Whenever I mix two-part epoxy, the combined volume appears to be 20%-30% less than the individual parts. Why is this?

I think this is woodworking-related, as I'm using epoxy to fill voids, but if I should ask elsewhere please point me in the right direction. I considered asking this on the chemistry SE site, but everything over there seems to be more about electron configuration, etc.

  • 1
    I've never noticed this! But then I've never mixed a large volume of liquid epoxy. I shall see if I can notice the same thing happening when mixing a small amount of standard off-the-shelf epoxy on a flat surface now that I know to look for it.
    – Graphus
    Sep 27, 2017 at 8:07

1 Answer 1


Trying to recall my grade 11 chemistry here, but the makeup of the two parts are not identical and one has molecules that fit in the empty spaces of the other part. You can observe the same behaviour with other liquids like mixing water and alcohol. Or an easier to visualize example is imagine pouring sand into a container filled with large marbles.

The mass of the two will equal the sum of both masses, but the volume is dependent on many aspects including the molecular structure, temperature, air pressure, etc.

  • Now it feels like a silly question. Thanks for the clear response.
    – 3Dave
    Sep 26, 2017 at 18:55

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