Tarp thickness is measured in mils. One mil equals one thousandth of an inch (0.001 inch), so a 20 mil tarp is 0.020 inch thick. As a quick rule, 5 to 8 mil suits light, short-term use, 10 to 12 mil covers general-duty jobs, and 16 to 20 mil or heavier is built for long-term outdoor and industrial protection. But mil is only part of the story, and it works differently across poly, vinyl, and canvas.
What Does Mil Mean for Tarps?
A mil is a unit of thickness, not weight or quality. One mil is one thousandth of an inch, so the higher the mil number, the thicker the material. For scale, a sheet of printer paper is roughly 4 mil, and a credit card is about 30 mil, which puts a 20 mil poly tarp right between the two.
Thickness matters because it tracks closely with puncture resistance, tear strength, and how long a tarp survives outdoors. A thicker tarp flexes less in the wind, resists abrasion against rough loads and edges, and holds up through more freeze and thaw cycles. That is why a contractor covering equipment for a season reaches for a heavier mil than someone throwing a cover over a woodpile for a weekend.
Mil is most useful when you are comparing poly tarps, where the number describes the finished sheet. Our heavy-duty polyethylene tarp is a full 20 mil, built for the demanding end of that range. For vinyl and canvas, thickness is described differently, which is where ounces and denier come in.
Mil, Oz, and Denier: How Thickness and Weight Are Measured
Tarp specs use three different numbers, and they do not measure the same thing. Reading them correctly is the difference between buying the right tarp and overpaying for the wrong one.
| Measurement | What it measures | Used for | A higher number means | Humphrys example |
|---|---|---|---|---|
| Mil | Material thickness (1 mil = 0.001 in) | Poly tarps and film coatings | Thicker, more puncture and tear resistance | True 20 mil heavy duty poly |
| Oz per square yard | Fabric weight | Vinyl and canvas tarps | Heavier, denser, more durable fabric | 18 oz vinyl, 18 oz canvas |
| Denier | Thickness and strength of individual fibers | Woven scrim inside coated tarps | Stronger base fabric, better tear strength | Reinforced poly and vinyl scrims |
The key takeaway: you cannot convert mil to ounces directly. A 20 mil poly tarp and an 18 oz vinyl tarp sit on different scales, so a higher number on one does not make it tougher than the other. Compare thickness within a single material, then compare materials by how they actually perform for your job. Canvas, for instance, is measured in ounces (our heavy duty canvas tarp is 18 oz) and is breathable and water-resistant rather than waterproof, which makes it a different tool than a waterproof poly or vinyl tarp of any thickness.
Tarp Thickness Chart by Application
The most reliable way to choose thickness is to start from the job, not the number. Use this chart to match a tarp to what it actually has to survive.
| Application | Recommended tarp | Why |
|---|---|---|
| Light, short-term covering (woodpiles, debris, weekend projects) | 5 to 8 mil poly | Low cost and easy to handle for brief or one-time use |
| General-purpose protection (materials, gear, seasonal storage) | 10 to 12 mil poly | The everyday heavy-duty starting point, durable through a full season |
| Long-term outdoor equipment and machinery covers | True 20 mil poly or 18 oz vinyl | Resists UV, abrasion, and repeated wind flexing over months or years |
| Trucking, dump truck, and load covers (asphalt, gravel) | 18 oz vinyl | Abrasion and tear resistance for heavy, shifting loads |
| Roofing, storm damage, and construction debris | True 20 mil poly or 18 oz vinyl | Holds up to wind, foot traffic, and sharp debris |
| Breathable covering (lumber, hay, vehicles, indoor drop cloth) | 18 oz canvas | Lets trapped moisture escape to prevent condensation and mildew |
| Welding and hot-work areas | Flame resistant poly, vinyl, or canvas | Self-extinguishing protection wherever sparks and slag are present |
For job-specific covers, our dump truck tarps are already built to the right spec for those loads and conditions, so you do not have to size the thickness yourself. Anywhere sparks, flame, or hot work are involved, move up to a flame-resistant tarp regardless of mil, since standard poly and vinyl will melt or burn.
What Mil Do You Need for Heavy Equipment Covers?
For machinery and equipment that live outdoors for months or years, a true 20 mil poly tarp or an 18 oz vinyl tarp is the right starting point. Both stand up to the sun, abrasion, and constant wind flexing that destroy lighter covers. If the equipment sits in direct sun long-term, lead with a UV-stabilized material first and thickness second. Our UV-resistant tarps guide breaks down which materials hold up best against sun damage. For a cover sized to the machine, a made-to-order heavy-duty tarp avoids the gaps and overhang that shorten a tarp's life.
True Mil vs. Advertised Mil
A 20 mil tarp is not always 20 mil. Mil is one of the easiest specs to overstate, and on many imported tarps, the number on the label is a nominal target rather than a guaranteed measurement of the finished product.
The gap shows up in a few ways. Some tarps are measured before the coating or lamination is added, so the advertised figure counts material that is not really protecting your load. Others are listed as "up to" a thickness, or measured at the thickest point rather than across the whole sheet. Loose manufacturing tolerances do the rest, and a tarp sold as 20 mil can measure several mil thinner by the time it reaches you.
That difference is not academic. A tarp that runs thin fails where it matters: it punctures on a sharp edge, tears in high wind, and breaks down faster in the sun. You replace it a season early, and the cheaper tarp ends up costing more than the right one would have.
A few ways to protect yourself:
Ask for the finished, or true, mil. A reputable manufacturer will give you the measured thickness of the completed tarp, not a target.
Cross-check the weight. Ounces per square yard are harder to fudge than mil, so a heavier oz weight is a good reality check on a thickness claim.
Buy from the source. When the company that sells the tarp also makes it, the spec on the page and the spec in your hands are the same thing.
This is the whole reason we publish a true 20 mil rating on our heavy duty polyethylene tarp. The finished tarp measures a full 20 mil, because we manufacture it ourselves in the USA, as we have since 1874. For the broader case on quality and accountability, our guide on American-made versus imported tarps goes deeper.
Match the Thickness to the Job
The right tarp thickness is the one that fits the work, not the highest number on the shelf. Start with the application, choose the material that suits it, confirm you are getting a true finished mil or oz weight, and you will buy a tarp that lasts instead of one you replace next season.
Still deciding between poly, vinyl, and canvas, or sizing a tarp to fit? Start with our Tarp Buying Guide, which walks through materials and sizes side by side.
When you know the spec you need, every Humphrys tarp is available in standard sizes or made to order, manufactured in the USA since 1874. Browse the options below, or call us at 800-457-4406 to spec a custom tarp for your job.
Frequently Asked Questions
How thick is a 20 mil tarp?
A 20 mil tarp is 0.020 inch thick, or twenty thousandths of an inch. That puts it at the heavy-duty end of the poly range, built to resist punctures, tears, and UV breakdown through long-term outdoor and industrial use. Just confirm you are getting a true 20 mil, since some tarps advertise a thickness they do not actually measure.
What is considered a heavy-duty tarp thickness?
For poly tarps, heavy-duty generally starts around 12 mil and runs up to 20 mil and beyond. For vinyl and canvas, which are measured by weight, heavy-duty falls in the 18 oz range. The right spec depends on the job, since longer outdoor exposure and rougher handling call for the higher end.
Is a thicker tarp always better?
Not always. Thickness drives puncture and tear resistance, but it is only one factor. Weave count, denier, the coating, and how the seams and hems are built all affect how long a tarp lasts. A well-constructed mid-weight tarp can outlast a thicker one that is poorly made, so judge the whole spec, not just the number.
What is the difference between mil and oz for tarps?
Mil measures thickness and oz measures weight per square yard, so they describe different things. Poly tarps are rated in mil, while vinyl and canvas are rated in oz. You cannot convert one to the other directly, which is why you compare thickness within a single material and compare materials by how they perform for your job.
