Step 1: Choose a Reference Object
Select an object you have nearby to calibrate your screen. Hold it against the bar below.
Step 2: Adjust the Bar to Match
Drag the edges of the teal bar until it matches the width of your reference object exactly.
Current bar width: 300 pixels = 85.6 mm
Your Calibrated Ruler
This ruler is calibrated to your screen. Hold objects against it to measure.
How to Use This Calibration Tool
- Choose a reference object – Pick something with a known size (credit card, dollar bill, or paper).
- Hold it against the screen – Place the object flat against the teal calibration bar.
- Drag to match – Adjust the bar width until it perfectly matches your object's width.
- Use the ruler – Once calibrated, the ruler below will show accurate measurements.
Tip: Your calibration is saved locally and will persist when you return. Different monitors require separate calibration.
Reference Sizes
| Object | Width (mm) | Width (inches) |
|---|---|---|
| Credit/Debit Card (ISO 7810) | 85.6 | 3.37 |
| US Dollar Bill | 156.1 | 6.14 |
| Euro Banknote (€20) | 133 | 5.24 |
| UK £10 Note | 142 | 5.59 |
| A4 Paper | 210 | 8.27 |
| US Letter Paper | 215.9 | 8.5 |
| iPhone 15 Pro | 70.6 | 2.78 |
| Standard Pencil Length | 190 | 7.5 |
Frequently Asked Questions
Why doesn't my browser know my screen's PPI automatically?
Browsers report "CSS pixels" which are standardized units, not physical pixels. This abstraction helps websites look consistent across devices, but it means the browser can't tell you the actual physical size of anything on screen without calibration.
Will my calibration work on a different monitor?
No – each monitor has different pixel density. If you use multiple displays, you'll need to calibrate each one separately. Your calibration is saved per-browser.
How accurate is this ruler?
Accuracy depends on your calibration. If you carefully match the bar to your reference object, the ruler should be accurate to within 1-2%. For precise work, use physical measuring tools.
What's a good PPI for a monitor?
Standard desktop monitors range from 90-110 PPI. Retina/HiDPI displays are typically 200-300 PPI. Phone screens often exceed 400 PPI.
Privacy & Limitations
- All calculations run entirely in your browser -- nothing is sent to any server.
- Results are estimates and may vary based on actual conditions.
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Screen Ruler Calibration Tool FAQ
What is Screen Ruler Calibration Tool?
Screen Ruler Calibration Tool is a free utility & everyday tool that helps you Calibrate your screen to display accurate physical measurements.
How do I use Screen Ruler Calibration Tool?
Enter your input values, review the calculated output, and adjust inputs until you reach the result you need. The result updates in your browser.
Is Screen Ruler Calibration Tool private?
Yes. Calculations run locally in your browser. Inputs are not uploaded to a server by default, and refreshing the page clears session data.
Does Screen Ruler Calibration Tool require an account or installation?
No. You can use this tool directly in your browser without sign-up or software installation.
How accurate are results from Screen Ruler Calibration Tool?
This tool applies standard formulas or deterministic processing logic for estimates. For medical, legal, tax, or investment decisions, verify with a qualified professional.
Can I save or share outputs from Screen Ruler Calibration Tool?
You can bookmark this page and copy outputs manually. Results are not persisted in your account and are typically not embedded in the URL.