Tarp shelter calculator

A tarp is the lightest shelter you can carry, but how much it protects depends on the pitch. Raise the ridgeline and you gain headroom but lose ground coverage; lower it and you stay dry in wind-driven rain but crawl in. This calculator shows the trade-off for the two most common pitches.

The geometry

In an A-frame, the ridgeline runs along the middle of the tarp and each half slopes to the ground. Each half is a right triangle whose sloping side is half the tarp width, so:

• Ground distance from the center = √((width ÷ 2)² − ridge height²)
• Covered width = 2 × that distance
• Roof angle = arcsin(ridge height ÷ (width ÷ 2))

In a lean-to, one edge of the tarp is on the ridgeline and the other on the ground, so the sloping side is the full width.

Example

A 10 × 10 ft tarp pitched as an A-frame with the ridgeline at 4 ft covers a strip 6 ft wide, so 60 sq ft, with a 53° roof: two people side by side. Drop the ridgeline to 3 ft and the covered width grows to 8 ft, with a flatter 37° roof that sheds rain less well.

Choosing the pitch

A steep roof of 45° or more sheds rain and snow better. In strong wind, pitch low with the narrow end into the wind.

Ridgeline and guylines

Choose trees a little further apart than the tarp length, so the ridgeline can be tensioned without the tarp touching the trees. Allow about 3 ft (1 m) of extra cord per tree for wraps and knots, and bring guylines and stakes for each corner and side tie-out. A taut-line hitch or a prusik makes the pitch easy to adjust.

Size guide

Safety

Check above for dead branches before pitching between trees, avoid hollows where water collects, and leave a gap at the ridge for ventilation to limit condensation.