For a flat surface, add your desired overhang to both ends and both sides. For a box-shaped object, add twice the object height as well. Round up only after calculating both minimum dimensions, and confirm whether the product listing gives cut size or finished size.
A tarp that is only “close” to the measured footprint often fails at the edges: tie-down points land too high, corners pull tight, or a small positioning error leaves part of the load exposed. Good sizing starts with the geometry of what you are covering, not with the nearest product name.
Sizing a flat cover
Measure the longest and widest points of the area. Decide how much tarp should extend past each end and each side. The allowance is applied twice because the tarp extends in two directions.
Flat-cover formula
Minimum length = object length + 2 × end allowance
Minimum width = object width + 2 × side allowance
Example: a 12 × 16 ft stack with 16 in at each end and 14 in on each side needs at least 14 ft 8 in × 18 ft 4 in. From the common sizes in our calculator, 16 × 20 ft is the first option that fits.
For a ground sheet beneath a tent, pool, or pile, the goal can be different: exposed tarp edges may collect water or create a trip point. In those cases, size the sheet to the footprint or fold excess material under rather than leaving a wide lip.
Sizing equipment and other 3D objects
Measure length and width at the widest points, not only at the base. Then measure the height from the highest point to the lowest level you want covered. If the tarp must reach the ground on both sides, add two full heights to each matching dimension.
Box-shaped object formula
Minimum length = object length + 2 × height + 2 × end allowance
Minimum width = object width + 2 × height + 2 × side allowance
This is conservative for a simple box and useful for equipment, stacked furniture, generators, and rectangular machinery. It does not model curved tops, handles, exhausts, sharp corners, or a tarp that stops halfway down. For irregular equipment, measure over the actual profile with a flexible tape or rope, then measure that rope on the ground.
Leave room for safe attachment
A few inches of exposed edge may be enough for dust protection indoors but not for an outdoor cover that needs to be secured below the object’s widest point. Plan where the tie-down force will act. Do not pull a tarp tight over sharp metal, hot parts, vents, or moving equipment.
Sizing a simple gable roof tarp
Do not use the flat building width as the tarp distance over a pitched roof. Measure or calculate the slope from ridge to eave on one side, double it, and add the desired eave overlap. Add gable-end overlap to the building length.
Our roof mode uses building span and pitch rise per 12 in of horizontal run to calculate the two slopes. It is intended for a symmetrical gable only. Hips, valleys, dormers, chimneys, multiple pitches, and damaged structures require a site-specific layout.
Roof work warning
Temporary roof covering involves fall hazards, wind loads, water routing, and possible structural damage. The calculator estimates fabric dimensions; it does not design an anchoring system or make a roof safe to access.
How to round up to a stock size
First compare both minimum dimensions. A 14 × 21 ft minimum does not fit a 16 × 20 ft tarp, even though the total areas are similar. Rotating a rectangular tarp can help only when both rotated dimensions clear the minimum.
| Common labeled size | Area | Useful scale |
|---|---|---|
| 8 × 10 ft | 80 sq ft | Small equipment or ground cover |
| 10 × 12 ft | 120 sq ft | Compact storage and projects |
| 12 × 16 ft | 192 sq ft | Medium stacks and equipment |
| 16 × 20 ft | 320 sq ft | Large equipment or roof sections |
| 20 × 30 ft | 600 sq ft | Large loads and broad temporary coverage |
Availability varies. More importantly, manufacturers may advertise a nominal cut size while the finished tarp is smaller after hems and seams. Look for “finished size” or an explicit tolerance in the product specification.
Common measuring mistakes
- Measuring only the footprint: the height or roof slope is missing.
- Adding allowance once: coverage is needed at both ends or both sides.
- Ignoring protrusions: handles, stacks, tires, and corners increase the real profile.
- Matching by area: the tarp has enough square feet but one dimension is too short.
- Assuming every labeled size is finished size: hems can reduce usable dimensions.
- Buying the largest possible tarp: unnecessary material is heavier, catches more wind, and is harder to secure without folds.
Before you buy
- Write down the minimum length and width separately.
- Check the product’s finished dimensions and tolerance.
- Confirm where reinforced corners and grommets will land.
- Check the estimated weight if one person must handle it.
- Choose material and reinforcement for abrasion, sun, temperature, and frequency of use.
- Plan a secure method that does not rely on overstretched grommets.
How this guide was prepared
We separate geometry and unit conversions from product claims, use exact standards where available, and flag specifications that vary by manufacturer. See our editorial policy for the review process.