Understanding how much moisture to put into a pile soon becomes an intuitive certainty.  Beginners can gauge moisture content by squeezing a handful of material very hard.  It should feel very damp but only a few drops of moisture should be extractable.  Industrial composters, who can afford scientific guidance to optimize their activities, try to establish and maintain a laboratory-measured moisture content of 50 to 60 percent by weight.  When building a pile, keep in mind that certain materials like fresh grass clippings and vegetable trimmings already contain close to 90 percent moisture while dry components such as sawdust and straw may contain only 10 percent and resist absorbing water at that.  But, by thoroughly mixing wet and dry materials the overall moisture content will quickly equalize.

Size of the pile. It is much harder to keep a small object hot than a large one.  That’s because the ratio of surface area to volume goes down as volume goes up.  No matter how well other factors encourage thermophiles, it is still difficult to make a pile heat up that is less than three feet high and three feet in diameter.  And a tiny pile like that one tends to heat only for a short time and then cool off rapidly.  Larger piles tend to heat much faster and remain hot long enough to allow significant decomposition to occur.  Most composters consider a four foot cube to be a minimum practical size.  Industrial or municipal composters build windrows up to ten feet at the base, seven feet high, and as long as they want.

However, even if you have unlimited material there is still a limit to the heap’s size and that limiting factor is air supply.  The bigger the compost pile the harder it becomes to get oxygen into the center.  Industrial composters may have power equipment that simultaneously turns and sprays water, mechanically oxygenating and remoistening a massive windrow every few days.  Even poorly-financed municipal composting systems have tractors with scoop loaders to turn their piles frequently.  At home the practical limit is probably a heap six or seven feet wide at the base, initially about five feet high (it will rapidly slump a foot or so once heating begins), and as long as one has material for.

Though we might like to make our compost piles so large that maintaining sufficient airflow becomes the major problem we face, the home composter rarely has enough materials on hand to build a huge heap all at once.  A single lawn mowing doesn’t supply that many clippings; my own kitchen compost bucket is larger and fills faster than anyone else’s I know of but still only amounts to a few gallons a week except during August when we’re making jam, canning vegetables, and juicing.  Garden weeds are collected a wheelbarrow at a time.  Leaves are seasonal.  In the East the annual vegetable garden clean-up happens after the fall frost.  So almost inevitably, you will be building a heap gradually.