Hi Zoe,
Thank you for posting to the forum! If you need to switch volume/mass/densities for properties of products, you can define these conversions as parameters.
The DATASMART Database comes with many Database parameters that include conversions between different units. SimaPro license holders through Long Trail Sustainability have access to the DATASMART Database. One can similarly define their own Project or Database parameters within SimaPro.
– Christina, Sustainability Analyst @ LTS