Uneven measurements

I am trying to draw a really simple 2 d drawing of a piece of joinery, I can draw a 100mm line. But then when I want to draw a second one and I type in 100mm, the line actually looks about twice the size of the first one. I am also constantly getting a squiggle before my measurements. I recently upgraded my sketch up to a newer version. please help. Its driving me nuts

Are you drawing your lines along an axis (is it turning to a color before you enter the value)? It is really hard to tell what is going on without seeing your screen. Maybe you can share a screenshot or two.

And do turn off Length Snapping (Model Info>Units)

If you share the .skp file that would help us quickly identify the problem.

thank you for your help

not sure you can see this image - I copied a screenshot onto this window

that length actually iOS about right but there is a squiggle before it?

same shot but I didn’t take my measurement all the way up to the line. The squiggle has gone?

The tilde ~ (squiggle) tells you that the displayed value is an approximation of the actuale measurement between endpoints (or other points), here the length of the edge you have drawn.
You can increase the displayed precision in ‘Model Info’.

Based on the color of the snap points, the first image shows you snapping from one end to the next while the second image shows you snapping from one ping and hovering over the line (not the end of the line).

Yes - but I don’t understand why there is a tilde when I have snapped to the end of the line?

Without sharing your model, neither do we.

As @Wo3Dan described, the tilde (~) character simply means that the actual length of the line cannot be shown with complete precision (decimal points or fraction) because of the limited display precision that has been set (by you explicitly or by the template that you are using). If you increase the display precision (via the Units portion of the Model Info window) then SketchUp will display finer values (more decimal digits or smaller fractional denominators).