Manual Scale Adjustment
Auto1 cm ≈ 37.8 px96.0 PPI
Inch (″)
1920 × 1080
PPI
Credit Card Size Guide (85.60 × 53.98 mm)

Hold any credit or debit card against this outline to verify physical 1:1 scale

85.60 × 53.98 mmPlace Credit Card / ID Here
4-Digit Scale Code:Automatic 4-digit calibration code
0960

Free Online Ruler — Measure Anything on Your Screen

Live Ruler transforms any screen — laptop, tablet, phone, or desktop monitor — into a precision measuring tool. Unlike physical rulers that get lost or break, your screen is always with you. Just open the page and start measuring in centimeters, millimeters, inches, or pixels. No app install, no signup, no nonsense.

How It Works

How to Use Live Ruler — Step-by-Step Guide

Follow these simple steps to measure any object on your computer or phone screen with 100% true physical accuracy.

1

Calibrate with Auto-Detection, Screen Size, or 4-Digit Code

Live Ruler automatically inspects your display resolution and pixel density in the background upon opening. To guarantee 100% physical accuracy, place any standard credit card (85.60 × 53.98 mm) over the open on-screen guide, or enter your screen’s diagonal size in inches (e.g. 24″, 27″, 15.6″). Once set, a 4-digit calibration code (e.g. 0825) is generated. You can copy and enter this code on any device or browser to automatically fetch and apply the exact scale instantly.

Pro Tip: Copy your 4-digit scale code (e.g. 0825) to save or share your calibrated scale without recalibrating.
2

Select Your Measurement Unit

Switch between Centimeters (cm) with millimeter tick marks, Millimeters (mm) for fine engineering and 3D printing tolerances, Inches (in) with 1/16″ subdivisions for carpentry and fabric, or Pixels (px) for web design and UI inspection.

Shortcut: Press 1 for cm, 2 for mm, 3 for inches, 4 for pixels.
3

Choose Ruler Edges or Activate All-Sides Screen Mode

Toggle the top, right, bottom, or left ruler strips. To measure physical objects directly against your screen’s outer glass bezels, click "Use Entire Screen as Ruler" above. This docks the controls to the center and activates all four edges, turning your entire display into a digital drafting table.

Shortcut: Press T for top, R for right, B for bottom, L for left ruler.
4

Use Precision Guide Lines, Crosshairs, & Compare Mode

Guide Lines (G): Click anywhere on the screen to drop reference lines, drag them to align with objects, and view exact distance badges. Press Esc to clear. Crosshair (C): Follows your pointer with real-time X/Y coordinates. Compare Mode: Drag two rectangular areas side-by-side to compare physical dimensions and see the exact difference.

Hotkeys: G for guides, C for crosshair, Esc to reset.
5

Download Printable Vector PDF or Go Fullscreen

Need a physical paper ruler? Click the Document icon to download a 100% true-scale vector PDF with calibration verification boxes. Press F anytime to go fullscreen and hide all browser distractions.

Press ? at any time to view the full cheat-sheet of keyboard shortcuts.

How Calibration Works

Every screen has different physical dimensions and pixel density (PPI — pixels per inch). A "pixel" on your phone is much smaller than a "pixel" on your 27″ desktop monitor. That’s why an online ruler needs to know your screen’s actual PPI to display measurements at their true physical size.

1. Automatic Screen Detection

Live Ruler automatically reads your display resolution and device pixel ratio in the background upon opening, applying the optimal scale instantly.

2. Screen Size Diagonal Input

Enter your screen’s diagonal size in inches (such as 24″, 27″, or 15.6″) directly in the manual scale card to compute physical PPI from screen geometry.

3. 4-Digit Scale Code (Generate & Fetch)

Every calibrated scale generates a portable 4-digit code (e.g. 0960 for 96.0 PPI). Enter this code in the restore box to automatically fetch and restore your exact scale.

4. Physical Credit Card Size Guide

Kept permanently open right on screen. Simply place any credit card, ID, or driver’s license (85.60 × 53.98 mm per ISO/IEC 7810) over the outline and fine-tune until it aligns 1:1.

Units Explained

Who Uses an Online Ruler?

Students measuring diagrams for homework. Tailors checking seam allowances from pattern PDFs. Designers verifying that a UI element is exactly the right size. Jewelers sizing rings against a reference chart. DIY enthusiasts checking bolt diameters when the caliper is in the garage. 3D printing hobbyists confirming tolerance before a 12-hour print. Parents helping kids with geometry. Engineers in the field needing a quick reference.

What Makes Live Ruler Different

Frequently Asked Questions (USA & Global)

How do I get a ruler on my screen?
Simply open Live Ruler in any web browser on your phone, tablet, laptop, or desktop computer. It immediately displays an actual-size ruler calibrated to your screen in centimeters, millimeters, inches, or pixels with no app download or installation needed.
Can I use my phone as a ruler?
Yes! Live Ruler turns your iPhone or Android phone into an accurate physical-size ruler. Tap "Use Entire Screen as Ruler" to align the scale with your phone’s outer edges, and place items directly against the glass to measure them instantly.
Is there a virtual ruler?
Yes, Live Ruler is a 100% free virtual on-screen ruler that provides true 1:1 scale physical accuracy. It includes draggable guide lines, crosshair coordinates, and multi-edge rulers directly in your browser.
Does the iPhone have a digital ruler?
While iPhones include a camera-based AR "Measure" app for room-scale distances, Live Ruler provides an exact physical millimeter scale directly on your iPhone display screen, making it far more precise for measuring small objects like screws, jewelry, coins, and cards.
How to activate screen ruler?
To activate the full screen ruler, visit Live Ruler and click the "Use Entire Screen as Ruler" button above the title. This centers the controls and turns all four screen borders into active, edge-to-edge rulers.
Is there a ruler in Chrome?
Google Chrome includes a developer pixel ruler in DevTools for web inspection, but it does not measure real-world physical inches or centimeters. Live Ruler works directly in Chrome on any device to deliver calibrated, real-life physical measurements.
How accurate is Live Ruler compared to a physical wooden or metal ruler?
After calibrating with a standard bank card, Live Ruler is accurate to within ±0.5 mm on most screens. The credit card method is based on real-world ISO standards, ensuring precision equal to or exceeding standard plastic school rulers.
How do I calibrate using my driver’s license or credit card?
Click the Target icon in the toolbar, select "Credit Card", place your card directly on the screen against the blue box, and adjust the slider until the outline matches your physical card exactly. All standard credit, debit, and driver licenses follow the universal ISO/IEC 7810 ID-1 standard (85.60 mm × 53.98 mm).
Can I print this ruler on US Letter or A4 paper without scaling distortion?
Yes! Click the PDF export icon. When printing, make sure your printer dialog is set to "Actual Size" or "100% Scale" (never select "Fit to Page" or "Shrink to Printable Area"). A 2-inch calibration check square is included on the printed page to verify accuracy.
How do I measure physical objects directly against my computer or phone screen bezels?
Click "Use Entire Screen as Ruler" above the title. This activates All-Sides mode, moving the ruler zero-points directly to the physical outer glass frame of your device, and docks the controls into the center.
Does Live Ruler save my device settings between visits?
Yes. Live Ruler stores your calibrated PPI and preferred measurement units in your browser’s local storage. Your data is stored 100% locally on your device and is never transmitted to any external server.