I have to agree with fearless leader here, 90% to 95% loss of
mass volume seems spot on.
I compost mostly manures mixed with lots of straw, twigs and leaves.
A 4 foot cube will reduce in 90 days to a 4x4x2 foot area, the soil under this mass will be totally awesome soil, very soft, full of worms and with a microbiome that is hard to measure because it is so stacked with bacteria, fungi and all the other good critters that it is very hard to count them.
Because of this I usually build heaps where I want to plant once the compost is ready, most of my compost goes around the orchard trees, except for the small amount (2 inches) that I leave for mulch on the new garden plot.
Over the years I've tried lots of methods and finally settled on this one for best fit all around.
If you use a scale and weigh your starting heap and your finished heap, the mass retains about 70% by weight dry, but volume shrinks tremendously no matter if you turn it or not.
I did a side by side way back in 1983 with two identical heaps that were 4' cubes, one turned and the other left alone.
Starting weights were 115.76 lbs. and 115.78 lbs. end weights were both 80 lbs.
Both heaps were layered with each layer being compressed with a 40 lb. flat tamper, then allowed to spring back for 1 hour before the next layer went on, this was repeated until the cubes were level full.
Layers were 14 inches thick before compressing with the tamper.
The components were; dried cow chips, fresh from a square bale straw, fresh cut grass clippings.
Each layer was misted via watering head and hose after compression.
The trial went on for 90 days with the turned subject cube being turned once every two weeks throughout the trial period.
At the end of the 90 days neither cube was sieved.
cubes were 1 inch thick pine lumber and they were weighed at start and finish with a certified truck scale.
I was actually surprised at the end results, I had anticipated the unturned cube to be less completely decomposed and to weigh more than the turned cube, I was wrong in that anticipation.
Error, I said mass when it should have been volume (the strike through)