Free actual-size online ruler
The Digital Ruler turns the screen of your phone, tablet or computer into a real ruler, with centimeters, millimeters and inches at actual size. Nothing to install and no account: open the page, calibrate once and measure.
It's handy when there is no ruler around and you need to check the size of a screw, a craft piece, a printed photo or any small object that fits on the screen.
Get started in under a minute
- Tap Calibrate and confirm your screen size (see below for which method to choose).
- Pick a unit: cm, mm or in.
- Place the object against the left edge of the screen, lined up with zero.
- Tap or drag on the ruler to the end of the object. The reading appears right below.
Need more room? Use Rotate for a vertical ruler (great on a phone held upright) or Fullscreen to hide the rest of the page.
Why most online rulers show the wrong size
Every screen has a different pixel density, measured in PPI (pixels per inch). A laptop may have 125 PPI, a large monitor 90 PPI and a phone over 150 PPI, once system scaling is taken into account.
Browsers don't report the physical size of the screen. By default they assume 96 pixels make 1 inch, which is rarely true. That's why an uncalibrated ruler can be off by 15% or more: a 10 cm object may show as 8.5 cm or 11.5 cm.
Calibration fixes this by telling the ruler how many of your screen pixels fit in one real millimeter. After that, measurements match real size.
Choosing the right calibration method
| Method | When to use it | Accuracy |
|---|---|---|
| Auto-detect | First visit. Identifies your device from its screen resolution. Works best on iPhone, iPad and Mac. | Good, when the device is recognized |
| Device | Your model is in the list (hundreds of phones, tablets, laptops and monitors). | Very good |
| Diagonal | You know the exact screen size in inches (e.g. 6.1 or 15.6), from the box or spec sheet. | Very good, if the diagonal is exact |
| Card | Any screen, including models not in the list. Use a bank card (credit or debit), which has the international standard size of 85.60 × 53.98 mm. | Most precise |
Tip: if the ruler was calibrated automatically, check it with a card. If the card doesn't match the rectangle, use the Card method.
How to read the ruler
Centimeters and millimeters
In cm mode, each number marks one centimeter. The medium tick between two numbers marks half a centimeter (5 mm), and the short ticks mark each millimeter. In mm mode the ruler is the same, but numbers are shown in millimeters (10, 20, 30…).
Example: if the object ends two short ticks after the number 4, it measures 4.2 cm, or 42 mm.
Inches and fractions
In inch mode, the longest ticks mark whole inches. Ticks get shorter for half an inch, 1/4, 1/8 and 1/16 of an inch. One inch is exactly 25.4 mm.
| Fraction | Decimal | Millimeters |
|---|---|---|
| 1/16" | 0.0625" | 1.59 mm |
| 1/8" | 0.125" | 3.18 mm |
| 1/4" | 0.25" | 6.35 mm |
| 3/8" | 0.375" | 9.53 mm |
| 1/2" | 0.5" | 12.7 mm |
| 5/8" | 0.625" | 15.88 mm |
| 3/4" | 0.75" | 19.05 mm |
| 7/8" | 0.875" | 22.23 mm |
| 1" | 1" | 25.4 mm |
Automatic reading
Don't want to count ticks? Tap or drag on the ruler: a red line marks the spot and the exact reading appears in the unit you chose.
Tips for accurate measurements
- Keep browser zoom at 100%. Any zoom changes the ruler size.
- Always start from zero, at the edge of the screen.
- Look at the screen straight on, not at an angle, to avoid parallax error.
- Measure twice to confirm.
- Recalibrate if you change system scaling, switch monitors or switch browsers.
- Protect your screen: avoid sharp or dirty objects on it.
Check your calibration with everyday objects
After calibrating, measure an object of known size. If the reading matches, the ruler is right.
| Object | Size |
|---|---|
| Credit or debit card | 85.60 × 53.98 mm |
| Brazilian R$1 coin | 27 mm diameter |
| Brazilian 50-centavo coin | 23 mm diameter |
| Brazilian 25-centavo coin | 25 mm diameter |
| Brazilian 10-centavo coin | 20 mm diameter |
| Brazilian 5-centavo coin | 22 mm diameter |
| SD memory card | 32 × 24 mm |
| microSD card | 15 × 11 mm |
| A4 sheet | 210 × 297 mm |
What you can measure
Rings
Measure the inside diameter of a ring that fits well, straight across the inside. Use that value in millimeters with a jeweler's size chart to find your ring size.
Screws, nuts and hardware
Metric screws are named by diameter and length: an M4 × 20 screw is 4 mm in diameter and 20 mm long. Measure the length from under the head to the tip.
Crafts, jewelry and sewing
Check the size of beads, buttons, clasps, interfacing and small patterns without hunting for a tape measure.
Documents, photos and prints
Check that a passport photo, a label or a document was printed at the right scale.
Know the limits
- The maximum length is the size of your screen. For larger objects, measure in parts or use the vertical ruler.
- Calibrated with a card, accuracy is usually around 1 mm per 10 cm.
- Curved or thick objects are harder to measure because of parallax.
- Don't use it where certified measurements are required, such as medical or engineering work.
Centimeter and inch conversions
One inch is exactly 2.54 cm (25.4 mm). To convert inches to centimeters, multiply by 2.54. To convert centimeters to inches, divide by 2.54.
- 1 inch = 2.54 cm
- 5 inches = 12.7 cm
- 10 cm ≈ 3.94 inches
- 15.6 inches (laptop screen) ≈ 39.6 cm diagonal
Frequently asked questions
What is a digital (virtual) ruler?
A ruler drawn on your phone, tablet or computer screen. Once calibrated for your screen, it shows centimeters, millimeters and inches at actual size, just like a real ruler.
Does the online ruler show actual size?
Yes, after calibration. Every screen has a different pixel size, so the Digital Ruler adjusts its scale to your device. That way 1 cm on screen is 1 real centimeter.
Can I use my phone as a ruler?
Yes. Open the Digital Ruler in your phone browser, calibrate by device model or with a card, and measure small objects. Use Rotate for a longer ruler with the phone upright.
How accurate is an online ruler?
Once calibrated, accuracy is usually around 1 mm per 10 cm. Without calibration it can be off by 10% or more, because every screen has a different pixel density.
How do I see 1 cm on my screen?
Calibrate the ruler, pick cm and look at the distance from zero to the number 1. That space is 1 real centimeter.
How do I find 1 inch on my screen?
Calibrate the ruler, pick in and measure from zero to the number 1. That space is 2.54 cm.
Why does the ruler look too big or too small?
It's probably not calibrated, or was calibrated for another screen. Check that browser zoom is at 100% and recalibrate with a card.
Does browser zoom or display scaling affect the ruler?
Yes. Always measure at 100% zoom. If you change system display scaling or switch monitors, calibrate again.
Can I use my phone's camera as a ruler?
Camera measuring apps (augmented reality) are fine for larger distances like rooms and furniture, but less precise for small objects than a calibrated on-screen ruler.
Is my calibration saved?
Yes. It's saved in this device's browser. If you clear site data or use another browser, you'll need to calibrate again.
Is the Digital Ruler free?
Yes. It's free, runs in your browser and needs no download or sign-up.