Overstuff is a percentage of fill added over the amount needed to just barely fill a volume. It is added density. So if you have a chamber that is 210ci and you fill it with only enough fill to fill 210ci, that is 0% overstuff. 30% overstuff would be 210 x 1.3. A mistake often made is that 150% overstuff would be 210 x 1.5, but that is actually 50% overstuff. 210 is “1” in the formula. 150% overstuff would be 210 x 2.5.
If you fill a space with the exact amount of down needed based on it’s fill power rating, it will only fill that space well if conditions are perfect. It has to be distributed just right with no moisture and like new loft. As we know, conditions out on the trail are often not perfect. Therefore it is best to have a at least a little overstuff so that if a space opens up inside your sleep system there is a little extra to push into that empty void. A bag with no overstuff will tend to clump up and shift around, leaving big cold spots. I go with 160% overstuff over the baffle height to create very dense chambers. This keeps all the space filled with down and limits shifting in the night.
Working hand in hand with overstuff is chamber size. A very large chamber contains a higher weight of fill. The higher the weight of total fill, the more potential it has to move and compress other fill. If you split the fill up into smaller chambers, each one has a lower weight and therefor doesn’t have the mass to move and compress the rest. There is a sweet spot of overstuff and reasonable chamber size that can sometimes be a challenge to find. If the chamber is really big, you’ll need a lot of overstuff to keep it from moving. Making tons of really tiny chambers is very labor intensive and adds weight from all the baffles.
Given the definition, overstuff is not a term that really applies to sewn through construction. A sewn through chamber starts out as a flat, two dimensional area. There is no base volume, so technically, by the definition, all the fill is overstuff, but really the term doesn’t apply. With box construction you have to fill a volume first, and then add overstuff density. With sewn through your filling and increasing density all in one go.