MM Ruler Online: Actual Size Ruler in mm, cm & Inches
A free mm ruler online that shows true millimeters, centimeters and inches on your screen. Calibrate once with a credit card, then measure small objects on your phone or computer. No sign-up, no download.
- 1. Calibrate onceMatch the ruler to a credit card. 10 seconds.
- 2. Place your objectPut one end on line A (the 0 mark).
- 3. Drag line BRead the length in mm, cm and inches.
Loading the ruler… If this message stays, JavaScript is not running on this page.
Length between A and B
Drag the red lines A and B to the two ends of your object. Line A starts at 0.
Calibrate the Ruler to Your Screen
Pick whichever method is easiest. Changes save automatically on this device.
On a phone, use the credit card. A dollar bill or A4 sheet is wider than most phone screens.
Best on a laptop or desktop at 100% zoom.
Default estimate for this device: PPI. Not sure of yours? Use the PPI calculator.
mm to cm to Inches Converter
Type a length to convert it. 10 mm = 1 cm. 25.4 mm = 1 inch.
What Is MM Ruler Online?
MM Ruler Online is a free, digital ruler that runs in your browser. It turns the screen you already have into an accurate on-screen ruler for millimeters, centimeters and inches. You do not need an app, a download or an account.
It works as an mm ruler online on iPhone, Android, tablets, laptops and desktops. Because it is a web page, there is nothing to install or update. Open it, calibrate once and measure. It is built mm-first, so every small tick is exactly 1 mm, which makes it a good metric ruler for small parts, jewelry, coins, screws and cards.
How to Use the MM Ruler Online to Measure in mm, cm and Inches
Three quick steps. The first one only needs doing once per device.
-
Calibrate once
In the calibration panel, choose a credit card, US dollar bill or A4 paper. Lay it on the screen with its left edge on the dark line, then drag the red line to its right edge. It saves automatically.
-
Place your object
Lay the object flat on the screen with one end on line A, which starts at 0. Look straight down at the screen to avoid reading errors.
-
Drag line B
Drag line B to the other end. The length shows in mm, cm and inches, with 1/8, 1/16 or 1/32 inch precision. Or type a length and press Set.
Why You Need to Calibrate an Online Ruler for Accurate mm Readings
A website cannot know the real size of your screen. Phones and monitors pack different numbers of pixels into every millimeter, so an uncalibrated online ruler is only a guess and can be off by 10% or more.
A credit card (85.6 mm wide), a US dollar bill (about 156 mm) and a sheet of A4 paper (210 mm) are fixed sizes worldwide. Matching the ruler to one of them fixes the scale for your exact screen, and readings are then typically within about 1%.
- Calibration is saved on your device, so you do it once.
- Using another phone, tablet or monitor? Calibrate that one too.
- The ruler tries to follow browser zoom, but if the size ever looks wrong, calibrate again.
- Check your result by measuring a credit card. It should read 85.6 mm.
How to Read a mm Ruler: Markings, Numbers and Inches
Every ruler follows the same pattern, on screen or on your desk. The shorter the line, the smaller the unit.
- Smallest lines: each one is 1 mm.
- Medium line: halfway between two numbers, it marks 5 mm.
- Long numbered line: every 10 mm, which is 1 cm. These are the ruler numbers: 1, 2, 3 and so on.
- Reading cm and mm: find the last number before your object ends, then count the small lines after it. If the edge is 7 lines past the 4, the length is 4 cm and 7 mm, which is 47 mm or 4.7 cm.
- Inch side: the longest numbered line is 1 inch, then 1/2, then 1/4 and 3/4, then 1/8, and the shortest lines are 1/16 inch. Switch to inch above for detail down to 1/32.
A standard ruler is 30 cm (about 12 inches) long, with metric on one edge and inches on the other. This one scales to your screen.
How Big Is 1 mm? Actual Size from 1 mm to 25 mm
These bars are drawn at true size on your screen, so calibrate first for them to be exact. Use them to see how big 2 mm, 3 mm, 4 mm, 5 mm or 6 mm really is.
| Size | What it looks like |
|---|---|
| 1 mm | About 10 sheets of printer paper stacked (one sheet is roughly 0.1 mm). |
| 1.5 mm | The thickness of a US penny. |
| 2 mm | The thread diameter of an M2 screw. |
| 3 mm | The thread diameter of an M3 screw. A headphone jack is slightly bigger at 3.5 mm. |
| 4 mm | The thread diameter of an M4 screw. |
| 5 mm | The thread diameter of an M5 screw. |
| 6 mm | The thread diameter of an M6 screw. |
| 10 mm | Exactly 1 cm. A US nickel is 21.21 mm across, about twice this. |
| 12 mm | The width of a USB-A plug. |
mm to cm and Inch Conversion Chart
Common lengths from 0.5 mm to 100 mm. 10 mm is 1 cm, 25.4 mm is 1 inch, and 1,000 mm is 1 meter.
| mm | cm | inch | Nearest 1/16 in |
|---|---|---|---|
| 0.5 | 0.05 | 0.020 | under 1/16 |
| 0.7 | 0.07 | 0.028 | under 1/16 |
| 1 | 0.1 | 0.039 | 1/16 |
| 1.5 | 0.15 | 0.059 | 1/16 |
| 1.8 | 0.18 | 0.071 | 1/16 |
| 2 | 0.2 | 0.079 | 1/16 |
| 3 | 0.3 | 0.118 | 1/8 |
| 4 | 0.4 | 0.157 | 3/16 |
| 5 | 0.5 | 0.197 | 3/16 |
| 6 | 0.6 | 0.236 | 1/4 |
| 7 | 0.7 | 0.276 | 1/4 |
| 8 | 0.8 | 0.315 | 5/16 |
| 9 | 0.9 | 0.354 | 3/8 |
| 10 | 1 | 0.394 | 3/8 |
| 12 | 1.2 | 0.472 | 1/2 |
| 15 | 1.5 | 0.591 | 9/16 |
| 20 | 2 | 0.787 | 13/16 |
| 25 | 2.5 | 0.984 | 1 |
| 25.4 | 2.54 | 1.000 | 1 |
| 30 | 3 | 1.181 | 1 3/16 |
| 40 | 4 | 1.575 | 1 9/16 |
| 50 | 5 | 1.969 | 1 15/16 |
| 60 | 6 | 2.362 | 2 3/8 |
| 70 | 7 | 2.756 | 2 3/4 |
| 80 | 8 | 3.150 | 3 1/8 |
| 90 | 9 | 3.543 | 3 9/16 |
| 100 | 10 | 3.937 | 3 15/16 |
Everyday Object Sizes in mm (Use to Check Accuracy)
Measure one of these to check your calibration, or to get a feel for how big a millimeter is.
| Object | Size |
|---|---|
| Credit card | 85.60 × 53.98 mm |
| Credit card thickness | 0.76 mm |
| US dollar bill | 155.96 × 66.29 mm |
| Nano SIM card | 12.3 × 8.8 mm |
| US penny | 19.05 mm across |
| US dime | 17.91 mm across |
| US nickel | 21.21 mm across |
| US quarter | 24.26 mm across |
| 1 euro coin | 23.25 mm across |
| AA battery | 14.5 mm across, 50.5 mm long |
| USB-A plug | 12 mm wide |
| Headphone jack | 3.5 mm across |
| A4 paper | 210 × 297 mm |
How to Measure Accurately with an Online Ruler (Diameter and Small Objects)
A few habits make the difference between a rough guess and a reading you can trust.
- Calibrate firstIt is the single biggest accuracy gain, and it takes about 10 seconds.
- Look straight downViewing from an angle makes the object look shifted against the ruler.
- Start at line ALine A begins at the 0 mark. Line your object up with it, not with the edge of the screen.
- Measure diameter across the widest pointFor a round object such as a coin, ring or bead, put line A on one edge and line B on the opposite edge.
- Use the mm scale for small thingsUnder 10 cm, millimeters are more precise than the inch scale, which resolves to 1/16 inch (about 1.6 mm).
- Check with a known objectMeasure a card, bill or coin from the chart. If it reads wrong, calibrate again.
What You Can Measure with an mm Ruler Online
Millimeters are the everyday unit for anything small.
- Rings and braceletsCheck the inner diameter of a ring you own in mm, then compare it with a size chart.
- Online purchasesCheck phone cases, watch straps, frames and screws against the listed size.
- Watch strapsStrap widths are given in mm, most often 18, 20 or 22 mm.
- Screws and boltsThe number after M is the thread diameter in mm, so an M4 screw is about 4 mm across.
- Pencil leads and paperMechanical pencil leads come in 0.5 and 0.7 mm. A4 paper is 210 × 297 mm.
- School, crafts and hobbiesMeasure beads, buttons, stamps, coins and 3D print details when no ruler is on hand.
Using the mm Ruler on Your Phone or Tablet
Open this page in your phone’s browser and use the credit card to calibrate. The ruler opens vertical on small screens so it fits, and you can switch to horizontal and rotate your phone for a longer ruler. Phones are small, so for anything longer than about 15 cm, measure in parts and add them up, or use a tape measure.
The same page works as an mm ruler online for iPhone, Android, iPad and desktop browsers. Each device needs its own calibration, which is saved separately.
MM Ruler Online: Frequently Asked Questions
What is MM Ruler Online?
MM Ruler Online is a free online mm ruler drawn on your screen in millimeters, centimeters and inches. Hold small objects against the screen, drag two red lines to the ends, and read the length.
How do I get a ruler on my screen?
Open this page and scroll to the ruler. It appears at once with no download or sign-up. Calibrate it once with a credit card so the millimeters are true size.
How do I calibrate the ruler?
In the calibration panel, choose a credit card, a US dollar bill or an A4 sheet. Lay it on the screen with its left edge on the dark line, then drag the red line to its right edge. The setting saves automatically on your device.
Why do I need to calibrate with an object of known size?
A browser cannot know how big your screen really is, because every phone and monitor has a different pixel density. A credit card (85.6 mm wide), a US dollar bill (about 156 mm) or an A4 sheet (210 mm) are fixed sizes, so any one of them gives the ruler a reliable reference.
How accurate is an online mm ruler?
After calibrating, it is typically accurate to about 1%, roughly 1 mm on a 10 cm length. Screen glass, cases and viewing angle add small errors, so use a physical ruler or a caliper when exact size matters.
How do I measure in mm on my phone?
Open this page on your phone, calibrate with a credit card, then lay your object on the screen with one end on line A. Drag line B to the other end and read the length in mm.
How many mm are in a cm and an inch?
There are 10 mm in 1 cm and 25.4 mm in 1 inch. A meter has 1,000 mm. The converter and chart on this page cover common lengths.
How big is 1 mm?
1 mm is one thousandth of a meter, about the thickness of 10 sheets of printer paper. Each small tick on a metric ruler is 1 mm. The actual-size bars on this page show 1 mm up to 25 mm.
How do I read a mm ruler?
Find the last numbered centimeter line the object passes, then count the small ticks after it. Each tick is 1 mm and the middle line is 5 mm. For example, 4 cm plus 7 ticks is 47 mm.
Can I measure the diameter of a ring or a coin?
Yes. Put line A on one edge and line B on the opposite edge at the widest point, then read the length. For a ring, measure the inside diameter and compare it with a ring size chart. A US size 7 ring has an inner diameter of about 17.3 mm.
Can I measure something longer than my screen?
Not in one go. Measure it in sections and add the parts together, or use a tape measure.
Why did the size change after I zoomed?
Browser zoom changes how big everything is drawn. The ruler tries to follow zoom, but if the size looks off, calibrate again at your usual zoom level.
Does it work on iPhone, Android and desktop?
Yes. It runs in any modern browser, including Chrome, Safari, Firefox and Edge. Each device or screen needs its own calibration, which is saved separately.
Is it free, and is my data stored?
The ruler is free and needs no account. Nothing you measure is sent anywhere. Only your calibration is saved, in your own browser on your device.
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Measurements on a screen are approximate. Do not use this tool for medical, safety or engineering decisions.