A pack rain cover is a waterproof shell that stretches over the outside of your backpack, like a shower cap for your pack, with an elasticated edge and sometimes a drawcord. It keeps rain off the main body of the pack and stops the fabric from soaking up water and gaining weight. A pack liner is a waterproof bag that goes inside your pack, and you place your gear inside the liner, rolling or sealing the top closed so water cannot reach anything even if the pack itself gets soaked.

The difference is where the waterproof barrier sits. A cover defends from the outside but leaves gaps, while a liner defends from the inside and has no gaps at all. That single design difference explains why experienced hikers so often prefer one over the other, and why many carry both in different situations.

Rain covers have a well-known weak spot: the back panel. The straps, hip belt, and the part of the pack that sits against your back are not covered, and rain runs down your back and shoulders straight into that gap. Wind makes it worse, blowing rain sideways under the cover and soaking the pack from angles the cover was never designed to block. A cover can also slip, snag on branches, or fill with water and sag, especially on a long wet day.

Covers are also fiddly in practice. You have to stop, take your pack off, fit the cover, and put the pack back on, which most hikers only bother to do once the rain is already heavy. By then, the pack fabric is wet, and a wet pack keeps wicking moisture inward for hours after the rain stops. A cover is better than nothing, and it does keep the outside of the pack cleaner and lighter, but as a sole defence it regularly disappoints in sustained rain.

A pack liner has no gaps to exploit. Once the top is rolled shut, water would have to get through the liner itself to reach your gear, and a good liner made from thick plastic or waterproof fabric does not let that happen, even if you drop your pack in a stream. Liners also protect against internal leaks, like a water bottle coming loose or a damp tent packed wet, which a rain cover cannot help with at all.

Liners are lighter than most rain covers, cheaper, and simpler: a heavy-duty plastic bag or a purpose-made dry bag liner does the job, and there is nothing to fit or adjust in the rain. You pack your gear inside the liner in the morning, seal it, and forget about it for the rest of the day. The main downside is that the pack fabric itself still gets wet and heavy, and a soaked pack takes a long time to dry in camp, which is where a rain cover can still help as a second layer.

A purpose-made pack liner weighs roughly the same as a small snack bar, while a rain cover is often a little heavier and bulkier, though both are light by backpacking standards. Cost is similarly low for both, and a simple plastic liner can cost almost nothing, which makes it an easy experiment: try a liner on your next wet hike and compare how dry your gear stays versus your previous rain-cover trips.

Durability differs. Plastic liners can puncture if you pack sharp items carelessly, so pack them thoughtfully and carry a spare on longer trips. Fabric dry-bag liners last longer and seal more reliably, but cost more. Rain covers, meanwhile, can tear on branches and lose their waterproof coating over time, so check yours before a trip rather than discovering a leak mid-storm.

If you must choose one, choose the liner. It is the only option that reliably keeps sleeping bags, clothing, and electronics dry in a proper downpour, and it works without any fiddling in the rain. If you hike often in wet climates, add a rain cover as well: the liner protects your gear, and the cover keeps the pack itself lighter and drier, which makes camp more pleasant. Together they cover each other's weaknesses, but alone, the humble liner does the job that actually matters.