30 cm Ruler — Actual Size
A standard 30 cm school ruler at actual size on your screen. Dual cm/mm and inch scale. Calibrate for precision. Drag 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
Hold the real object against your screen to verify calibration.
About This 30 cm Ruler
This ruler measures exactly 30 centimeters (300 mm / 11.81 inches) — the standard school and office ruler length used worldwide. The top half shows centimeter and millimeter marks; the bottom half shows the inch scale with 1/16" precision.
For best accuracy, click Calibrate and use a credit card to match your screen's exact PPI. Once set, your calibration is saved so every visit is accurate automatically.
Frequently Asked Questions
Is this 30 cm ruler actual size?
Yes — when calibrated to your screen. By default it uses 96 PPI, which is accurate on most desktop monitors. Click "Calibrate" and use a credit card to set your screen's exact PPI for maximum precision.
How many inches is 30 cm?
30 cm = 11.811 inches (approximately 11 and 13/16 inches). You can see both scales on this ruler. A standard school ruler is 30 cm or 12 inches long.
How many mm is 30 cm?
30 cm = 300 mm exactly. This ruler shows all 300 individual millimeter marks — each small tick is 1 mm, every 5th is slightly taller, and every 10th (1 cm) is the tallest with a number label.
What is the standard ruler length?
The most common ruler length worldwide is 30 cm (approximately 12 inches). This is the standard size sold in schools and offices. Longer rulers include the 45 cm (18 inch) and 60 cm (24 inch) versions used in drafting.
How do I measure something with this ruler?
Place your object directly on screen and align one edge with the 0 mark. Read where the other edge falls. You can also drag on the ruler to measure any span — the exact measurement appears in the badge above the ruler.
Can I use this as a school ruler replacement?
Yes — after calibrating to your screen, this 30 cm ruler is as accurate as a physical school ruler for most tasks. It's ideal when you don't have a physical ruler nearby. For critical measurements, always calibrate first.