 # How to calculate volume of a mesh object

With the 3D print addon. Under Rigid Body -> Mass i have put the value of 5Kg. And Under object -> Rigid Body -> Calculate mass with a custom i have change value to 7200 (cast iron) as density given. Click on the value it suddenly change to 44.5kg. And i want 5Kg. How it works. Any suggestion whats going wrong, or help. Thanks

how calculate weight.blend (2.3 MB)

i doubt blender can calculate the mass of such a complicated shape
with some open volume
so it will be an approximation anyway!

happy bl

Density =mass / volume
kg/m^3 = kg / m^3
kg/m^3 = 5 / 0,006172

Density - 7.2 kg/dm3
Mass - 5kg
V -?
V = D/M (7.2 / 5) — 1.44 dm3

1 m3 = 1000 dm3
1 m3= 1.000.000 cm3
Density = kg / m3

Sorry getting confuse can u plz make it clear. What will the final result and how it works to my model. I have attach my file . Do i have to scale or 3D addon do so. If i have to scale all the measurement will go wrong. Thanks

I don’t have much experience with the 3D print addon
1 m3 = 1000 dm3
1 m3= 1.000.000 cm3
Volume = 6172.6843 cm3 = 6172,6843 / 1.000,000 = 0,0061726843 m3
Blender Density = kg / m3
Density = 5 / 0,0061726843 = 810,02

V = M / D
V = 5 / 7,2 = 0,694 dm3 = 694 cm3
V = 0,000694 m3 = 0,694 dm3 = 694 cm3

You have to set the Mass first…then in your Ridgid body set 5kg, then in the 3dprint select volume…you will get 6172.68cm3…if you hit scale to volume with that setting…nothing will change on the mesh…the 5kg will stay.

Its not working can plz make it more clear or a gif or video. How it works. Thanks

As per our PM’s, what you are doing is drawing an object and then trying to say it weighs 5kg ( which is OK but you can’t expect the math to work in that way)…The object needs to be scaled to fit its weight.

A solid disk of cast iron needs to be 159.5mm x 35mm to weight 5kg
if you draw up a cylinder with these dimensions and calculate the mass it will show 4.99kg in the solid body slot…

Like I said in the PM…If you want this object to fit a certain weight you are going to have to do ALL the calculations…I am not sure if it could be done with a ratio or not…I haven’t taken math for 40 years…

Is where I got the size of a disk to find the radius…I left 35mm thick and adjusted the diameter till I got 11.2lbs =5kg
Then in Blender…made a cylinder to those dimensions…
Select the disk and add the ridged body… don’t enter anything in the Mass
Bring up Calculate Mass-Cast Iron…and check the KG in the soft Body…4.99Kg

PROBLEM !! The object is a batch of different parts…you have to subtract all those volumes from the overall and still try to keep 5Kg ???
( I only have half the math skills you would need to do that!! )

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How the mass is calculate with 3D print addon

I have 2 mesh with same size and same verts, Why the mass is showing different. To right it is showing 5Kg and To left it is showing 20Kg.

Before What i did is i put the value 5Kg in rigid body -> mass and in object -> rigid body -> calculate mass i have selected custom and put the value of 7200.
In the 3d Addon -> Transofrm -> I click on Scale to Volume. A pop-up with some value just click on OK. This way i have got different value. 5Kg and 20 Kg.

Without changing the size and same verts how the mass is set? Any suggestion. Did my mesh scale but it did not how the mass is set to different values. Thanks

different mass.blend (1.1 MB)

You set the Mass Value After you set the Mass in Ridgid Body…It has to be set first then you set in Ridgid Body and in 3d print only select Scale to Volume. Remember you are taking an object and just Saying it is 5Kg…not actually modeling an object to be 5Kg
when you select calculate Mass again with these selected it will revert…Scale to Volume will not change the Mass.

Confusion 1st later. Can you plz a small video or gif so that i can understand how it works. Thanks

Not sure this will get the point to you…
Look at it this way…the Mass in the Ridgid body setting that you set to 5kg is an arbitrary mass that will be used in the simulation,only, it does not change the mass of the object…Calculate Mass is the calculated Mass of your object as modeled. To get an object to be 5kg you HAVE to scale the object. You have 3 choices, change the diameter, change the thickness or add mass to the object.
NOTE: If you do not apply scale it does not work…all cubes reguardless of their size will have the same mass, untile you apply scale…then each will reflect the actual mass.

Thanks for detail answer. I have a question i have model with all the measurement. After scaling (point 4) the mesh get reduce. All my measurements go wrong.

your model scaled by density and volume
d = m / v
v = m / d
m = 5 kg ( you want it to be 5 kg)
d = 7150 kg / m3 (cast iron)
v = 699,3 cm3

If you don’t want your measurements to change,
volume must remain constant, ( v = 1966,5 cm3 )
mass or density must change.

any option or way without changing the volume or measurements and increasing only the mass i.e. 5kg to 10 and 15kg.

v = 1966,5 cm3 = 0,0019665 m3

Calculate mass (Custom)
m = 5kg
Density = d = m / v = 5 / 0,0019665 = 2542,588 kg / m3

m = 10 kg
d = 10 / 0,0019665 = 5085,176 kg / m3 ( 2542,588 x 2 )

m = 15 kg
d = 15 / 0,0019665 = 7627, 765 kg / m3 (2542,588 x 3 )

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