When we estimate we're making a smart guess about something. Estimating isn't always completely accurate but can give us a rough idea of a length.
Just like when measuring, it's important to think about the units we're using to estimate.
If something is really tall, like a skyscraper, would it be best to use millimetres, metres or kilometres to estimate the height? Millimetres would be too small, and kilometres would be too big. The best unit to use would be metres.
Sometimes it's easiest to think of other things that might be a similar length.
Choose the best estimate for the length of a classroom.
Write down the measurement marked with an X.