A tarp and bivy bag is two objects doing what a tent does as one, and the choice turns on ventilation and pitching options rather than on grams alone. The weight saving is real, but it is not the reason experienced solo walkers pick one over the other.
What the tarp gives you
A tarp is the lightest roof there is, and what you buy is a shape and a set of anchor points. A rectangle is the most versatile and the noisiest in wind; a hexagonal or catenary cut tarp has curved edges sewn to pitch taut with fewer wrinkles, which is quieter and sheds better, at the cost of the flat-pitch options a rectangle keeps.
Count the tie-outs and look at how they are reinforced. Each one is a pitch you can improvise and each one is also a stress point. And read what is in the bag before you compare a tarp weight with a tent weight: many tarps are sold without guylines, pegs or poles, and several are designed to be pitched on trekking poles.
What the bivy has to handle
The specification everyone reads first on a bivy bag is the least likely to be the problem. Hydrostatic head, published in millimetres, describes resistance to water pushing in from outside, and essentially everything on sale keeps rain out.
What makes people damp is the moisture they produce themselves. The figure to weigh against the waterproofing is breathability — a moisture vapour transmission rate in grams per square metre per twenty-four hours, or a RET value — together with the test the maker names, because the scales are not interchangeable. Taped seams and a waterproof zip are the difference between a rated fabric and a waterproof bag.
Shape decides the rest. A flat bag packs smallest and lies against your face; a hooped version carries a short pole over the head end, adds weight and packed bulk, and buys the space that makes a long night bearable.
What a tent gives back
A tent is one object, pitches to a known shape every time, and gives you a vestibule — somewhere to leave boots, somewhere to sit out weather. It also gives you a groundsheet with a stated hydrostatic head, whereas a tarp-and-bivy setup relies on the bivy floor for that job, and the two figures are published differently.
Against it: a solo tent with its own poles is usually heavier than a tarp and bivy of similar quality, and it packs to a fixed length set by the poles.
Compare them honestly
The comparison is only fair when both sides are weighed the same way. Add up, for the tarp system: canopy, guylines, pegs, any pole you need, plus the bivy. Set that against the tent’s packed weight — the figure that counts pegs, guylines and stuff sack — rather than its trail weight, which counts only what stands it up. Makers publish one or the other and the gap runs to several hundred grams.
Then decide on the nights you actually have. A tarp and bivy suits dry, still, sheltered pitches and short trips; a tent earns its extra grams in wind, in insects, and on the morning you have to pack up in rain.
What people ask before buying
Is a bivy bag on its own enough?
It is a complete shelter in the sense that it keeps rain off, but there is nowhere to cook, nowhere to sit out a wet morning and no space to keep kit dry. Most people who bivy regularly carry a tarp as well, which is why the two are usually compared as a pair against a tent.
Why is my bivy bag damp inside when it did not rain?
A sleeping person produces moisture all night, and a bivy has very little volume for it to go anywhere. That is why breathability figures matter more than hydrostatic head here. Compare the moisture vapour transmission rate or RET value the maker publishes, along with the test it names.
Last reviewed 6 September 2026