About a third of a bale I,d say.
We wrapped ours in some old net curtain.
The bale was very cheap from a local farm shop.
I understand that as it breaks down over time it emits enzimes that inhibit the growth of algae, but we find the koi eat the algae now they are bigger

Barley straw bales round here are only moveable by machine, they are 2 feet by 18 inches by about 4 feet, the size of 4 of the old bales one used to see stacked up in barns. A third of one of them would cover about 1/5 of the pond.

takes up to 6 weeks but one yr I forgot and the pond cleared in 5 weeks any way. So don't think it works personally.

Well, last year the pond never cleared at all, so since I have Barely straw I was just going to give it a try.

Or Barley straw even!

Those bales are enormous, the one we had was the old haybale size thatI could just about lift onto a wheel barrow
Our pond didn't clear last year either but did the latter part of the winter, its greening up a bit now, the koi seem to be grazing on the walls and not asking for food yet