Optimized Fill — How Much Down an Underquilt Needs

Every down quilt has a number it needs to hit and a number it actually ships with. Almost nobody publishes either one. This page explains how to tell the difference — and why every Jacks R Better underquilt leaves the shop with more down in it than its temperature rating requires, at no extra charge.

The short version

• A quilt needs down in proportion to the internal volume its baffles enclose — not in proportion to its length, its weight, or its price.
• Volume ÷ fill power = the base fill rate: the ounces that specific quilt needs to reach its rating on the day it is sewn.
• Most quilts ship at the base fill rate. The extra down — the industry calls it overstuff or overfill — is sold back to you at $10 to $15 per ounce.
• That extra down does not make the quilt warmer out of the bag. It does two other things: it keeps the down evenly distributed — down that migrates to one side of a chamber leaves the other side thin, and a thin chamber is a cold spot — and it keeps the quilt at its rating in year five.
• We include it as standard: 36% above base fill rate in our differential-baffle quilts, 25% in the box-baffle Nests. One price, no dropdown.

Why "overstuff" is the wrong word

The word only works if you accept the premise buried underneath it: that a quilt's standard fill should be the bare minimum that hits the number on the day it ships, and that anything past that is a luxury you pay extra for.

We don't accept that premise. A stock quilt should perform at its rating out of the box and still perform at its rating in its fifth season, with nothing added and nothing upsold. That isn't a deluxe option. That's the job.

So we build every quilt to the fill it should have had in the first place and put one price on it. We use the industry's word on this page only because you need it to run the comparison — it's the label on their dropdown, not ours. On our bench it isn't overstuff. It's just the fill.

How to compare two down quilts honestly

Fill weight on its own means nothing, and any shop quoting it without dimensions is quoting a number you can't use. Ten ounces in a narrow 3/4-length quilt is a completely different quilt from ten ounces spread across a full-length shell. Same number, opposite outcome.

Here is the sequence that actually works. It takes about two minutes per quilt.

1. Start with the geometry

Width, length, baffle wall height, and how many chambers. Multiply them out and you have internal volume — the space that has to be filled with lofted down.

Our Mt Washington 3 is 42″ wide and 77″ long, in 11 chambers each 7″ long at 2″ of baffle wall height. That is 588 cubic inches per chamber and 6,468 cubic inches of internal volume. Get the same figure for anything you are comparing, or you are not comparing anything.

Baffle wall height is the number most often left out, and it matters more than any other. Our Mt Washington 4 has the same 42″ × 77″ footprint as the Mt Washington 3 and the same 11 chambers — but 3″ walls instead of 2″. That single inch takes internal volume from 6,468 to 9,702 cubic inches, and it is the entire reason one is a 20°F quilt and the other is a 0°F quilt. Same shell, same down, same construction.

2. Divide by fill power

Fill power is the number of cubic inches one ounce of down fills when fully lofted. Volume ÷ fill power = the base fill rate — the ounces that quilt needs to hit its rating.

For the Mt Washington 3: 6,468 ÷ 850 = 7.6 oz.

Higher fill power means fewer ounces for the same volume. That is where legitimate weight savings come from — not from putting less down in the quilt.

3. Then ask how far above the base fill rate it is actually stuffed

This is the one number that compares across quilts of different sizes, because it is a ratio rather than a raw weight. It is also the number that tells you how long the quilt will hold the rating printed on its label.

Ours is +36% on every differential-baffle quilt we build. Most quilts ship at 0% over as standard.

If a maker will not give you chamber count, baffle wall height and base fill rate, you cannot complete this exercise — and that is worth knowing on its own.

What the extra down actually buys you — and what it doesn't

It does not make the quilt loftier. We'll be straight about that, because plenty of marketing implies otherwise. A chamber filled to exactly its base fill rate reaches the same loft on day one as a chamber filled 36% over. Lay two quilts side by side out of the bag and you would not see a difference.

The difference is everything that happens after that.

A fuller chamber keeps the down evenly distributed

Even distribution is what a temperature rating actually depends on, and extra fill is what holds it. A chamber filled only to its base rate has slack in it. Every night you shift in the hammock, every time the quilt gets stuffed and unstuffed, the down migrates — drifting to one side of the chamber and leaving the other side thin. A thin chamber is a cold spot, and that is where most of them come from — it is why people describe a quilt as one that "used to be warmer." Packing a chamber fuller leaves the down far less room to travel, so it stays distributed the way it was sewn to be.

Down loses loft as it ages

Body oils, trail dust and humidity collect in the plumules over time, and loft falls off years before a quilt looks worn out. A quilt filled to exactly its rating has no margin for that; it starts sleeping colder than its label within a couple of seasons.

The extra fill is that margin. It is the reason we will still call a 20°F quilt a 20°F quilt in its fifth winter.

So you are not buying a warmer quilt than the number on the label says. You are buying a quilt that is still that warm once you have actually used it.

Why 36% on some quilts and 25% on others

Because they are not the same quilt underneath, and there is no single correct fill percentage that applies to every construction.

Our Dual Differential Contoured Radial baffles — the construction we invented in 2004 — curve to follow the hanging shape of a loaded hammock. The inner shell is cut smaller than the outer shell so the quilt conforms to that curve without the two shells pulling together at the edges and compressing the down. That geometry behaves differently under load, and it needs 36% to do its job properly.

The Nest models are built with box baffles, shaped specifically around the bottom entry of a Hennessy hammock. A box baffle is fully and correctly filled at 25%.

The smaller number is not a smaller effort or a cheaper build. It is the figure that construction calls for. We would rather publish two different percentages and tell you why than pick one number, print it across the whole catalogue, and hope nobody asks.

The numbers for every JRB underquilt

Every figure below is published on the product page it links to, alongside the chamber count and baffle wall height the calculation depends on.

All six use 850 fill power RDS-certified down behind a 1.1 oz taffeta shell with a DWR coating, and all six are sewn in Pennsylvania.

What it costs to match this elsewhere

Take the Mt Washington 4 as the example, because it carries the most down. Its base fill rate is 11.4 oz and it ships with 15.52 oz — a difference of 4.1 oz.

At the $10 to $15 per ounce that major US quilt makers publish for overstuff on their standard catalogue quilts, matching that fill costs $41 to $62 on top of their listed price. And because they hold no stock, you wait six to eight weeks either way — the overstuff just costs more on top of the wait.

We are not naming names. Go price one up and check the arithmetic yourself. That is the entire point of this page.

Per-ounce upgrade rates reflect published figures as of August 2026.

The one thing to take away

Ask any quilt maker for three numbers: internal volume, base fill rate, and how far above it the quilt is actually filled. Ours are printed on every product page. If you cannot get those three numbers from someone selling you a down quilt, the fill weight they are quoting you is not information — it is decoration.

Still deciding which underquilt you need? Match your coldest night with the Hammock Underquilt Temperature Guide, compare the range in the Best Hammock Underquilts guide, or browse all six in Hammock Underquilts.