A tarp that stops short of the trailer deck leaves the lower portion of the load exposed to road spray, wind, and shifting weather. One with too much drop can drag, flap, and wear through before its time. Knowing how to measure tarp drop gives flatbed operators a better fit from the start, whether they are covering lumber, machinery, steel, palletized freight, or building materials.
Tarp sizing is more than picking a length and width that looks close. The height and shape of the load, the desired deck coverage, the trailer setup, and the tarp's built-in features all affect the fit. A few accurate measurements before ordering can prevent a tarp from becoming an expensive compromise.
What Tarp Drop Means
Tarp drop is the vertical distance from the top edge or crown of the covered load down the side of the trailer. On a standard flatbed tarp, it is the portion of material designed to hang along the load's sides. A tarp listed with a 6-foot drop is built to provide roughly 6 feet of side coverage from the top of the load toward the deck.
The drop is not the same as the total width of the tarp. Tarp width must cover the top of the freight plus the drop on both sides. For example, an 8-foot-wide load requiring a 6-foot drop on each side needs approximately 20 feet of tarp width: 8 feet across the top, plus 6 feet down the driver side and 6 feet down the passenger side.
That calculation is a starting point, not a substitute for checking the tarp's stated finished dimensions. Some tarps include reinforced hems, end flaps, tail flaps, or construction details that affect usable coverage. Always review the manufacturer's measurements before placing a fleet order.
How to Measure Tarp Drop on a Flatbed
Start with the load fully positioned and secured on the trailer. Measure after the freight is blocked, braced, and tied down as required. A tarp cannot replace proper cargo securement, and its fit should be evaluated around the load as it will actually travel.
Measure from the highest point to the deck
Use a tape measure to find the distance from the highest point of the load down to the trailer deck on each side. If the freight is uniform, such as a stacked lumber bundle, this measurement may be the same across the full length. If the load has changing heights, measure the tallest and widest sections instead of relying on an average.
The key question is where you want the tarp edge to land. For most flatbed applications, the tarp should reach the deck or sit close enough to it that bungee straps can pull the lower edge down securely. A tarp that only reaches halfway down the load may be workable for certain protected freight, but it will not provide the weather coverage expected for a full side tarp.
If the load sits on dunnage, include the full distance to the deck in your measurement. Do not measure only from the top of the cargo to the bottom of the cargo. The exposed space below the freight still needs to be considered when determining whether the tarp will reach the rub rail area and provide adequate tie-down points.
Account for load width and shape
Next, measure the load's widest point. The width across the top determines how much tarp material is used before the side drop begins. Square, rectangular freight is straightforward. Irregular machinery, crated equipment, and rounded loads require more attention because the tarp must travel over contours rather than across a flat surface.
For a load with sharp corners, allow enough material for corner protection and a natural drape. Pulling a tarp tight over an unprotected sharp edge creates stress points that can tear under wind load. Corner protectors, edge guards, or added padding can reduce wear, but they also add a small amount of bulk that should be considered.
A load that is 8 feet wide with a desired 6-foot drop generally calls for a tarp around 20 feet wide. If the load is wider than legal deck width, rounded at the top, or has protrusions, additional width may be needed. The best fit depends on the cargo profile, not just the nominal width listed on the bill of lading.
Check the front and rear coverage
Drop is a side measurement, but tarp length matters just as much. Measure the freight from the front edge to the rear edge, then allow material for front and rear coverage. Many flatbed tarps use end flaps or tail flaps to close these areas, and the amount of extra length needed depends on the tarp style and how the load is configured.
A tarp that has the correct drop but is too short can leave the front or rear exposed. One that is substantially too long can create loose material that catches wind and makes securement harder. When a load has a raised front section, a stepped profile, or a rear overhang, measure each area rather than assuming a standard tarp length will close properly.
Measure both sides when the load is not centered
Freight is not always centered perfectly on a trailer. A load may sit closer to one side because of weight distribution, securement requirements, forklift loading, or trailer equipment. In that case, measure the drop needed on both sides separately.
If one side needs 5 feet to reach the deck and the other needs 6 feet, select for the longer side. Trying to save material by sizing to the shorter side can leave the opposite side exposed. Extra manageable material is preferable to a tarp that cannot be tensioned to the deck.
Use the Tarp Width Formula
For a standard rectangular load, use this simple formula:
Required tarp width = load width + desired drop on driver side + desired drop on passenger side
An 8-foot-wide load needing 6 feet of coverage per side requires a tarp width of about 20 feet. A 10-foot-wide piece of equipment needing 8 feet of coverage per side requires about 26 feet of width.
This formula works well for common flatbed loads, but it does not account for every situation. Tall loads, uneven freight, covered coils, top-mounted equipment, and bulky corner protection can require extra material. When the load is near the limit of a tarp's dimensions, size up rather than forcing the fit.
Choose Drop Based on the Freight and Trailer
A 4-foot drop may work for lower-profile cargo where the tarp only needs to cover the upper portion of the load. A 6-foot or 8-foot drop is more common when the goal is to reach the deck on standard flatbed freight. Taller cargo may require a specialized tarp, multiple tarps, or a different coverage plan altogether.
The trailer also matters. Step decks, lowboys, trailers with side kits, and equipment with unusual deck heights change the distance from the load's top to the securing point. Measure on the actual trailer whenever possible. A tarp that fits a standard 48-foot flatbed may not fit the same freight on a lower deck or a trailer with added equipment along the sides.
Pay attention to where bungee straps will hook. A tarp may technically reach the deck but still be difficult to tension if its D-rings, grommets, or rubber rope placement do not line up with usable securement points. Good coverage requires enough drop and a practical way to hold that material down under road conditions.
Avoid the Most Common Tarp Measurement Errors
The most common mistake is measuring only the cargo height. Height tells you part of the story, but tarp drop is based on the distance from the top of the covered load to the deck or intended tie-down level. Ignoring dunnage, trailer deck height, or an uneven load can leave you short.
Another frequent issue is sizing from a clean, square drawing rather than the actual freight. Tarps lose usable coverage when they must pass over corners, bundled edges, tarped accessories, or irregular shapes. If the load has a high point, measure from that point.
Do not assume a larger tarp is always the better buy. Excess material adds weight, handling time, wind exposure, and opportunities for chafing. The right tarp is large enough to protect and secure the load, but not so oversized that it becomes difficult to control.
Before ordering replacements or building a fleet tarp package, record the load types your trucks haul most often, the deck heights in use, and the drop that consistently reaches the rub rail. Those measurements turn tarp purchasing from guesswork into a repeatable operating standard, helping every truck leave the yard better prepared for the road ahead.
