struggling with scale bars?

I know quite a few of you were having some difficulty calculating scale bars for your diagrams today...

Some of you may have done one big scale bar representing the entire length of your zebrafish embryo, like I have done in this example diagram of a cell:

In the future it would be good for you to know how to make a nice little neat scale bar that represents a nice round number (like 10 μm 100 μm or 1000 μm).

There are multiple ways to think your way through this problem, but here is what I would do:

So, say you measured your zebrafish embryo and it was 3032 μm long. And say you have drawn a zebrafish embryo that measures on your page 24 cm long...

Actual Zebrafish embryo length =  3032 μm
Length of embryo you have drawn = 24 cm

If you wanted to draw a scale bar that represents 1000 μm, then how would you calculate how long this scale bar should be?

The way I look at this is to first ask, how much smaller than  3032 μm is 1000 μm?

To find this you would do 3032/1000 = 3.032
So, 1000 μm is 3.032 times smaller than  3032 μm

Now, this means that your scale bar representing 1000 μm will need to be 3.032 times smaller than the length of the embryo you have drawn on your page - so 3.032 times smaller than 24 cm.

You can work this out by doing:

24/3.032 = 7.9 cm (the same as 79 mm)

So, you can draw a scale bar that is 79 mm long, and label this up as 1000 μm.