Osprey Ultralight Zipper Sack Review

Overview

The Ultralight Zipper Sack is a thin nylon pouch with a full-width zipper and a small D-ring, built to corral the loose stuff that migrates to the bottom of a pack: socks, cables, a first-aid kit, a headlamp. It weighs 17 g in the Small and 21 g in the Medium, so the penalty for organization is roughly the weight of a few sheets of paper. The trade-off is that it is a container, not a shield. It organizes gear but does little to keep water out.

Key Specs

Spec	Details
Weight	17–21 g depending on size
Volume	2 L (Small) to 5 L (Medium)
Dimensions	Small: about 6 x 7.6 in (15 x 19 cm); Medium: 20.5 x 25 x 20.5 cm (8 x 9.84 x 8 in)
Main fabric	40D recycled high-tenacity nylon
Fabric credentials	bluesign approved, GRS-certified 100% recycled main body fabrics
Water resistance	PFAS-free durable water-repellent finish (not waterproof)
Closure	Zippered opening
Attachment	D-ring for hanging
Warranty	Limited lifetime

Performance

Weight and bulk. At 21 g for 5 liters, the Medium works out to about 4 g per liter of capacity. The Small is less efficient by that measure, about 8.5 g per liter, but at 17 g it is still a rounding error in a base weight. The nylon is thin enough that an empty sack folds down to something close to a folded receipt, which matters for hikers who carry one only on resupply-heavy sections.

Access. The zipper is the reason to pick this over a drawcord stuff sack. A zipper opens wide, so contents are visible at a glance, and a hand can reach the bottom without pulling everything out. For small, easily lost items (a repair kit, a charging brick and cable, toiletries), that matters more than shaving a gram. Cinching a drawcord sack with cold fingers is a small annoyance. A zipper on a quarter-full pouch is easier.

Fabric and weather. The 40D recycled nylon is a step up from the gossamer fabrics on the lightest pouches, and Osprey finishes it with a PFAS-free water-repellent treatment. That handles a damp pack bottom or a brief shower, but a zipper is a seam, and nobody should expect this sack to survive a river crossing or hold up through a day of steady rain with a sodden pack. Hikers who need dry electronics or a dry sleeping layer still want a roll-top dry bag or a pack liner. This sack is for sorting, not sealing.

Durability. Owners of earlier generations of Osprey’s zip sacks praised the weight and the way the pouches tidied suitcases and packs, while wondering how long the zippers would hold up. That is the right question for any zippered ultralight pouch: the zipper is the failure point, not the fabric. Osprey backs the sacks with its limited lifetime warranty, which takes some of the sting out of a worn slider. In practice, the way to extend their life is to avoid overstuffing; a sack packed to its drum-tight limit strains the zipper at the corners.

The D-ring. It’s a small convenience. It lets the sack hang from a tent loop or clip to a pack for use as a camp-kitchen or toiletry caddy, but the ring is not a load-bearing feature, and it adds a little hardware to an otherwise featureless pouch.

Materials. The bluesign approval and GRS-certified recycled content are real selling points for hikers who weigh environmental impact alongside ounces, and the fabric doesn’t pay a visible performance penalty for them.

Choosing a Version

The two sizes differ by only 4 g, so weight shouldn’t drive the decision. Volume should.

• Small (17 g, about 2 L): Sized for a single category of small items: electronics and cables, a med kit, toiletries, or a pocket-sized repair kit. It holds a phone, battery bank and cables with room left over, and is the better pick for someone who wants several color-coded or labeled pouches rather than one catch-all.
• Medium (21 g, 5 L): The more flexible size. It swallows a clothing layer such as socks and base layers, a day’s worth of snacks, or a full kitchen kit with stove, lighter and spoon. Hikers who want one sack as a general-purpose “small stuff” bin should start here.

A reasonable pairing for a long trail is one of each: the Small for electronics and first aid, the Medium for a clothing group or food bag liner.

Pros & Cons

Pros

• 17–21 g, with almost no weight difference between sizes
• Full zipper opening gives easier access than a drawcord sack
• 40D recycled nylon is sturdier than the thinnest ultralight pouch fabrics
• PFAS-free water-repellent finish, bluesign approved and GRS-certified recycled fabrics
• D-ring allows hanging in camp
• Limited lifetime warranty

Cons

• Not waterproof; the zipper is an entry point for water
• The zipper is the likely point of wear on an overstuffed sack
• Heavier per liter than a basic drawcord stuff sack, especially in the Small
• No internal pockets, dividers, or handle beyond the D-ring

Who Should Buy This

Weekend and thru-hikers who run a tidy pack and want fast access to small items without rummaging, along with travelers who use the same sacks in a carry-on, will get the most from it. It pairs well with a pack liner, which handles the weather side while the sacks handle order. Hikers who expect it to keep gear dry, or who count every gram and already sort their gear with drawcord sacks or zip bags, should look elsewhere: a roll-top dry bag for the first group, and a bundle of freezer bags for the second.

Verdict

The Ultralight Zipper Sack does one job, keeping small things findable, at a weight few hikers will notice. It earns a place in a pack for anyone who values access over waterproofing, with the Medium as the better single purchase and the Small as the natural companion. Keep it away from anything that must stay dry, and don’t stuff it to the seams.