Fill Power Explained: What Makes a Quilt Warm (and Light)

Sep 20

3 min read

If you’ve ever shopped for a high-end backpacking quilt, you’ve probably seen a number like 900FP or 900 Cuin. That’s your fill power, in short, a key measure of how warm, light, and compressible your down is.

Let's dig deeper!

What Is Fill Power?

Down and other insulators function by trapping air between the down fibers. Fill power is a measurement that indicates the volume occupied by 1 ounce (27 g) of down in cubic inches (cuin - cubic inches per ounce). To simplify, how much down you need to fill up a space depending on the quality.

A higher fill power means more loft per gram, which results in more trapped air and, consequently, greater warmth per gram.

Important: Fill power alone does not determine warmth. It is also essential to consider the amount of down present in your quilt.

Fill Power Ranges

Fill Power	Typical Use
400–550FP	Budget quilts, heavier & less compressible
600–700FP	Mid-range, moderate warmth and loft. A normal range in outdoor jackets or puffers.
750–850FP	High-end three-season gear
900FP+	Ultralight premium quilts and sleeping bags — maximum warmth per ounce (what we use!)

So to conclude, fill power means lighter, smaller, and more efficient quilts.

Fill Power Is a Range — Not an Exact Number

Fill power is often presented as a single number, but in reality it’s a measurement of a natural material, which means there’s always some variation. When down is tested in the lab, the result represents an average loft measurement from a sample, and batches can contain clusters that loft slightly more or slightly less than that number. Because of this, very high ratings like 950FP or 1000FP should be taken with a bit of context. In some cases these numbers reflect peak laboratory measurements rather than the typical performance of the entire batch.

What ultimately matters more than chasing the highest number is consistent, high-quality down that reliably meets its rating which is always our goal at Vilse. It's always a good idea to read up on some reviews, trusted forums or reddit comments if a manufacturer really lives up to their promises in terms of weight and warmth.

Overfill

While, fill power is a great measurement for down quality, it's based on perfect lab conditions. In real life the down will move a around inside the baffels which can create cold spots, areas where down is missing. Which means warm air will go right through and get you cold. To combat this, a lot of high end manufacturers use "overfill", which is simply adding more down than technically needed to fill the baffles. All Vilse bags comes with a standard 30% overfill.

In our bag customization tool you can choose to add even more overfill. This will be on top of the already 30% to further increase warmth.

However...

The more down you add without increasing the volume of the baffles that store it, the less efficient per weight it gets. Remember, it's the air between the fibers that keeps you warm, not the down itself. Stuffing it with too much down just gives you diminishing returns. For example, ordering a -1°C bag with 60 g overfill will give you a comfort rating of about -4°C but will have almost the same weight/amount of down as a -6°C bag.

RDS Certification: Ethical & Reliable

All the down we use at Vilse is RDS-certified, which stands for "Responsible down standard" guaranteeing:

Ethically sourced geese
No live-plucking and humane treatment of birds.

This ensures high-quality, consistent, and responsible down you can trust.

TL;DR

Fill power = loft per ounce. Higher = lighter, warmer for its weight, more compressible.
900FP goose down = premium, ultralight warmth.
Overfilled baffles prevent cold spots.
RDS-certified = ethical, traceable, consistent quality.
Warmth = fill power × fill weight x fit x loft, not fill power alone.

With 900FP RDS-certified Polish down, Vilse quilts give you maximum warmth, minimum weight, and ethical sourcing — everything you need for ultralight adventures.