12 Inch Ruler — Actual Size
A real ruler on your screen, calibrated to your monitor. Dual inch and centimeter scale. Drag on the ruler to measure any span.
Place a physical credit card on your screen. Drag the slider until the dashed box matches the card exactly, then click Apply.
Common Objects at Actual Size
These are rendered at your screen's calibrated size. Hold the real object against the screen to verify your calibration.
About This Ruler
This 12 inch ruler is drawn at actual size on your screen using your monitor's pixel density (PPI). The top edge shows inch markings with 1/16" precision; the bottom edge shows centimeter and millimeter markings. At 12 inches, this matches a standard school or drafting ruler.
For the most accurate measurements, click "Calibrate" and use a physical credit card to set your screen's exact PPI. Once calibrated, your setting is saved — the ruler will stay accurate on every visit.
Frequently Asked Questions
Is this ruler actual size?
Yes — when calibrated to your screen. By default it uses 96 PPI (the web standard), which is accurate on most desktop monitors. Click "Calibrate" and use a credit card to set the exact PPI for your screen for maximum accuracy.
How do I calibrate the ruler to my screen?
Click the "Calibrate" button below the ruler. Place a physical credit card on your screen and drag the slider until the on-screen box matches the card exactly. Click Apply. The ruler will instantly resize to match your screen's actual pixel density.
How long is 12 inches in cm?
12 inches equals exactly 30.48 centimeters (or 304.8 mm). A standard school ruler is typically 12 inches or 30 cm. You can see both scales on this ruler — inches on the top edge, centimeters on the bottom edge.
How do I measure something with this ruler?
Place the object directly on your screen and align one edge with the 0 mark. Read the measurement where the other edge falls. You can also click and drag on the ruler to measure a custom span — the measurement appears above the highlighted area.
Can I print this ruler?
To print: open your browser's print dialog and set scale to exactly 100% (not "fit to page" or "shrink"). After printing, verify with a physical ruler. Most home printers print at 96 DPI so the default settings should be accurate.
How many mm is 12 inches?
12 inches = 304.8 millimeters. The formula: inches × 25.4 = mm. So 12 × 25.4 = 304.8 mm. This ruler shows mm marks on the bottom edge (1 cm = 10 mm).