What size tarp do you need for camping?

Tarp size comes down to two separate choices. The width across the ridge sets the floor width and how much room you have beside your pad. The length along the ridge sets how far the open ends are from your head and feet. Choose the width for the space you need beside your body, then choose the length from the pad length plus the margin you want at each open end.

Width and length answer different questions

In an A-frame pitch, the width (W) runs across the ridge and the length (L) runs along it. The ridge height (h) decides how much of the width ends up as floor. Changing the length changes only the floor area and the fabric area. A 3 m wide tarp gives the same floor width and wall angle whether it is 3 m or 4 m long. Changing the width changes the floor width and the wall angle together, so settle the width first.

The A-frame tarp geometry guide sets out the equations and the headroom strip under the ridge. This guide applies the same model to five sizes at one ridge height, so the rows can be read against each other.

Floor width and wall angle for five sizes

The table fixes the ridge at 0.8 m (80 cm) for every row. That height is below half the width of the narrowest tarp, which is 1 m for the 2 m width, so one ridge height works for all five. Floor area is the floor width multiplied by the length along the ridge. Fabric area is the width multiplied by the length.

The arithmetic behind the table

Every row uses the same steps. Half the width is the sloping edge from the ridge to the ground, and the ridge height is one short side of a right-angled triangle. For a 3 m tarp at 0.8 m, the working is:

half width  = 3 ÷ 2 = 1.5 m
half floor  = √(1.5² − 0.8²) = √(2.25 − 0.64) = √1.61 = 1.269 m
floor width = 2 × 1.269 = 2.54 m
wall angle  = asin(0.8 ÷ 1.5) = 32.2°
floor area  = 2.54 × 3 = 7.61 m²
fabric area = 3 × 3 = 9.00 m²

For the 2 m tarp, half the width is 1 m, so the floor is 2 × √(1 − 0.64) = 2 × 0.6 = 1.20 m. Turning the 3 × 4 m tarp so that its 4 m side runs across the ridge changes more than the length. The 4 × 3 m row has the same 12 m² of fabric as the 3 × 4 m row. Its floor is 1.13 m wider (3.67 − 2.54 = 1.13 m), its walls are 8.6° shallower (32.2 − 23.6 = 8.6°), and its floor area is 0.85 m² larger (11.00 − 10.15 = 0.85 m²).

Sleeping length sets the length along the ridge

Start from the sleeping pad, not from a size on a product listing. Measure the pad's length, and the width you need at the shoulders. This guide uses a pad 1.83 m long and 0.5 m wide as an explicit example input. Those figures are worked numbers, not a standard pad, so replace them with your own measurements.

The length along the ridge should be longer than the pad, so that each open end sits some distance from the sleeper. The spare length is the tarp length minus the pad length. Splitting it evenly puts the pad in the middle. A 3 m long tarp leaves 3.00 − 1.83 = 1.17 m spare, so 1.17 ÷ 2 = 0.585 m, about 0.6 m at each end. A 4 m tarp leaves 2.17 m, or about 1.1 m at each end.

spare length       = 4.00 − 1.83 = 2.17 m
each open end      = 2.17 ÷ 2 = 1.085 m, about 1.1 m
floor width        = 2.54 m (table row, 0.8 m ridge)
ground floor beside the pad = 2.54 − 0.50 = 2.04 m, 1.02 m each side if centred

The last line is the floor at ground level. Because the walls slope, the room above the pad is less than that, and the A-frame geometry guide covers how to measure it.

Fabric area and weight

A 3 × 4 m tarp has 12.00 m² of fabric, and a 3 × 3 m tarp has 9.00 m². The ratio is 12 ÷ 9 = 1.33, so the longer piece has a third more fabric. For the same material, weight scales with area, so the longer piece weighs about a third more before ropes, stakes and seams are added. That is arithmetic only. The mass per square metre depends on the fabric, so use the figure the maker gives for your tarp.

Fabric weight is one line of a pack total. For how a shelter fits into the total, see base weight explained.

Choosing between a 3 m and a 4 m length

Four checks settle most of the choice, and each one uses numbers from the table.

• The sleeping length plus the margin you want at each open end. A 3 m tarp gives a 1.83 m pad about 0.6 m of spare length at each end.
• The ground you have. The footprint is floor width × length, which is 7.61 m² for 3 × 3 m and 10.15 m² for 3 × 4 m at 0.8 m. A campsite with a narrow flat strip may take one size and not the other.
• The fabric you will carry, from the area ratio above.
• Whether anyone will sit up or move crosswise. That needs floor width more than length.

When a wider tarp is the better answer

A wider tarp at the same ridge height gives more floor and shallower walls. The turned 4 × 3 m tarp in the table is that case. The space is wider and shorter, which suits a pad that sits crosswise to the ridge or a pair of people who want a wider working area under the tarp. The turned 4 × 3 m setting in Shelter studio shows the same ridge height with the sides swapped, so you can compare the two pitches on screen.

What these numbers leave out

The model uses the width, length and ridge height you enter. It does not estimate fabric weight, sag, wind loading, condensation or comfort, and it has no door, beak or catenary cut. A tarp that fits a pad on paper can need a different ridge height once the guylines are tied and the fabric sags. Re-measure after a test pitch.

Checks before you buy or cut

Write down the pad length, the width you need at the shoulders and the ridge height you expect to use. Run those values in the Shelter studio, check the floor width against the pad, and check the line allowance against the ties you plan. The ridge height also sets the wall angle, which the A-frame geometry guide explains. For the lines that hold the shape, see tarp guyline length, and for the pitch in wind and rain, see pitching a tarp for wind.