screen matches the object in your hand.
Online inch ruler at real size
Inch mode uses 1/16-inch ticks. Eight small divisions equal half an inch. The unit relationship is exact: 1 inch = 25.4 mm.
Accurate by design.
Start with calibration. Automatic detection and device profiles are estimates, not a guarantee of physical accuracy. Browser zoom and display scaling can change the result.
Measure directly on your screen
Online Ruler in CM, MM and Inches
OG Online Ruler is a free ruler online that turns any screen edge into a practical measuring tool. Choose centimeters, millimeters, inches, or pixels, calibrate your display, and compare an object with a scale shown at actual size.
Online centimeter and millimeter ruler
Use the unit selector to switch between centimeters, millimeters, inches, and pixels. In cm mode, every numbered mark is one centimeter and each smaller division is one millimeter. For help with a particular reading, open the centimeter ruler and reading helper or the millimeter ruler with offset measurements.
To measure a specific length such as 7 cm or 10 cm, calibrate the screen, align the object with the starting edge, and read the numbered endpoint. The scale can appear on the top, bottom, left, or right. Turn on more than one edge for horizontal and vertical references together. You can also pin a guide, drag it to a new position, adjust it with the arrow keys, or remove it when finished.
Calibrate the ruler to actual size
An accurate online ruler must account for differences between screens. Browser pixels do not always match physical pixels, and two displays with the same resolution can have different dimensions. OG Online Ruler offers several calibration choices: automatic device detection, a known device profile, screen diagonal entry, and credit card calibration. Matching the on-screen card outline to a standard bank card is usually the most dependable option.
After calibration, the online ruler actual size scale is calculated using your screen’s pixel density. This makes the tool more reliable than assuming every display uses the browser default. For the best result, set browser zoom to 100 percent, keep the browser maximized, and recalibrate after moving the window to another monitor. Your calibration is stored only in your browser and is not uploaded.
Online inch ruler at real size
Select inches for a fractional scale. A true-to-size ruler depends on calibration because browser pixels are not a dependable physical measurement by themselves. Once calibrated, each whole-inch mark and fractional tick is scaled to your display. The dedicated inch ruler explains sixteenths and fractional rounding; the dual-scale screen ruler compares centimeters and inches together.
Need to see a full 12-inch span? Fullscreen mode makes as much scale visible as your monitor allows, but it cannot enlarge the physical display. A phone cannot show twelve physical inches at once, while many desktop monitors can. Calibration keeps every visible inch correctly sized even when the complete length does not fit. Verify a known span with the actual-size ruler check before relying on it.
Fast, private, and easy to control
The ruler runs directly in the browser with no account, download, or upload. Unlike an online ruler camera experience, it does not need camera permission or analyze images. Place the item gently against the screen without pressing, choose the nearest ruler edge, and read the endpoint. The crosshair displays live horizontal and vertical coordinates, while Measure Box lets you drag between two points and see both dimensions together.
Keyboard shortcuts make repeated measurements faster: press 1 for centimeters, 2 for inches, 3 for pixels, 4 for millimeters, G for guides, C for the crosshair, F for fullscreen, and D for dark or light mode. These controls make OG Online Ruler a flexible online ruler tool for quick checks as well as careful comparisons. Start with calibration whenever physical accuracy matters, then choose a unit and measure directly against the scale.
How to read a ruler in cm and mm
To measure 2, 5, 7 or 10 cm at actual size, calibrate first, select cm and align the object with the starting edge. Read the endpoint, not the number of lines.
| Centimeters | Millimeters | Inches |
|---|---|---|
| 1 | 10 | 0.3937 |
| 2 | 20 | 0.7874 |
| 5 | 50 | 1.9685 |
| 7 | 70 | 2.7559 |
| 10 | 100 | 3.937 |
The table converts lengths; it is not a true-size drawing. Inch values are rounded to four decimal places. Only objects that fit on your screen can be measured against it.
A worked ruler reading
Three small divisions after the 2 cm mark mean 2.3 cm, or 23 mm. The starting edge is zero even though its number is hidden.
Built for real measurements
How to use
1 cm2 in3 px4 mmG GuidesC CrosshairF FullscreenD ThemeH How to use
Online ruler questions
Frequently asked questions.
Can I use my phone as a ruler?+
Yes. Calibrate the screen, choose a unit, and align a small object with an active ruler edge.
Is there a ruler online?+
Yes. OG Online Ruler measures in centimeters, millimeters, inches, and pixels without an installation.
How can I identify 1 inch?+
One inch equals exactly 2.54 centimeters. Calibrate the display and measure to the first inch mark.
How do I measure centimeters online?+
Select centimeters, calibrate the screen, align the object at the edge, and read its endpoint.
Can a smartphone measure objects?+
Yes. A calibrated on-screen ruler is useful for small objects that fit on the display.
Can I use my camera as a ruler?+
Camera measuring apps exist, but OG Online Ruler measures on screen and does not request camera access.
Can we measure objects online?+
Yes. Calibrate first, keep browser zoom at 100 percent, and align the object carefully.
How do I read a ruler online?+
Start at the edge and read the numbered mark at the other end of the object.
How do I view the ruler in Word Online?+
Open the View tab and enable Ruler when the option is available in Word for the web.
How do I add a ruler in Word Online?+
Choose View and enable Ruler. If unavailable, open the document in the desktop Word application.
How do I show the ruler in PowerPoint Online?+
Check the View tab for Ruler. Use the desktop application if the web option is unavailable.
How do I use an online ruler?+
Set zoom to 100 percent, calibrate, select a unit and edge, then align and measure.
How do I measure 10 cm with an online ruler?+
Select centimeters, calibrate, align at the starting edge, and read the numbered 10 mark.
Is the inch ruler shown at real size?+
It is actual size after screen calibration. Without calibration, physical size may differ.