An inch ruler on your screen
This page opens the ruler in inches, with marks for halves, quarters, eighths and sixteenths of an inch, the same as a standard US ruler. After calibration, one inch on screen is one real inch, so you can hold small objects against the edge of your screen and read their size. For longer lengths, convert inches to feet.
Related guides: How to Read a Ruler (Inches and Centimeters) · How to Read a Tape Measure
How to use it
- Tap Calibrate and match a bank card for the best accuracy, or let the ruler detect your device.
- Hold the object against the ruler with one end at 0.
- Read the whole inches from the numbers, then count the small marks: each one is 1/16 inch.
Good to know
- 1 inch = 2.54 cm = 25.4 mm, exactly.
- Between two numbers, the longest mark is 1/2 inch, then 1/4, then 1/8; the shortest marks are 1/16 inch.
- On a computer, hover over the ruler to see the position as a fraction, such as 2 3/8″.
Inch fractions on this ruler
Each inch on the ruler is split into 16 equal parts. The table shows the marks you will use most, with the decimal value and the size in millimeters.
| Mark | Fraction (in) | Decimal (in) | Millimeters |
|---|---|---|---|
| Shortest line | 1/16 | 0.0625 | 1.59 |
| 4th-longest line | 1/8 | 0.125 | 3.18 |
| Shortest line | 3/16 | 0.1875 | 4.76 |
| 3rd-longest line | 1/4 | 0.25 | 6.35 |
| Shortest line | 5/16 | 0.3125 | 7.94 |
| 4th-longest line | 3/8 | 0.375 | 9.53 |
| Shortest line | 7/16 | 0.4375 | 11.11 |
| 2nd-longest line | 1/2 | 0.5 | 12.7 |
| 4th-longest line | 5/8 | 0.625 | 15.88 |
| 3rd-longest line | 3/4 | 0.75 | 19.05 |
| 4th-longest line | 7/8 | 0.875 | 22.23 |
| Numbered line | 1 | 1 | 25.4 |
- To read 3/8 inch, count 6 of the smallest spaces from the start of the inch: 6/16 is 3/8.
- One sixteenth of an inch is 1.59 mm, so reading to the nearest mark is accurate to about ±0.8 mm.
- Always start at the 0 line, and look straight down at the mark rather than at an angle.
Three ways to calibrate
Auto-detect your device
The ruler recognizes about 130 phones, tablets, laptops and monitors, including iPhones, Galaxy phones, Pixels and MacBooks.
Enter your screen size
Enter your screen's diagonal size (for example, 15.6" for a laptop), and the ruler works out the scale from your screen resolution.
Calibrate with a bank card
Hold any bank card against the outline on screen and move the slider until they match. Bank cards are the same size everywhere: 85.60 × 53.98 mm.
How accurate is the ruler?
How accurate the ruler is depends on how well it knows your screen's pixel density, meaning how many pixels fit in an inch. There are three ways to set it, from most to least accurate:
Bank card (±0.5%): Bank and credit cards are made to one worldwide size, 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Because every card is exactly the same, matching one on screen gives the most accurate result.
Auto-detect (±1%): The ruler looks up your device's published screen size and resolution. Screens of the same model can differ very slightly, so this is a little less exact. It covers about 130 devices.
Screen size (±2%): You type in your screen size, and the ruler works out the rest from your resolution. Advertised sizes are rounded (a "15.6-inch" screen may really be 15.59 inches), so this is the least exact method. It's a good fallback when you don't have a card.
Real-life objects to check your measurements
Use these everyday objects to confirm the ruler is calibrated correctly.
Bank or credit card: 85.60 × 53.98 mm (3.37 × 2.13 in)
A4 paper: 210 × 297 mm (8.27 × 11.69 in)
US Letter paper: 8.5 × 11 in (215.9 × 279.4 mm)
US quarter: 24.26 mm (0.955 in) across
1 euro coin: 23.25 mm (0.915 in) across
AA battery: about 50.5 mm long and 14.5 mm wide
More online rulers
Measuring guides
- How to Read a Ruler (Inches and Centimeters)
- How to Read a Tape Measure
- Fraction to Decimal Inch Chart (with Millimeters)
- US Coin Sizes: Diameter, Thickness and Weight
- How to Measure Your Foot for Shoe Size
- How to Measure Your Wrist for a Bracelet or Watch
- Lumber Sizes: Actual vs Nominal Dimensions Chart
- Necklace Length Chart: How Each Chain Length Fits
- Free Printable Ruler Worksheets with Answer Keys
- Screw and Bolt Size Chart: US and Metric
- Drill Bit Size Chart: Inch, Metric and Tap Drill Sizes
- Nail Size Chart: Penny Sizes Explained
- Envelope Sizes Chart: US and International
- Business Card Size: Standard Dimensions by Country
- Wrench Size Chart: SAE vs Metric, Sockets and Allen Keys
- Wire Gauge Chart: AWG Sizes in Inches and mm
- Pipe Size Chart: Nominal vs Actual Sizes
- Sandpaper Grit Chart: Which Grit for Which Job
- Thread Pitch Chart: UNC, UNF and Metric Threads
- Sheet Metal Gauge Chart: Thickness in Inches and mm
- Extension Cord Gauge Chart: Length, Amps and AWG
- Rebar Size Chart: US Bar Sizes, Diameter and Weight
- Plywood and Drywall Thickness: Nominal vs Actual
- Tap Drill Size Chart: UNC, UNF and Metric
- Number and Letter Drill Size Chart
- Bolt Torque Chart: SAE Grades and Metric Classes
- Washer Size Chart: SAE, USS and Metric Washers
- How to Show the Ruler in Word
- How to Show the Ruler in Google Docs
- How to Show the Ruler in PowerPoint
- Is an Online Ruler Accurate? How to Calibrate It
- How to Measure Inches on Your Phone
- How Big Is 1 Inch? Everyday Objects That Are an Inch
- All guides →