How to Use Screen Ruler Online
Quick-start guides for every Screen Ruler Online tool. Jump to the tool you need below — each section covers what it does, who it's for, and a 5-step walkthrough.
Tool-by-Tool Quick Start
Each section below covers one tool — what it does, who should use it, and a 5-step quick start. Calibration and pixel-density basics are explained at length in the Screen Ruler section, since those concepts apply across the measurement tools.
Screen Ruler
An accurate on-screen ruler in cm, mm, and inches. Calibrate once with a credit card and you can measure real objects against the screen edge to within half a millimeter. Most useful for designers checking print sizes, shoppers verifying product dimensions, and students measuring small objects.
Quick start
- Open the Screen Ruler. The tool loads a vertical ruler on the left side of the screen with markings in your selected unit.
- Tap the calibration panel and place a credit card flat on the screen. Drag the slider until the on-screen outline matches your card edge-to-edge.
- Switch units between centimeters, millimeters, and inches with the unit selector at the top.
- Lay the object you want to measure against the ruler. Align one edge with the zero mark and read the other edge.
- Use fullscreen mode for the longest possible ruler, especially on small screens. Calibration is saved per device for 30 days.
Printable Ruler
A free printable ruler PDF in cm, mm, or inches at true 1:1 scale. No calibration needed — print it, lay it flat, and measure. Ideal when you need a physical ruler for crafts, woodworking, sewing, or kids' homework and don't have one in the drawer.
Quick start
- Open the Printable Ruler page and pick your unit (cm, mm, or inches) and length.
- Click Generate PDF. The download is true 1:1 scale at 300 DPI.
- In your printer dialog, set Scaling to 100% (or Actual Size) — never Fit to Page, which silently shrinks the ruler.
- Print on standard paper. Use the included calibration square to verify the printer didn't rescale the file.
- Cut along the outline and you have a physical ruler. Laminate it for repeat use.
Device Specs
A filterable database of 69+ phones, tablets, and laptops with screen size, resolution, PPI, and aspect ratio. Useful for developers checking responsive breakpoints, shoppers comparing models, and designers picking target devices.
Quick start
- Open the Device Specs page. You'll see a filterable table with 69+ devices.
- Filter by brand, OS, year, type, screen size, or use the search box for a specific model.
- Click any device row to open its detail page with full specs and a side-by-side related-devices list.
- Use the Open Ruler CTA on a device page to launch a pre-calibrated ruler tuned to that device's PPI.
- Bookmark a comparison or copy specs into a spec sheet for procurement and design work.
Random Spinner
A customizable wheel spinner for fair random picks. Type any list of options, hit Spin, and the wheel lands on one at random. Used by teachers picking students, families settling dinner arguments, and teams randomizing draft order.
Quick start
- Open the Spinner. It loads with a default short list of options.
- Click an entry to edit it, use Add to add a new option, and Remove to delete one.
- Each option gets its own color automatically as the wheel re-segments.
- Press Spin. The wheel rotates with smooth deceleration and lands on one segment at random.
- Every option has the same probability of winning. Your list is stored only in your browser and disappears when you close the tab.
Dead Pixel Test
Full-color test screens that surface dead, stuck, and hot pixels on any monitor, phone, or TV. Run before accepting a delivery, after a screen repair, or when you suspect a fault — a 60-second pass tells you if your panel is healthy.
Quick start
- Open the Dead Pixel Test page on the screen you want to check.
- Maximize your browser to fullscreen so the test fills the whole panel — F11 on most browsers.
- Cycle through the color screens (red, green, blue, white, black). Look closely for any pixel that doesn't change with the background.
- A pixel that stays black is dead, a pixel stuck on one color is stuck, and a pixel always white is hot.
- Run the gradient test for grid uniformity issues like banding. Repeat after a stuck-pixel fix attempt to confirm recovery.
Protractor
A digital protractor that measures any angle from 0° to 360° on screen. Drag two rays to the angle you need, with optional snap to 1°/5°/15°/45°. Used in geometry homework, woodworking miter cuts, sewing darts, and roof-pitch checks on photos.
Quick start
- Open the Protractor. The tool loads a circular dial with two draggable rays at a default 90° angle.
- Drag either ray by its endpoint handle to rotate it around the center. The angle and shaded sector update in real time.
- For exact angles, toggle Snap on and pick a 1°, 5°, 15°, or 45° increment. Rays click to the nearest step.
- Place a printed drawing flat against the screen to measure angles drawn on paper.
- Use Reset to return to 90°. The protractor measures the inner angle by default — subtract from 360° for the reflex angle.
Screen Size Calculator
Calculate physical screen size from resolution and PPI, or check the right TV size for a viewing distance. Used by home theater builders sizing TVs, designers picking monitors, and shoppers comparing 65-inch vs 75-inch panels.
Quick start
- Open the Screen Size Calculator and pick your input mode — diagonal size, width and height, or resolution and PPI.
- Enter your numbers. The calculator returns physical width, height, area, and recommended viewing distance.
- For TV sizing, use the TV size for room sub-page: enter your couch-to-screen distance and pick a standard (THX, SMPTE, or 4K immersive).
- Compare two screens by running the calculator twice and noting the diagonal vs viewable area — a 16:9 65-inch is much larger than a 21:9 65-inch.
- Cross-check the physical size with the Device Specs database before buying.
Pixel Converter
An instant CSS unit converter — px to mm, cm, inches, rem, em, pt, %, vw, and vh. Designed for front-end developers, designers, and anyone tuning typography or layout against a target DPI.
Quick start
- Open the Pixel Converter. Pick your source unit (px, mm, cm, inches, rem, em, pt, %, vw, vh).
- Enter your value. All target units update simultaneously.
- Set the base font size (default 16px) and DPI for accurate rem and physical-unit conversions.
- Use the cheat sheet for common conversions like 1rem = 16px or 1pt = 1.333px.
- Copy the result into your CSS or design tool. The converter is stateless — refresh resets the form.
Aspect Ratio Calculator
Calculate aspect ratios from pixel dimensions or find a missing dimension when one side and the ratio are known. Built for video editors, social media designers, and developers picking image sizes for Instagram, TikTok, YouTube, and LinkedIn.
Quick start
- Open the Aspect Ratio Calculator. Enter pixel width and height to get the simplified ratio.
- Or pick the find-missing-dimension mode: enter width and a target ratio (16:9, 4:3, 1:1, 9:16) and the calculator returns the height.
- Use the platform cheat sheet to look up the right size for Instagram posts, TikTok video, YouTube thumbnails, or LinkedIn banners.
- Verify your image with the Pixel Converter if you need to convert pixel sizes to physical units for print.
- The simplified ratio is the GCD of width and height — 1920×1080 reduces to 16:9, 1080×1080 to 1:1.
PPI Calculator
A free PPI / DPI calculator that derives pixel density from screen diagonal + resolution, judges Retina at the viewer's actual distance, reverses pixels needed for print at 300 DPI, and compares devices side-by-side.
Quick start
- Open the PPI Calculator. Pick a mode: Auto-detect (current display), Screen (manual), Print (reverse from physical size), or Compare (multi-device).
- Enter the screen diagonal in inches plus resolution as width × height (e.g. 1920×1080).
- Set your typical viewing distance — the calculator scales the Retina threshold against it.
- Read the PPI result, Retina verdict (Below / Meets / Exceeds), and the physical screen size in mm/inches.
- Copy the result, share the calculated link (URL state-encoded), or jump to the Ruler tool with the same calibration.
Calibration Basics
Calibration is the most important step for any measurement tool on the platform. Without it, the ruler shows approximate measurements based on your device's reported pixel density, which is often inaccurate. With calibration, measurements become precise to within half a millimeter.
Using a Credit Card
A standard credit or debit card is the best calibration object because its dimensions are standardized worldwide at 85.6mm x 53.98mm (ISO/IEC 7810). Place your card flat on the screen and use the calibration slider to adjust the on-screen outline until it matches your card exactly.
Using Other Objects
If you don't have a credit card handy, you can calibrate with US quarters (24.26mm), Euro coins (23.25mm), UK pound coins (22.5mm), and more. Select the object from the calibration panel, place it on the screen, and adjust the slider.
Understanding Pixel Density (PPI)
PPI stands for Pixels Per Inch. It tells you how many tiny dots of light fit in one physical inch of your screen. A phone with 460 PPI has much smaller, more densely packed pixels than a desktop monitor with 96 PPI. This means the same number of pixels takes up less physical space on the phone.
Modern screens also use device pixel ratios. A MacBook with a Retina display might report a device pixel ratio of 2, meaning each "CSS pixel" in the browser actually spans 2x2 physical pixels. The ruler, protractor, and pixel converter all account for this automatically during calibration.
Device-Specific Rulers
We maintain pre-configured ruler profiles for popular devices. These profiles use the device's known PPI to provide a starting point before manual calibration. Visit a device-specific ruler for the best starting accuracy:
Questions?
Full answers to common questions about every tool — ruler accuracy, protractor precision, spinner fairness, device support, privacy — live on the FAQ page.