What Is a Compostable Bag? | The Certified Breakdown

That’s the simple version, but the word “compostable” carries real weight in the U.S. Unlike “biodegradable,” which any plastic can claim, compostable is a certified standard. A bag can’t legally call itself compostable for industrial facilities without meeting specific criteria. Understanding what is a compostable bag means knowing the standards, the certification logos, and where the bag is actually allowed to break down.

What Are Compostable Bags Made Of?

Compostable bags are typically made from plant-based or bio-based materials. The most common ingredients include corn starch, potato starch, cellulose, PLA (polylactic acid), PBAT, and vegetable oils. These materials are designed to be broken down by microorganisms in the right conditions, turning into carbon dioxide, water, biomass, and compost—not microplastics that persist for centuries.

You’ll find these bags used as kitchen caddy liners, food-scrap bags, yard-waste bags, and produce bags. They’re short-lived by design, meant for collecting organics and hauling them to a facility that can process them properly.

One critical point: a bag being “plant-based” doesn’t automatically make it compostable. Some bio-based plastics are formulated to resist breakdown, and some compostable bags contain a blend of materials that only break down under specific conditions. Composition alone never guarantees certification.

The U.S. Standard: ASTM D6400 and BPI Certification

In the United States, the most cited standard for compostable plastics is ASTM D6400. This standard was written for products designed to break down in municipal or industrial aerobic composting facilities. Bags meeting this standard must achieve about 90% biodegradation within 180 days under controlled conditions, disintegrate into fragments no larger than 2 mm, and leave compost that’s safe for plant growth with no toxic residue.

The BPI Certified seal is the most trusted third-party certification for compostable packaging in the U.S. When you see the BPI logo on a bag, it means an independent lab verified the product meets ASTM D6400. New York State’s procurement guidance goes further, requiring certified bags to have the certification logo and the word “COMPOSTABLE” printed directly on the bag.

So when you’re checking whether a bag is genuinely compostable, don’t look for marketing language—look for the certification mark from BPI or an equivalent body.

Industrial vs. Home Compostable: Know the Difference

The single most common mistake gardeners make is assuming one compostable bag works everywhere. A bag certified for industrial composting (ASTM D6400) may not break down in your backyard pile, and a home-compostable bag might not be accepted at a municipal facility.

Industrial composting runs hot—typically 130–160°F—with controlled moisture and aeration. Home compost piles run cooler and slower. That’s why home-compostable standards exist separately. In Europe, EN 17427:2022 covers carrier bags suitable for well-managed home composting, with a cycle of normally at least 12 months. Australia’s AS 5810 serves a similar purpose. If your city’s green-bin program requires ASTM D6400-certified bags, a home-compostable bag won’t cut it, and vice versa.

If you’re shopping for liners for your own compost pile, check your city’s rules first—some programs accept certified liners, while others reject bags even when certified. The acceptance is always program-specific.

Common Mistakes and How to Verify a Compostable Bag

The biggest trap is confusing “biodegradable” with “compostable.” Biodegradable is a loose claim with no standard behind it—a bag labeled biodegradable could take decades to break down. Compostable, by contrast, is a certified, standards-based promise. Treat any bag that only says “biodegradable” as conventional plastic.

Three quick checks will keep you honest:

• Look for the standard, not the adjective. ASTM D6400 for industrial, EN 17427 or AS 5810 for home composting.
• Match the bag to the facility. An industrial-commercial bag won’t reliably break down in a backyard pile, and a home-compostable bag may be rejected by a municipal program.
• Check the packaging for the seal. BPI or an equivalent certification logo should be printed on the box, not just claimed in fine print.

Also remember: compostable bags belong in a composting system, not a landfill. In a landfill, the oxygen-free environment means the bag won’t break down any faster than conventional plastic. And if you’re looking for a tested roundup of compostable sandwich bags for everyday use, that guide covers the certified options worth buying.

One final caution: home-compostable bags are not automatically industrial-compostable, and industrial bags aren’t automatically home-compostable. Read the certification scope on the package before you commit, and when in doubt, ask your local organics facility what they accept.