If you keep all the ashes in one urn, the general rule of thumb for volume is one cubic inch per pound of body weight. So, the average adult urn has a storage capacity of around 200 cubic inches. Children’s urns and urns for pets can be a little more miniature, about 100 lbs.
However, the size of your chosen urn will depend on your needs. For example, urns don’t have to be used for ashes – you may want to keep a few small memorial items instead. Companion urns hold the ashes of two people, a thoughtful resting place for couples or close family members.
Rather than keeping all of someone’s ashes in one urn, you might prefer to scatter some and keep a small amount with you. You can also split the ashes between different urns for different family members. It’s up to you, as there is an urn for every eventuality.