Compressing sleeping bag

I am considering purchase of a new down sleeping bag to lighten my pack and facilitate use of a smaller pack as well. However, it is difficult to compare different sleeping bags without knowing how much they can be compressed within a compression sack. I believe most manufacturers list a "pack size" but this is the size of the stuff sack they provide and not a sack with compression straps.
How far can I compress a down sleeping bag? I am looking specifically at the Feathered Friends Swallow 20 degree regular bag. This bag comes with a 9L sack but if anyone knows how small this bag can be compressed I would greatly appreciate it.

I’ve never really worried about how small I can compress a down bag. (A 25 degree bag with 800-fill power will, I think, compress to about the size of a softball or one-liter bottle?) However, if I use a compressor sack to get to that size, I’m left with a very stiff, unyielding item that can be difficult to pack around. As a result, I’m left with little empty spaces and an overall load that isn’t any more compact. (Those empty spaces undo all the space saving of compressing the bag - and compression probably doesn’t do the down fill any good.)

I tend to put my sleeping bag in the bottom of the bag, uncompressed. As I add gear, it compresses the bag more naturally, filling all the voids, and I can shift the shape as needed to accommodate long or bulky items. Until recently, I didn’t use a stuff sack (but was very careful about not setting my pack in puddles or mud.) Recently, I’ve begun using a 40 liter stuff sack that doubles as a pump sack for my sleeping mat. This works about the same as putting the bag in loose.

One alternative to using a compression sack is to stuff your jacket, assuming you carry a down or synthetic jacket, in with the sleeping bag. I have a WM Ultralite (very similar to the Swallow) and the regular stuff sack will accommodate a light down jacket without too much trouble, though it's tight. This works for me because I normally only use the down jacket in the mornings or evenings when I already have the bag unpacked.

I've read that a compression sack is capable of compressing the bag to the point of damage, which is obviously to be avoided with a $500 bag.

My WM Ultralite, size small, stuffs down to less than the size of a soccer ball. However, as Glenn points out, you really don't want to do that, because the result will be as hard as a soccer ball.

After some years of stuffing my sleeping bag into a dry bag, resulting in a small but hard, unyielding sausage, I went back to using a waterproof pack liner and packing as Glenn recommends. My pack liner is the "schnozzle" that came with my Exped UL7 Downmat. It's so much easier to pack!

You really don't want to add the extra weight of a compression sack, which will offset part of the weight savings of the sleeping bag. It's not necessary.

Also, when at home, be sure to fluff up the bag and store it loose--don't leave it compressed for long periods. I manually fluff mine and then give it a short session in the clothes dryer at low temp before storing it.

Storing it loose is good advice, as is drying it after use.

Down is remarkably resilient, though. I have a Moonstone bag that was stored stuffed for well over a year and within an hour of unstuffing was back to full loft (I have too many bags and not enough space.) This leads me to believe that it's probably OK to store a bag partially compressed, that is, in a bag maybe three or four times the size of the stuff sack. I keep one of those "Moisture Grabbers" (desiccant bags) in with them, too.

Thanks for the feedback everyone. I'm going to try stuffing my existing down bag into my pack without a stuff sack but I still want to figure out how much smaller volume I can get with a new ultralite bag in a compression sack. Does anyone have a photo and/or dimensions of their WM Ultralite or FF Swallow or Hummingbird in a tight compression sack?
The Western Mountaineering website FAQs recommends "a compression sack that has a compressed volume of half the included stuff sack" and also states "though there are no disadvantages to temporarily compressing your bag".

Hi Jellyman...I have just done this with my sleeping bag. I bought a 5L ultralight compression bag and put all my sleeping kit inside it, namely, my ultralight (700 grams) Vango Venom 200 two-season down mummy style sleeping bag, my Klymit Intertia X-Frame Recon ultralight inflatable sleeping mat, my Klymit Pillow X Recon inflatable pillow and a folded space blanket. Compressed down, the dimensions came to 25cm x 17cm (about 9 3/4" x 6 3/4") and the total weight is about 1.1 kg. (about 38.8 oz). The compression bag only cost me £5 ($7.10), on ebay, with free postage. I might put my thermal top, thermal leggings and a warm pair of socks in there too as I plan to wear them inside the bag...Hopefully this will not increase the dimensions too much when compressed. Update: I added the thermals and socks and when compressed down the length only increased to about 32cm, with the diameter staying the same. The total weight has gone up to just over 1.5kg (about 53 oz).

Down bags will loose their loft if you compress them. I just use the stuff sack it came with. I use a 38 liter pack and can get all my gear and 5 to 7 days worth of in it. I have quilts, an EE rev 20 and hammockgear burrow 0 and dont have any issue carrying it in a small pack. Maybe you should look at the other stuff you are carrying. Maybe ditch some stuff and get smaller gear

If you need to choose a really good sleeping bag then you need to look for articles with reviews of different sleeping bags from professional tourists. I usually find a lot of useful information, full characteristics and an honest opinion on the Internet on portals about hiking and camping. It really helps to choose the best option for me.