What Are Compostable Bags Made Of? | Materials Breakdown

Most people assume “compostable” means the bag will vanish in their backyard pile or a landfill. That assumption leads to a lot of disappointment and contaminated recycling bins. The truth is more specific: In the United States, compostable bags are designed to break down in municipal or industrial aerobic composting facilities, where temperature, moisture, and microbial activity are tightly controlled. Understanding what these bags are actually made of — and what those materials require — matters for anyone tossing them into a compost cart or considering them for yard waste.

The Main Materials In Compostable Bags

Compostable bags come from several material families, and manufacturers often blend them to hit the right balance of strength and breakdown speed. The most common ingredients you’ll see on product specs include:

• Plant starches — usually corn, potato, or cassava starch that forms the structural base.
• PLA (polylactic acid) — a bioplastic made from fermented plant sugars, typically corn. It’s rigid and works well for bags that need to hold their shape.
• PBAT (polybutylene adipate terephthalate) — a biodegradable synthetic polymer often blended with PLA to add flexibility and tear resistance.
• Vegetable oils — used as plasticizers or base ingredients in some resin blends.
• Kraft paper — a common choice for yard waste bags, sometimes coated with compostable materials to improve moisture resistance.

PLA and PBAT are frequently paired in retail product listings because the combo gives a bag that feels like conventional plastic but actually breaks down under the right conditions.

What “Compostable” Actually Means In the U.S.

The word “compostable” isn’t just marketing. In the U.S., it typically refers to a product’s ability to break down in municipal or industrial aerobic composting facilities — not in a landfill or most backyard piles. The recognized benchmark is the ASTM D6400-23 standard, which covers plastics and products made from plastics intended to be composted under aerobic conditions in municipal and industrial facilities, where thermophilic (high-heat) conditions are achieved.

The same standard also addresses labeling. A bag claiming “compostable in aerobic municipal and industrial composting facilities” has a specific meaning: it should break down under those controlled, high-temperature conditions, not just anywhere you leave it.

Some products claim both home and industrial compostability, but those are separate claims. ASTM D6400 itself is framed around municipal and industrial facilities, so a “home compostable” label needs independent verification from the manufacturer.

How Certification And Labeling Work

Not every bag labeled “compostable” is certified under ASTM D6400. Regulatory bodies have stepped in to make labels more honest. Washington state, for example, has specific rules for film bags that meet the standard. A qualifying bag must be clearly identifiable — typically tinted or uniformly colored green, beige, or brown with “compostable” labeling, or marked with the word on both sides following specified color and band rules. Smaller bags under 14 by 14 inches get proportional marking requirements.

Here’s a rule that catches people off guard: A qualifying film bag is prohibited from displaying a chasing-arrow resin identification code or recycling symbol. That’s intentional — putting a recycling symbol on a compostable bag confuses sorting systems and consumers alike.

At the federal level, the FTC requires that compostability claims be backed by competent and reliable evidence, and ASTM D6400 is recognized as a benchmark in practice. In short: if a bag doesn’t reference a recognized standard, its compostability claim is worth questioning.

Common Mistakes To Avoid

Two errors cause most of the frustration around compostable bags. First, assuming a compostable bag will break down in any environment. These bags need the heat, moisture, and microbial activity found in managed composting facilities. Toss one in a dry landfill and it can persist for years. Second, treating “biodegradable” and “compostable” as interchangeable. They are not. “Biodegradable” is a weaker claim that simply means something will eventually break down under unspecified conditions — it carries no guarantee about timeframe or safety. “Compostable” implies a specific, tested breakdown process under defined conditions.

If you’re ready to add compostable bags to your routine — whether for kitchen scraps or yard waste — our roundup of the best compostable bags covers options that actually meet the standards discussed here.