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The simplest and safest way to estimate how many cubic inches of space you will need is to take the weight of the individual and convert it directly into cubic inches. So a 150 lbs individual will generally require about 150 cubic inches of space inside the cremation urn. The industry standard size for a single adult urn is around 200 cubic inches, which will generally hold any individual who weighed 200 lbs or under.
1 pound of healthy body weight generally equals 1 cubic inch of cremated remains.
1 pound = 1 cubic inch
This isn’t an exact formula, but a good rule of thumb which will ensure that you always have more space in the urn than you need. A closer formula would involve determining the deceased’s ideal body weight, and converting that number to cubic inches. This works because the amount of remains is determined more by bone structure than by physical weight, since tissue is completely incinerated and what is left is bone matter. You can use this ideal weight calculator to determine how many cubic inches you may need if the person was significantly over or underweight.
If you’re looking at a smaller urn, which holds, as an example, 100 cubic inches, and the individual weighed only a little more than that (say, 120-130 lbs), you might be ok but you’ll definitely want to have a contingency plan in case there is a portion of the remains that will not fit. For instance, you can get smaller “sharing” keepsake urns and share the remains among family members, or you can scatter some of the remains on your property, in a lake or ocean, or at a favorite outdoor area (it’s legal to scatter almost anywhere; most national forest areas are fine with scattering, and for local parks you may want to contact the local authorities to get the ok). See more info on scattering here.