Cardboard breaks down in compost, but only if you prepare it correctly
Yes, cardboard is compostable. It is made from wood fibers and will decompose in a compost pile or bin, turning into usable compost in three to six months depending on how you shred it, how wet your pile stays, and what else you add. The catch is that cardboard needs to be prepared — whole boxes take much longer to break down, and some cardboard should not go in compost at all.
The cardboard that works best is plain kraft cardboard with no glossy coating, no plastic tape, and no ink that rubs off when wet. Corrugated cardboard (the wavy kind in shipping boxes) breaks down faster than solid cardboard because it has more surface area. Shredding or tearing it into strips speeds decomposition significantly — pieces smaller than 2 inches wide will integrate into your pile in weeks rather than months.
Key Takeaways
- Plain kraft cardboard without glossy coating, plastic tape, or heavy ink will compost successfully when shredded into strips smaller than 2 inches wide.
- Corrugated cardboard (the wavy kind) decomposes faster than solid cardboard because water and microbes can reach more surface area.
- Cardboard acts as a brown (carbon-rich) material in compost, so balance it with green materials like food scraps or grass clippings to keep your pile active.
- Cardboard coated with plastic, wax, or glossy finish will not break down and should be removed before composting.
- Wet cardboard compacts and can slow air flow in your pile, so layer it loosely and mix it in rather than packing it down.
Which cardboard is safe to compost
Plain kraft cardboard — the tan, uncoated kind used for shipping boxes and moving boxes — is the safest choice. It contains no chemical coatings and will fully decompose. Corrugated cardboard (the ridged kind with air pockets) is ideal because the wavy interior creates channels for water and air to move through, speeding breakdown.
Cardboard printed with black soy-based ink is generally safe, though you should test it first: wet a small piece and rub it with your finger. If the ink comes off on your skin, it is water-soluble and will not harm compost. If it does not rub off, the ink is likely mineral-based and safe as well. Avoid cardboard where the ink is thick or glossy — that usually means plastic-based ink.
Newspaper and cardboard egg cartons also compost well. Egg cartons are already thin and break down quickly. Newspaper is pure cellulose and adds carbon to your pile without the bulk of cardboard. Shred both into small pieces to speed decomposition.
Cardboard to keep out of your compost
Do not compost cardboard with a glossy or waxy coating. This includes pizza boxes (the inside is usually wax-coated to resist grease), takeout containers, and some shipping boxes with a shiny finish. The coating does not break down and will remain in your finished compost as plastic-like particles.
Remove plastic tape, plastic windows, and foam packing material before composting any cardboard. These will not decompose and will end up in your finished compost where they can harm soil or plants. Styrofoam packing peanuts, plastic air pillows, and plastic-wrapped items should all be removed and recycled or thrown away separately.
Cardboard that has held pesticides, fertilizers, or other chemicals should not go in compost. This includes boxes from garden supply stores if they held treated products. When in doubt, check the box for warning labels or ask the seller whether the contents were chemical-treated.
How to prepare cardboard for faster composting
Shredding is the single most important step. Whole cardboard boxes can take six months or longer to break down because microbes can only work on the outside surface. Shredded cardboard — torn or cut into strips 1 to 2 inches wide — exposes far more surface area and will decompose in four to eight weeks.
You can shred cardboard by hand, which takes time but requires no equipment. Tear along the grain of the cardboard, which is easier than tearing across it. A paper shredder designed for office paper will also work, though it may jam on thick corrugated cardboard. A leaf shredder or wood chipper will handle larger quantities quickly.
Wet the cardboard before adding it to your compost pile. Dry cardboard repels water and will sit in your pile without breaking down. Soak shredded cardboard in water for a few minutes, then layer it into your pile. Wet cardboard also compacts easily, so mix it loosely with other materials rather than packing it down — air flow is essential for decomposition.
Balancing cardboard with other compost materials
Cardboard is a brown material — it is carbon-rich and breaks down slowly on its own. To speed decomposition and keep your pile active, balance it with green materials like food scraps, grass clippings, or plant trimmings. A good ratio is roughly three parts brown to one part green by volume, though compost will work with a wider range.
If you add a lot of cardboard at once, your pile may cool down and slow decomposition. Add cardboard gradually, layering it with green materials as you go. This keeps the pile warm and active. If your pile has slowed and feels cold, add more green material or turn the pile to introduce oxygen.
Cardboard alone will eventually compost, but it will take much longer — six months to a year or more. In a balanced pile with regular turning and moisture, shredded cardboard will integrate completely in two to three months.
Common mistakes that slow cardboard breakdown
Packing cardboard down tightly is the most common mistake. Compacted cardboard excludes air, which slows the microbes that break it down. Layer cardboard loosely and mix it with other materials so air can flow through. If your pile feels dense and smells sour or like ammonia, turn it to add oxygen.
Adding cardboard to a dry pile will not work. Cardboard needs moisture to decompose. If your pile is dry, water it before adding cardboard, and water again after layering it in. The cardboard should feel damp to the touch, not soggy.
Using glossy or coated cardboard is another common error. Many people assume all cardboard is the same and do not check for coatings. Spend 30 seconds examining each box — look for a shiny finish, feel for a waxy texture, and check the inside of pizza boxes. Coated cardboard will not break down and will contaminate your finished compost.
How long cardboard takes to compost
Whole cardboard boxes take four to six months in an active compost pile, and longer in a passive pile. Shredded cardboard takes two to four weeks in a hot, well-maintained pile with regular turning and balanced green-to-brown ratio. In a slower pile with less turning, shredded cardboard takes six to eight weeks.
Temperature matters significantly. A pile that reaches 130 to 150 degrees Fahrenheit (54 to 65 degrees Celsius) will break down cardboard faster than a cooler pile. This temperature range is reached when you have enough green material, adequate moisture, and regular turning. A pile that sits undisturbed will decompose more slowly but will still work — it just takes longer.
You will know cardboard is fully broken down when you cannot identify individual pieces. The shredded strips will have turned into dark, crumbly material that looks like soil. At that point, the cardboard has become part of your finished compost and is ready to use in gardens or potted plants.
Frequently Asked Questions
Can I put a whole pizza box in my compost?
No. Pizza boxes have a wax or plastic coating on the inside to resist grease, and this coating does not decompose. Remove the cardboard sides (which are usually uncoated) and compost those, but throw away or recycle the bottom and sides that touched the pizza.
Will cardboard ink hurt my compost or garden?
Black soy-based ink is safe and will not harm compost or plants. Most cardboard boxes use this type. Glossy or colored inks are usually plastic-based and should be avoided, but you can test by wetting the cardboard and rubbing it — if the ink comes off, it is water-soluble and safe.
How much cardboard can I add to my compost pile at once?
Add no more than one-third of your pile's volume as cardboard in a single week. Adding too much at once can cool the pile and slow decomposition. Layer it gradually with green materials, and your pile will stay active and warm.
Can I compost cardboard in a small apartment compost bin?
Yes, but shred it into very small pieces — no larger than 1 inch wide — and add it in small amounts mixed with food scraps. Small bins have less volume and lower temperatures, so cardboard takes longer to break down. Expect four to six months instead of two to three.
What should I do with cardboard that has tape or labels on it?
Remove all plastic tape, plastic labels, and plastic windows before composting. These do not decompose. The cardboard underneath is fine to compost once you have stripped away all non-cardboard materials.