To get from cubic feet from cubic yards divide by 9 so 3.5 cubic feet is about .39 cubic yards. How much that will cover I don't know

I'm sorry the conversion I gave you was from sqft to sqyd. To go from cubic ft to cubic yards divide by 27 so 3.5 cubic ft is about .13 cubic yards.

I think 24 bales will give you a 1/4 inch covering over 4000 square feet.

There are on-line calculators for dirt, gravel, sand etc. Assuming that dirt and peat are the closest in density.

Using an on-line "planting fill dirt calculator" I found that...to cover 4000 sqft .25 inches deep you need 3.34 cubic yards of material. So...

3.34 cubic yards is 90.18 cubic feet of material (3.34 X 27)

If your bales of peat moss are 3.5 cuft then you will need to buy 26 bales. (90.18 / 3.5 = 25.76)

If you want to cover it .50 inches deep you will need twice as much, conversely if you want to cover it .125 inches you will need only half as much.

However...

It could be that you would need less material as I believe that the weight/density of peat is less that fill dirt.

Are you going to mix the peat 50/50 with sand as top dressing? Or just use the peat with out the sand?

Yes, coverage at 1/4 inch. 24 to 26 bales sounds like my average response. Thanx so much for your help.

I'm advising her to use compost or peat moss as a topdressing but w/o sand.

Is there any way for her to buy in bulk? It might be a lot less expensive (even delivered) that way.

Thanx so much for reminding me, BP. I forgot I had a load delivered before.