Online Ruler Tool
* Before use, make sure to calibrate the ruler by entering in screen size or adjusting the ruler size to match a standard card (ex. credit/debit/gift card).
How to use the online ruler
- Set the Screen Size slider to the diagonal size of your screen in inches. You can use the fine tune buttons to adjust it in steps of 0.1.
- Check the size by holding a bank card against the 0 end of the ruler. Its edge should line up with the dashed Standard Card Width line. If it does not, fine tune the screen size until it does.
- Choose centimeters or inches, and pick a horizontal or vertical ruler depending on the shape of the object and your screen.
- Line one end of the object up with the 0 mark, then drag the red line to the other end. The measurement is shown in the circle on the line.
Calibrating the ruler to your screen
Screens have varying pixel densities, so a ruler drawn in pixels is only accurate once it knows how big your screen is. The screen size is the diagonal measurement from one corner of the display to the opposite corner. It is usually listed on the box, in the product specifications, or on the manufacturer's website for your model.
The quickest way to confirm the setting is with a bank card, library card, or ID card. These are all a standard 85.6 mm (3.375 in) wide. Place the card flat against the screen at the 0 end of the ruler and adjust the screen size until the far edge meets the dashed line.
Ways to use it
- Measuring small items like screws, buttons, jewellery, or paper cut-outs when you do not have a ruler to hand.
- Teachers showing students how to read centimeter and inch scales on a projector or an interactive whiteboard.
- Using the ruler as a backlight to trace your desired measurements onto a sheet of paper.
- Measuring an angle rather than a length? Our online protractor does the same thing for angles in a photo.
Why we made this tool
We wanted an online ruler that was simple, accurate and easy to carry around. Why be forced to carry around a ruler when you can just use an online version and get the same result. All you need to know is your device's screen size. The ruler then scales itself to the screen size you enter, and the card line lets you check that it is right before you measure. That makes it accurate enough for everyday measuring on laptops, monitors, tablets, and phones.
Frequently asked questions
Is the online ruler free?
Yes. The tool is free to use, with no limits. Your settings are also saved in your browser, so you only need to calibrate once per device.
How accurate is an online ruler?
It is as accurate as the screen size you enter. To verify that the scale is correct, you can use a standard size card. All credit, debit and gift cards follow strict size guides, so they make sure the ruler is accurate as well.
Why does the ruler look wrong on my monitor?
The screen size is almost certainly off. Hold a bank card against the 0 end of the ruler and fine tune the screen size until the card's edge meets the dashed line. External monitors are a common culprit, since the ruler cannot detect them on its own.
How do I find my screen size?
Look up your device model and its specifications or check on the manufacturer's website. Most phones, laptops and other devices all have their screen sizes publicly available. As such, a simple search will give you your exact screen size to use for the ruler.