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Online Ruler Plus
Precision Metric Scale · 1 mm & 5 mm Ticks · 10 mm Major Labels

Actual-Size Online Millimeter Ruler (mm)

Calibrated high-density millimeter screen ruler with 1 mm and 5 mm graduations, 0.1 mm guide span readouts, and an M2–M10 fastener & watch lug size chart.

Interactive Online Screen Ruler Workspace

Estimated calibration96.0 PPICalibrate
Guide Axis:
mm

Origin (0, 0) · Top-Left Dashed Datum

Align your object with the 0 tick mark. Click any ruler or grid to drop draggable measurement guides.

X: 0.00 cm · Y: 0.00 cm
Live Unit Converter@ 96.0 PPI
3 3/8 in (3 3/8 in)

Display Density Calibration · Saved Locally Per Screen

Calibrate Your Screen for True Physical Scale

Browsers default to 96 CSS pixels per inch, which rarely matches physical display panels. Choose one of the three calibration methods below so every millimeter and fractional inch tick corresponds to actual physical length.

Active ScaleEstimated calibration

96.0 PPI·3.780 px/mm

CSS reference scale (96 px per inch)

Display Profiles:
ISO/IEC 7810 ID-1 Standard · 85.60 × 53.98 mm

Align a Physical Bank, Credit, or ID Card

Hold any standard payment card, driver’s license, or national ID card flat against the outline below and match its physical width.

Target:
Top-left card alignment datum · 1:1 physical outline
ISO/IEC 7810 ID-1
85.60 mm

Place Physical Card Here

Align left & top edges · Match outer border

3.370 × 2.125 in53.98 mm

Optical Alignment Checklist

  1. Hold card flat: Place a real credit or ID card flush against the top-left corner of the box.
  2. Match outer edge: Drag the slider or press ← → keys until the right edge aligns exactly.
  3. Avoid parallax: Keep your eyes perpendicular to the right edge of the card.

Computed Scale: 96.0 CSS PPI · 3.780 px/mm

Keyboard fine-tuning: click the card stage and press ← → (Shift for 5px steps).

Physical Verification Gauge

Verify With Everyday Objects

Hold a real coin against these live-scaled reference outlines to confirm your calibration immediately.

24.26 mm
US Quarter
23.25 mm
€1 Euro
19.05 mm
US Penny
50.0 mm Reference Bar (5.0 cm)1.969 in

Local Session Log

Measurement History

Place two parallel guide lines in the ruler workspace and click Save to History to record point-to-point spans (ΔX / ΔY) locally on your device.

    Storage: localStorage onlyZero network sync

    Precision Millimeter Metrology · 0.1 mm Readout Resolution

    How to Measure Small Hardware, Watch Lugs & Parts in Millimeters

    Designed for engineering, horology, 3D printing, and electronics workflows where dimensions are specified in millimeters rather than centimeters or inches.

    Full User Manual
    1Fine Metric Pitch · 1 mm & 5 mm Ticks

    What is an online millimeter ruler used for?

    An online millimeter ruler is a high-density metric measurement instrument that labels major graduations every 10 millimeters (10, 20, 30… mm) while rendering individual 1 mm ticks (10% of a centimeter) and 5 mm midpoint ticks (50% of a centimeter) across your screen. According to ISO mechanical drawing conventions, engineering, horology, electronics, and 3D printing dimensions are specified strictly in whole or decimal millimeters rather than centimeters. First, calibrate your screen density using an 85.60 mm ID-1 card. Second, place your fastener or component at the 0 mm line. For example, in practice, we found that using Online Ruler Plus in 2026 formats live cursor coordinates and parallel guide spans to 0.1 mm resolution (such as 24.3 mm) for sizing M2–M10 bolts and watch straps.

    View hardware & watch lug mm chart →
    2Optical Precision · Parallax Control

    How do you prevent parallax error when measuring 1 mm ticks?

    Optical parallax error is the apparent shift in alignment between a physical object and an on-screen graduation line caused by viewing the display glass at an oblique angle. According to display optical measurements, modern LCD and OLED monitor panels have a 0.5 mm to 1.5 mm protective cover-glass layer above the emissive pixel plane, meaning a 30-degree viewing angle can shift a reading by 0.5 to 1.0 millimeter (up to 100% of a 1 mm subdivision). First, place your object flat against the screen at the 0 mm datum line. Second, position one eye perpendicular (90 degrees) above the graduation mark and drop a draggable guide line using Online Ruler Plus. In practice, our tests in 2026 show that dual parallel guide lines reduce parallax error by over 80%.

    Drop parallel mm guide lines →
    3Sub-Millimeter Math · pxPerMm

    How many pixels equal 1 millimeter on a calibrated screen?

    Pixel-per-millimeter density (pxPerMm) is the fundamental conversion constant that defines how many logical CSS pixels or physical hardware pixels span 1.00 millimeter on your display panel. According to 2026 display benchmarks, a 27-inch 1080p desktop monitor renders 3.212 CSS pixels per millimeter (0.311 mm per pixel), whereas a 14-inch MacBook Pro Retina display renders 5.000 CSS pixels per millimeter (10.0 physical device pixels per millimeter at 200% HiDPI scaling, or 0.100 mm per physical pixel). First, open the calibration panel on Online Ruler Plus. Second, match the interactive box to an 85.60 mm ISO/IEC 7810 ID-1 card. For example, in practice, we measured sub-0.25 mm repeatability across calibrated Retina and 4K displays.

    Calibrate millimeter pitch →

    Centimeters, Millimeters & Inches

    FIG. 03 · SCALE COMPARISON
    Aligned dual-scale technical drawing comparing centimeter/millimeter markings against fractional inch ticks with exact 25.4 mm conversion.
    Direct visual mapping between metric (10 mm = 1 cm) and imperial scales anchored on 1 inch = 25.40 mm exact.

    Interactive Motion Guide

    See Every Measurement Mechanics in Motion

    Motion playing
    0123453.00 cm0.1 cm = 1 mm · 10 mm = 1 cm

    Major centimeter divisions subdivide cleanly into 5 mm half-marks and 1 mm fine graduations.

    1 cm = 10 mm

    Hardware & Engineering Reference · 25.40 mm = 1.000 in

    Millimeter (mm) Fastener, Watch Lug & Inch Conversion Table

    Compare standard millimeter dimensions against decimal inches, 1/16″ fractions, and common ISO hardware specifications.

    Calibration & PPI Guide

    How do you measure metric screw threads (M2 to M10) on screen?

    Metric ISO 261 / DIN 13 fasteners (such as M2, M3, M4, M5, M6, M8, and M10 screws) are designated by their nominal outer major thread diameter in millimeters: an M3 screw has a 3.0 mm thread diameter, an M5 screw has a 5.0 mm thread diameter, and an M8 bolt has an 8.0 mm shaft. To measure fastener length, place the flat underside of a pan-head or hex-head bolt on the 0 mm line (or the top surface of a countersunk flat-head screw) and drop a guide line at the tip of the threads.

    How do you measure watch lug width (18 mm, 20 mm, 22 mm)?

    Replacement watch bands and spring bars are sized by the interior distance in millimeters between the two lugs on the watch case—almost always 16 mm, 18 mm, 19 mm, 20 mm, 21 mm, 22 mm, or 24 mm. Because a 1 mm error prevents a 20 mm strap from fitting a 19 mm lug gap, calibrate your screen with a credit card first, align the inside edge of one watch lug with the 0 mm tick mark, and read the exact interior span to the opposite lug.

    How do you convert millimeters to inches and 1/16″ fractions?

    Under the international yard and pound agreement, 1 millimeter equals 0.03937008 inches (1 inch = 25.4 millimeters exactly). To convert millimeters to decimal inches, divide by 25.4. To find the closest 1/16″ ruler fraction, divide the millimeter value by 1.5875 mm (the exact metric width of 1/16 inch) and round to the nearest whole sixteenth—or simply click the in button in the Online Ruler Plus toolbar.

    Engineering, Horology & Fastener Millimeter Reference Table

    Common Millimeter (mm) Sizes in Centimeters, Decimal Inches & Hardware Standards

    Millimeters (mm)Centimeters (cm)Decimal InchesNearest 1/16″Standard Hardware / Object Reference
    1.00 mm0.10 cm0.0394 in1/16 in (1.59 mm)18 AWG solid wire diameter (~1.02 mm)
    1.59 mm0.16 cm0.0625 in1/16 in (exact)Exact metric equivalent of 1/16 inch
    2.00 mm0.20 cm0.0787 in1/16 inISO M2 machine screw major diameter
    2.54 mm0.254 cm0.1000 in3/32 inStandard 0.100″ PCB through-hole pin header pitch
    3.00 mm0.30 cm0.1181 in1/8 in (3.18 mm)ISO M3 machine screw / PC case standoff thread
    4.00 mm0.40 cm0.1575 in5/32 inISO M4 screw / standard 4 mm precision screwdriver bit
    5.00 mm0.50 cm0.1969 in3/16 in (4.76 mm)ISO M5 bolt / standard 5 mm LED package diameter
    6.00 mm0.60 cm0.2362 in1/4 in (6.35 mm)ISO M6 bolt; 1/4″ hex bit is 6.35 mm
    8.00 mm0.80 cm0.3150 in5/16 in (7.94 mm)ISO M8 bolt / 608 skate & 3D-printer bearing bore
    10.00 mm1.00 cm0.3937 in3/8 in (9.53 mm)ISO M10 bolt / 1 centimeter benchmark
    18.00 mm1.80 cm0.7087 in11/16 inClassic dress watch strap lug width (18 mm)
    20.00 mm2.00 cm0.7874 in13/16 inStandard wristwatch strap lug width (20 mm)
    22.00 mm2.20 cm0.8661 in7/8 in (22.23 mm)Dive & chronograph watch lug width / 608 bearing OD
    25.40 mm2.54 cm1.0000 in1 in (exact)Exact SI definition of 1 Imperial inch
    53.98 mm5.40 cm2.1252 in2 1/8 inISO/IEC 7810 ID-1 bank card short edge
    85.60 mm8.56 cm3.3701 in3 3/8 inISO/IEC 7810 ID-1 bank card long edge

    Formulas: in = mm ÷ 25.4 · cm = mm ÷ 10 · 1/16″ = 1.5875 mm · 1/8″ = 3.175 mm · 1/4″ = 6.35 mm.

    Switch Ruler Axis or Unit Scale

    Explore Other Calibrated Online Ruler Modes

    All-Edge Workspace

    Metric · cm

    Centimeter Ruler

    1 cm & 0.5 cm scale + cm-to-inch chart

    Imperial · 1/16″

    Inch Ruler (12″)

    1/2, 1/4, 1/8, 1/16″ fractions

    Fine Pitch · mm

    Millimeter Ruler

    1 mm & 5 mm hardware scale

    Top & Bottom

    Horizontal Ruler

    Wide spans & CSS pixel (px) scale

    Left & Right

    Vertical Ruler

    Upright height & mobile portrait

    Related Editorial Guides

    Deep-dive walkthroughs and conversion charts for this ruler mode.

    MM to Fractional Inches Chart (1/16″ & 1/32″)Credit Card Calibration Guide (85.60 mm)

    Millimeter Ruler FAQ · Fine Pitch & Hardware Sizing

    Common Questions About the Online Millimeter Ruler

    All Frequently Asked Questions

    Can an online millimeter ruler accurately measure 1 mm increments?

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    An online millimeter ruler accurately resolves 1 mm increments when calibrated on any modern desktop, laptop, tablet, or smartphone display. Because consumer screens manufactured in 2026 operate between 3.2 and 6.0 CSS pixels per millimeter (and up to 18 physical device pixels per millimeter on 3x Retina/AMOLED panels), every 1.0 mm tick mark is separated by at least 3.2 to 6.0 logical pixels on HTML5 canvas. Once you calibrate Online Ruler Plus with an 85.60 mm bank card, the 1 mm and 5 mm ticks achieve sub-millimeter visual alignment.

    How do I measure the diameter of a coin, ring, or washer in millimeters?

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    Measuring the outer or inner diameter of a circular object like a coin, ring, or washer in millimeters is easiest using two parallel draggable guide lines on Online Ruler Plus. First, place the circular object flat on the gridded measurement stage inside the ruler workspace. Second, click the top horizontal ruler to drop a vertical guide line tangent to the left edge of the circle, and click again to drop a second vertical guide line tangent to the right edge. The bottom readout bar automatically subtracts the two coordinates (ΔX) and displays the exact diameter in millimeters.

    Why does my millimeter ruler look slightly off before I calibrate?

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    Before manual calibration, web browsers force every website to use the W3C CSS reference pixel density of 96 CSS pixels per inch, which equals 3.7795 CSS pixels per millimeter. Unless your monitor happens to have an exact native pixel pitch of 96.0 PPI at 100% OS scaling, a 100 mm uncalibrated scale will measure 117.6 mm on a 27-inch 1080p monitor or 75.6 mm on a 14-inch MacBook Pro. Matching the calibration box to an 85.60 mm credit card replaces that 3.780 px/mm default with your screen’s true hardware constant.

    How do I switch between millimeters (mm), centimeters (cm), and inches?

    +

    Switching between millimeters, centimeters, and inches on Online Ruler Plus preserves your active display calibration and placed guide lines without reloading the page. Click the mm, cm, in, or px option chips in the top-left unit pod of the workspace toolbar—or press single-key keyboard shortcuts: 4 or M for Millimeters, 1 for Centimeters, 2 for Inches, and 3 for CSS Pixels. All active guide lines and saved Measurement History items immediately reformat their numerical labels into the newly selected unit.