TOOLGENIEOnline Ruler – Actual Size in mm, cm & Inches
Free virtual ruler for actual-size measurements on your screen. Measure any object in millimeters, centimeters, and inches. Calibrate with PPI for true accuracy — no app install required.
Screen resolution: 0 × 0 px
Visible ruler width: 212 mm / 8.33 in
Click the ruler to place a red measurement mark
Ruler Settings
Don't know your PPI? Calibrate with a credit card or other known object below. PPI is saved in your browser for next time.
Calibrate to Actual Size
Place a known object against the screen, click the ruler at its edge, then press Calibrate. That sets PPI so the online ruler matches real life.
Credit / debit card
85.6 mm (3.37 in)
US dollar bill (length)
156.1 mm (6.15 in)
A4 paper (short side)
210 mm (8.27 in)
iPhone 14 / 15 width
71.5 mm (2.81 in)
AA battery (length)
50.5 mm (1.99 in)
How to Use the Online Ruler
- Keep zoom at 100% — Do not zoom the browser page in or out. Zoom changes pixel density and will make actual-size readings wrong.
- Calibrate PPI — Place a credit card, banknote, or other known object against the screen. Click the ruler at the object's edge, then use Calibrate or adjust Pixels per inch (PPI) until the scale matches.
- Measure your object — Align the item with the left edge of the ruler (zero). Read millimeters/centimeters on the top scale or inches on the bottom scale.
- Mark a length — Click the ruler to drop a red line so you can read the exact mm, cm, and inch values after you move the object away.
- Choose inch graduation — Switch between 1/8", 1/16", and 1/32" for coarser or finer inch ticks. Use Restore PPI anytime to reset saved settings.
About the Online Ruler
The Online Ruler by ToolGenie is a free virtual ruler for actual-size measurements in millimeters, centimeters, and inches — right in your browser. You no longer need to carry a physical ruler: open the tool on a phone, tablet, laptop, or desktop and measure height, width, or length of everyday objects.
Unlike a static image, this ruler scales to your screen width and uses pixels per inch (PPI) so one inch on screen can match one real inch after calibration. The top edge shows cm/mm; the bottom edge shows inches with selectable graduations (1/8, 1/16, 1/32). Click to mark a red measurement line and see the length in all three units at once.
Everything runs client-side. Your PPI preference is stored only in your browser's local storage so it is ready next time you visit. No login, no upload, and no install — just a practical screen ruler for craft, DIY, design mockups, and quick measurements when a physical ruler is not nearby.
How to Adjust the Virtual Ruler to Actual Size
Displays vary, so calibrating makes the online ruler match real life:
- Take a standard object with a known length — smartphone, wallet, credit card, A4 paper, or paper money. You can confirm exact sizes with a quick search if needed.
- Place the object on the online ruler and compare lengths. If it does not match, adjust PPI up or down (or click Calibrate after marking the edge) until the ruler and object agree.
- Once calibrated, measure other objects. PPI is saved in your browser for future visits until you clear site data or tap Restore PPI.
Frequently Asked Questions (FAQ)
Are online rulers accurate?
Yes — an online ruler can be highly accurate once you calibrate it to your display. Our tool reads your screen size and lets you set pixels per inch (PPI) so the scale matches real millimeters, centimeters, and inches. After a quick calibration with a credit card or other known object, measurements are reliable for everyday use.
How can I check that the ruler is correct?
Calibrate with any object of known length — a credit card (85.6 mm), banknote, A4 paper, or smartphone. Place it against the screen, click the ruler at the object's edge to mark the length, then adjust PPI (or use Calibrate) until the mark matches the real size. When they match, your virtual ruler is correct.
How to read inches on a ruler?
On this online ruler, the inch scale is on the bottom. Whole-inch numbers are labeled; smaller ticks show fractions based on your graduation setting (1/8, 1/16, or 1/32 of an inch). Read from the left (zero) to your measurement mark for the length in inches.
How to read centimeters on a ruler?
Centimeters and millimeters are on the top of the ruler. Each numbered mark is 1 centimeter; the smaller ticks between them are millimeters (10 mm = 1 cm). Read from the left edge to your mark for the length in cm or mm.
How to convert centimeters to inches?
1 cm equals approximately 0.393701 inches. To convert centimeters to inches, divide the centimeter value by 2.54. Example: 10 cm ÷ 2.54 ≈ 3.94 inches. Our online ruler shows both units at once so you often do not need a separate conversion.
How do I use my phone as a ruler?
Open the ToolGenie Online Ruler on your phone's browser, keep zoom at 100%, and calibrate PPI with a credit card or another known object. Then place the item you want to measure against the screen and read the mm, cm, or inch scale — or tap to place a red mark for a clear reading.
How many centimeters are in a 12-inch ruler?
A 12-inch ruler is 30.48 centimeters long (12 × 2.54 = 30.48). That is also 304.8 millimeters.
What is PPI and why does the online ruler need it?
PPI (pixels per inch) is how many screen pixels fit in one real inch. Displays differ, so the same number of pixels is not the same physical length on every device. Setting the correct PPI makes the virtual ruler match actual size. Your PPI is saved in the browser until you clear site data or tap Restore PPI.
Can I change the inch graduation on this virtual ruler?
Yes. You can choose 1/8, 1/16, or 1/32 inch graduations. Finer settings (like 1/32) add more tick marks between whole inches so you can measure smaller increments more precisely. The default is 1/16.
Is this online ruler free and private?
Yes. The ToolGenie Online Ruler is free, runs entirely in your browser, and needs no login. PPI settings stay in local browser storage only — nothing is uploaded to a server.
Tips for Accurate Online Ruler Measurements
- Calibrate once per device: Laptops, phones, and monitors have different PPI. Save settings after calibrating each screen you use.
- Use a rigid reference: A credit card or ID card is usually more accurate than a soft wallet or folded paper.
- Avoid browser zoom: Stay at default 100% page zoom. Pinching or Ctrl/Cmd + scroll will skew results.
- Prefer finer inch ticks for small parts: Switch to 1/32" when measuring tiny gaps or craft pieces.
- Mark before moving the object: Click to place the red line so you can read mm, cm, and inches clearly after removing the item from the screen.