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I just want to scale the entire object proportionally down to 120mm in length.

I have already set the units in the scene to be mm.

I see Object Mode > Transform > Dimensions but I don't see how to lock those dimensions to move with each other when I set a length.

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I don't think there's a way of doing it directly.

I do it by:

  • CtrlA > Applying the scale of the object
  • Adjusting the known dimension in the 'Dimensions' panel
  • Copy and pasting the scale from the known dimension to the others in the 'Scale' panel
  • (CtrlA > Applying the scale of the object, again)

EDIT: 2.8 makes one way a bit easier...

  • Select the known dimension in the 'Scale' panel
  • Right-click and select 'Copy as Driver'
  • Right-click in the other scale fields and 'Paste Driver'

Now the known dimension will drive the other 2, when you adjust the known Dimension.

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  • $\begingroup$ at least I know I'm not crazy for not finding it. o_o;;;; $\endgroup$ – user84702 Nov 29 '19 at 23:13
  • $\begingroup$ @user84702 ...sort of way, for doing it in bulk from 1 dimension.. see edit. $\endgroup$ – Robin Betts Nov 30 '19 at 6:36
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Quite a few ways to achieve this, but the simplest ways are as follows.

Take the current length dimension of you object and calculate the ratio as:

ratio = 120 / current_length

Then press the S key then type in the ratio into the number pad and hit Enter.

Another way to do this is press the S key then press the / key then type in the current_length value, then press enter, this will scale the object to be 1mm long. Then press the S key and type 120 in to the number pad and hit Enter to scale the object up to 120mm.

The other option is to just hit the S key and move the mouse until you are roughly around 120mm then hold the Shift key to give you fine grained control of the scaling and set to exactly 120mm.

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    $\begingroup$ You can also do the calculations on the fly with the numerical input while in scaling operation. If you press * or = while in operation, the numerical input switches to Advanced Mode and you can do lots of math in there. $\endgroup$ – Martynas Žiemys Nov 30 '19 at 10:25
  • $\begingroup$ That @MartynasŽiemys this is a really useful feature that I was unaware of. I will play with it :) $\endgroup$ – Moog Nov 30 '19 at 10:30
  • $\begingroup$ Yeah, that's one of the secret cheat codes in Blender that is very well hidden. :D Very useful for converting measuring units. $\endgroup$ – Martynas Žiemys Nov 30 '19 at 10:41
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You can also press N in viewport. Look at item Tab. There is a dimensions part. You can select all X,Y,Z values with mouse. Then grab and move the mouse so it will scale. Scale up so You will reach the desired value for X Y or Z. The rest will just follow (if you select all of them)

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  • $\begingroup$ I tried this in 2.81 and it seems that it only selects identical values this way then begins scaling, this is easily solved by hold shift while selecting the values, then releasing the mouse before releasing shift. Note that this will also take fractions as well as decimal numbers $\endgroup$ – Moog Nov 30 '19 at 10:33

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