I'm not positive but I think I'd start with 4 cu ft to get 1.
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I use 8 cu. ft. of dry oak and hickory leaves to end up with 1 cu. ft. of finished leaf mold.
To get the leaves to partially decompose I build a "chicken wire" tube, mine is one meter (or 3 feet works for English measuring) diameter and 4 feet tall, I fill this to the top with the dry leaves and then I water them with a sprinkler until all leaves are saturated.
From that point I walk away for a week and when I come back I first check moisture content and adjust it, then I pack the leaves down with a stick and walk away for another week of letting them do their thing.
This is repeated until the leaves end up packed down to about a foot in height (for me this is about 8 weeks), at that point I remove my containing form and set it up for another batch if I need more than the one that is working.
I use a pitch fork to turn the leaf mold so I can check it for signs it is ready for use (white fungi threads(hyphae) are what I use to make the determination) when it looks like 1/3 of the pile has hyphae running through it I take the new leaf mold and use it where needed.
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I have piled up leaves 4 feet high and 25 ft in diameter. A year later when you begin piling the following years leaves you will have a hard time finding evidence of those leaves. I guess you might have an inch or less. So at that rate it's 48 to 1.
You have to remember though that you can keep compressing leaves possibly endlessly. If you drag your leafs in a tarp and after dumping those leaves you flip the tarp and walk over the tarp you can seemingly keep walking over it and they just keep compressing. If you walk over them with just your booted feet your feet will sink thru the leaves. With the tarp it spreads your weight over a larger area. You can test this by walking thru a raked pile and judge how much your foot print compresses the leafs under your foot.
I'm pointing out the compression factor to point out how hard it is to give a number. But it also gives you an estimate of what will be left a year later. If you keep raking the compost pile you'd be fluffing the pile up. If you mix them into soil they will recompress to an unfluffed volume.
Like John, my ratio is closer to 50 to 1 if it's well decomposed.
I would suspect your ratios have a lot to do with the degree to which the leaves have disintegrated. Like compost, sometimes its best to use it before it reaches such a state of decomposition that a lot of that carbon has begun to gas off and return to the atmosphere from which it first came. Plants pull that carbon from the air by way of photosynthesis. Decomposition returns much of it back to the atmosphere. Only what is sequestered in the ground remains for the long haul.
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