I have a pretty steady access to about 4 pots of coffee worth of coffee grounds each day. I have heard coffee grounds are great for a compost pile, but I am wondering how much is too much? Is there a good ratio to maintain between coffee grounds and other compostable material?
If you add enough carbon, then you can't really put too many grounds in.
Coffee grounds are 20 carbon to 1 nitrogen or 20/1. A good ratio is 30/1 for aerobic composting, So lets say you use leaves to compost the grounds with. leaves are 50/1, so a mix of 55% coffee grounds to 45% leaves would be a good mix. Remember that this is by weight, so shredding the leaves will help make it easier to measure. Of course the condition of the material can change the ratio, but it is always safer to put in too much carbon than too much nitrogen.
Late last year (September or so) I started getting about 2 gallons a week of grounds. I now have a big pile of them in one compost bin, mostly collected since the weather turned cold.
On the one hand, I haven't seen them act "hot" at all, even though I haven't been mixing them with anything since November. I think this is just from a dry November making a dry pile, and cold weather since.
On the other hand, folks who've written on this from actual experience suggest mixing with leaves at 1 part grounds to 2 parts leaves (by volume), or no more than 25% of the pile (also here) (by volume, after some compaction?). I suspect that aeration may be the limiter - grounds are obviously a lot denser than most materials, so the pile is likely to be oxygen-starved if there are too many. I'm just guessing now, but I'll probably know in a year when my compost is done.