Digital Ruler

Online screen measurement tool achieving accurate millimeter-level object measurement through calibration, supporting fullscreen mode and real-time coordinate display, suitable for design verification, object size measurement, and other scenarios.

Features

Accurate Calibration

Ensure measurement precision through physical object calibration. Supports multiple standard reference objects (credit card, A4 paper, currency, etc.) or custom length, establishing precise correspondence between screen pixels and actual millimeter units.

Fullscreen Measurement

After calibration automatically enters fullscreen mode, screen edges display millimeter scale rulers for measuring horizontal and vertical dimensions. Grid background provides visual reference for object alignment.

Real-time Coordinates

Shows crosshair and current coordinates when moving mouse, displaying horizontal and vertical positions in real-time (dual units of millimeters and pixels), convenient for precise positioning and recording.

Adjustable Scale

Supports showing/hiding millimeter-level fine graduations, automatically adjusts main graduation intervals (5mm, 10mm, 20mm, or 50mm) based on screen resolution, ensuring graduations are clear and readable.

How to Use

Initial Calibration

1. Prepare physical object of known length (credit card, ruler, banknote, etc.)
2. Select preset reference object or enter custom length
3. Place object horizontally on screen calibration area
4. Drag red marker to actual length of object
5. Click "Complete Calibration"

Recommended reference objects:

• Credit Card: 85.6mm (standard size)
• A4 Paper Width: 210mm
• A4 Paper Height: 297mm
• US Dollar Bill: 155.956mm
• Chinese Yuan: 155mm

Measure Objects

1. After calibration enters fullscreen measurement mode
2. Place object to measure on screen
3. Read dimensions from scale rulers on edges
4. Move mouse to view precise coordinates
5. Click "Recalibrate" when needing to remeasure

Coordinate Reading

When moving mouse displays:

• Crosshair marks current position
• Coordinate popup shows horizontal and vertical distance (millimeters)
• Pixel coordinates shown in parentheses
• Coordinate origin is top-left of screen

Technical Principles

Calibration Mechanism

By measuring pixel count of known-length object on screen, calculate pixel density (pixels per mm). For example, 85.6mm credit card occupies 342 pixels on screen, pixel density is 342 ÷ 85.6 ≈ 4.0 px/mm.

Resolution Adaptation

Pixel density varies greatly across different screens:

• Standard monitors: approximately 2-4 px/mm
• High-resolution laptops: approximately 4-8 px/mm
• Retina displays: approximately 8-12 px/mm

System automatically adjusts graduation intervals based on calculated pixel density, ensuring clear readability at any resolution.

Measurement Precision

Precision depends on:

1. Calibration object accuracy
2. Screen resolution (higher is more precise)
3. Object placement flatness
4. Viewing angle (needs perpendicular viewing)

Typically can achieve ±0.5mm measurement precision.

Suitable Scenarios

• Design Verification: Verify actual dimensions of printed materials, samples
• Product Measurement: Quickly measure small object length and width
• Educational Demonstration: Demonstrate length units and measurement methods
• DIY Crafting: Assist size confirmation in handicrafts and model making
• Screen Size Calculation: Calculate actual display area of monitors

Notes

• Before calibration ensure screen is clean, no dust or stains affecting visual judgment
• Use rigid objects for calibration, avoid using soft rulers or easily deformed objects
• Keep object parallel to screen during calibration, avoid tilting
• View screen perpendicularly during measurement, avoid viewing angle errors
• Touch screen devices support touch operations, mobile devices can drag calibration markers with finger
• Press ESC to exit fullscreen mode
• Different browser zoom ratios affect measurement precision, recommend using 100% zoom
• This tool only suitable for measuring planar objects, three-dimensional objects require professional tools

Mobile Device Use

Mobile devices (phones, tablets) also supported:

1. Place device horizontally, ensure screen faces up
2. Use touch drag for calibration
3. After calibration can measure small objects (such as keys, batteries, etc.)
4. Note mobile device screens usually curved or have protective cases, may affect precision

Common Questions

Why is measurement result inaccurate after calibration$1

Possible reasons:

1. Calibration object has size error, recommend using standard objects
2. Object not completely adhered to screen during calibration
3. Browser zoom ratio not 100%
4. Object not flat on screen during measurement

Can measure curves or irregular shapes$2

This tool only supports measuring straight-line distances, curves or irregular shapes need decomposition into multiple straight-line segments for measurement.

Will calibration data be saved$3

Current version does not save calibration data, refreshing page or switching tools requires recalibration. Recommend recording calculated pixel density value, can directly use custom length for quick calibration next time.

Why are my screen graduation intervals very large$4

Low resolution or larger monitors automatically use larger graduation intervals (such as 20mm or 50mm) to ensure graduation clarity. This is normal phenomenon, does not affect measurement precision.