How to Measure for a Tarp? | Get The Right Fit

Measure the full coverage area—top plus drop on every side—and add overhang for rain protection and tie-downs before ordering.

Ordering a tarp that’s too small is a wasted purchase, and a tarp that’s too tight won’t stay tied down in the wind. Getting the fit right starts with one rule that most people get wrong: you’re measuring the complete coverage area, not just the flat surface. Whether you’re covering a woodpile, a truck bed, or a roof, the steps below walk through what to measure, how much extra to add, and the mistakes that lead to wrong-size orders.

What Should You Measure For A Tarp?

Start with the longest and widest points of the object, not the average. If you’re covering something with irregular edges or protrusions, measure across the widest spots and include the height so the tarp can drape around them. For flat covers, the top dimensions matter most. For anything with sides, the drop becomes the critical number.

For a 3D object, Derflex’s measurement guides recommend beginning with the top length and width, then adding the drop on each side so the tarp reaches the edges and can be secured. So if you want a tarp to drape down 2 metres on all four sides, you add 4 metres to both the length and the width. That jump from flat-top thinking to full-coverage thinking is where most sizing errors happen.

How Much Overhang Should You Add To Each Side?

Most applications need extra allowance beyond the exact dimensions of the object. The commonly recommended range is roughly 12 to 24 inches of overhang per side, though some applications call for as much as 30 to 60 cm depending on what you’re covering and how much rainfall you need to shed.

The overhang serves two purposes: it keeps rain from running down the sides of the object, and it gives you room to tension the tie-downs without pulling the tarp off the edges. For roof tarping, some sources recommend adding 3 to 4 extra feet on every side for secure anchoring with sandbags or lumber—though that’s application-specific, not a universal rule. For trucks and trailers, you measure the outside-to-outside distance across the top and add about 1 inch for play, plus the drop needed to reach the tie-down points.

How To Measure For A Truck Or Irregular Shape

Truck and trailer tarps use different logic than a flat ground cover. For a dump truck or trailer mesh tarp, you’ll need the bed length, the inside width, the outside width, and the cab shield length and rise—then add 24 inches (2 feet) to the tarp-length calculation to account for secure fastening.

For irregular shapes, divide the area into simple rectangles and record the maximum height plus any stress points where the tarp will stretch or wear. A surprisingly common error is using the average width instead of the widest point; that leaves the tarp short where the object bulges. Also measure the height and account for slopes, protrusions, and tension hardware before you order.

How To Order The Right Tarp Size Without Mistakes

Before ordering, recheck the numbers and confirm that you’ve measured the longest and widest points—not just the flat top. Mixing units (feet with metres) or failing to recheck dimensions are the two mistakes that lead to ill-fitting covers, and ordering too tight is the third. Add that extra allowance for install tolerance and tie-downs; that’s the buffer that makes the difference between a tarp that fits securely and one that fights you on installation day.

Use a steel tape measure rather than a cloth one for accuracy; it won’t stretch or bend at awkward angles. For large objects, bring a helper to hold the other end and keep the tape straight. And one important note: some sellers state that their measured sizes are the finished tarp sizes, while others want the object size first and compute the tarp from that. Match the ordering method to the seller’s spec sheet, and check whether opened tarps can be returned—for shelter or custom replacement tarps, many suppliers will not accept returns once the tarp has been used.

Once you’ve confirmed your measurements, the fit depends on matching them to the right product for your job. For large ground covers, our tested roundup of 20 ft tarps breaks down the top options that hold up to weather and tie-down strain.

Before you order, do a final walk-through: recheck every dimension with the steel tape, confirm the overhang covers your tie-down points, and verify the seller’s sizing method matches yours. That single check saves the headache of a tarp that arrives too short or too tight to secure.