For a flat surface, add overhang to both ends and both sides. For a box-shaped object, add two height drops as well. Round up only after both minimum dimensions clear, and confirm whether the listing is cut size or finished size. Boats, cars, hay, roofs, and pools each break the simple box in a different way; use the child guide for that job.
A tarp that is only “close” to the footprint fails at the edges: tie-downs land too high, corners pull tight, or a small placement error leaves the load exposed. Geometry first, product name second.
Pick the job first
The three formulas on this page cover a lot of rectangular work. They are the wrong starting point for a gable roof, a hull, a car with mirrors, a hay crown, or a pool anchor line. Use this table to jump to the guide that already did that measuring, then come back here for stock-size round-up.
| Job | What to measure | Guide | Tool |
|---|---|---|---|
| Flat stack, ground sheet | Length, width, overhang | This page | Size calculator, Flat Cover |
| Roof | Sloped path over both faces, not the floor span | Roof tarp size guide | Size calculator, Roof Area |
| Equipment / machinery | Stored profile, four drops, chafe points | Equipment tarp size guide | Size calculator, 3D Object; Plan Builder |
| Boat | Centerline, beam, drop below the gunwale | Boat tarp size guide | Size calculator, then add windshield path by hand |
| Car | Length, mirror width, cover-line height | Car tarp size guide | Size calculator, 3D Object |
| Hay / straw | Top footprint or crown path; sides stay open | Hay tarp size guide | Size calculator, Hay Stack |
| Pool | Waterline to the real anchor line | Pool tarp size guide | Pool cover calculator |
| Camping | Pitch geometry, not a flat ground sheet | Camping tarp size guide | Camping configurator |
| Container | ISO envelope as a check, then the real unit | Container tarp size guide | Size calculator, Flat or 3D |
If you would rather not retype the same numbers in four tools, the Tarp Plan Builder runs size, fabric, weight, and a planning price from one measurement set. It currently speaks equipment and firewood/hay-style top covers; the job guides above still own the odd geometries.
Sizing a flat cover
Measure the longest and widest points. Decide how far the tarp should extend past each end and each side. That allowance is applied twice because the sheet 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 the calculator, 16 × 20 ft is the first option that fits.
For a ground sheet under a tent, pool, or pile, exposed edges can collect water or trip people. Size to the footprint or fold the extra under. Hay and firewood are flat covers with a different rule: cover the top, leave the sides open. That is in the hay and firewood guides, not in this formula.
Sizing equipment and other 3D objects
Measure length and width at the widest points, not only at the base. Measure height from the highest point to the lowest level you want covered. If the tarp must reach that line on both sides, add two full drops 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 crate, generator, or rectangular machine. It does not model a hull, a car with mirrors, a windshield, or a tarp that stops halfway down. For those, use the boat, car, or equipment guide and measure the real path.
Plan where the tie-down force will act. A few inches of exposed edge may be enough indoors. Outdoors the cover has to reach below the widest point. 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 distance over a pitched roof. Measure or calculate the slope from ridge to eave on one side, double it, and add eave overlap. Add gable-end overlap to the building length. The full method, safety notes, and a worked example are in the roof tarp size guide.
The calculator’s roof mode uses building span and pitch rise per 12 in of run for a symmetrical gable only. Hips, valleys, dormers, chimneys, and damaged structures need a site layout. Temporary roof covering involves fall hazards, wind loads, and water routing. The calculator estimates fabric; it does not design anchors or make a roof safe to walk on.
How to round up to a stock size
Compare both minimum dimensions. A 14 × 21 ft minimum does not fit a 16 × 20 ft tarp, even when the areas look similar. Rotating a rectangle helps only when both rotated dimensions still clear.
| 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. Manufacturers may advertise a nominal cut size while the finished tarp is smaller after hems. Look for “finished size” or an explicit tolerance. White Duck, for example, states that a cut-size listing finishes 3–5% smaller than the labeled dimensions. Check the actual listing, then run the size through the Listing Checker if you want that supplier’s own finished-size rule applied to a real product.
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: mirrors, windshields, handles, stacks, and corners change the profile.
- Matching by area: enough square feet, one dimension too short.
- Treating a boat, car, or hay pile as a box: use the job guide.
- Assuming every labeled size is finished size: hems reduce usable dimensions.
- Buying the largest possible tarp: extra fabric is heavier, catches more wind, and needs a fold plan.
Before you buy
- Write down the minimum length and width separately.
- Open the job guide if the shape is not a rectangle or a box.
- Check the product’s finished dimensions and tolerance.
- Confirm where reinforced corners and grommets will land.
- Check estimated weight if one person must handle it.
- Plan a secure method that does not rely on overstretched grommets.
Cut vs finished size: White Duck Outdoors (cut size is the panel before hems; finished size is 3–5% smaller). Treat that as one manufacturer’s rule, not an industry constant. Roof safety: OSHA Roof Tarping (Blue Roof) Safety Fact Sheet.
Frequently asked questions
How much bigger should a tarp be than the object it covers?
Add the coverage you actually need on every side: drops down the sides, overhang at the ends, and working room for hems and attachment. For a draped 3D object, each side drop counts twice in the matching direction. Sizing to the object’s footprint alone always comes up short.
Is the advertised tarp size the size I receive?
Often not. Many consumer tarps are sold by cut size, before hems reduce the finished dimensions. Look for a stated finished size and tolerance, and plan coverage from the finished number. The Listing Checker applies each supplier’s own finished-size rule to real listings.
Should I round up to the next stock size?
Usually yes, as long as you have a plan for the extra fabric. An oversized cover can be folded and tensioned; an undersized one fails. Excess material needs management so it does not flap, pool water, or reach wheels and traffic.
How do I size a tarp for an irregular shape?
Break the shape into the largest rectangle that contains it, measure the extreme points in each direction, and plan folds for leftover material. For roofs, boats, cars, hay, pools, and containers, use the job-specific guide instead of forcing the box formula.
What is the difference between drop and overhang?
A drop is the deliberate vertical coverage down the side of an object, chosen to reach an attachment line or protect the lower body. Overhang is extra material beyond the coverage line. You need the first; you manage the second.
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.