Is it a real-size ruler?

No, and it could not be: a whiteboard is nearly two metres across and every display is a different size, so a centimetre on screen is not a centimetre. It is a ruler for reading, not for holding against something. Use the plastic ones on the tables for that.

Why does the strip start at zero?

Because that is the technique, and a tool that opened on a strip floating in the middle of the ruler would be teaching the mistake first. Dragging it off zero is the deliberate next step, and the board writes out the subtraction when you do.

Can I measure in metres?

Not yet. A metre stick and a 30 cm ruler are different objects with different marks on them, and drawing one as the other would teach that a metre is 30 of something. It is worth its own tool.

Why can I not move the ruler instead of the strip?

It would be the same measurement, and one thing to drag is better than two. Moving the strip is also what makes the zero mistake reachable, which is the half of this that a picture in a book cannot do.

Interactive Ruler

Measure · Year 1–Year 5

Measure in cm and mm, and read the same length three ways.

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About the ruler

A ruler with something on it to measure. Drag the strip along the ruler, or drag either end to change how long it is, and every position is a whole number of millimetres.

The strip can be dragged OFF zero, and that is the point. Starting from the end of the ruler rather than from zero is the mistake every class makes, and a ruler that lined the two up for you could not show it. Off zero, the board writes the subtraction out: it starts at 3 cm, it ends at 9 cm 7 mm, so it is 6 cm 7 mm long. Line up puts it back.

The same length reads as 67 mm, 6 cm 7 mm, or 6.7 cm, and switching between them does not move the strip. That is converting between units with the measurement standing still, which is the Year 4 objective and hard to show any other way.

The 10 cm ruler is drawn as wide as the 30 cm one, so its millimetre marks are three times further apart. A class reading millimetres for the first time needs marks they can see from the back of the room, and a 10 cm ruler drawn to scale would be a third of the length.

Hide the answer and the board asks the question instead. The class measures, then the tool says.

How to use it on your whiteboard

1Drag the middle of the strip to slide it along the ruler.
2Drag either end to make it longer or shorter. The buttons do the same, a millimetre at a time.
3New Length gives something else to measure, lined up at zero and never a whole number of centimetres.
4Drag the strip off zero to set the harder question, and Line up puts it back.
5Units switches between mm, cm and mm, and cm as a decimal.
6Answer hides the measurement, so the class reads the ruler first.
7Tap Fullscreen to fill the board.

Questions

Is it a real-size ruler?: No, and it could not be: a whiteboard is nearly two metres across and every display is a different size, so a centimetre on screen is not a centimetre. It is a ruler for reading, not for holding against something. Use the plastic ones on the tables for that.
Why does the strip start at zero?: Because that is the technique, and a tool that opened on a strip floating in the middle of the ruler would be teaching the mistake first. Dragging it off zero is the deliberate next step, and the board writes out the subtraction when you do.
Can I measure in metres?: Not yet. A metre stick and a 30 cm ruler are different objects with different marks on them, and drawing one as the other would teach that a metre is 30 of something. It is worth its own tool.
Why can I not move the ruler instead of the strip?: It would be the same measurement, and one thing to drag is better than two. Moving the strip is also what makes the zero mistake reachable, which is the half of this that a picture in a book cannot do.