Why Shredding Paper Adds Two Percent Surface Area

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It does not. Shredding a sheet of office paper adds between 1.6 and 2.7 percent to its surface area, and the reason is obvious as soon as you measure it: a sheet is already almost entirely surface. Its two faces come to 1,206 square centimetres and all four of its edges together come to 0.99, so the edges are 0.082 percent of the sheet. Cutting it up makes more edges. There was never much there to multiply.

What shredding actually changes is the space between the pieces, and that changes by a factor of fifteen. It takes the void fraction of the material from about 6 percent to 92, which takes its oxygen diffusivity up by a factor of 38, which takes the time for oxygen to reach the middle of a garden bin from five days to three hours. The surface area answer is folklore. The porosity answer is the mechanism, and it is a hundred times larger.

The audit, which takes one minute

A sheet of US Letter at 20 lb bond is 215.9 by 279.4 millimetres and a tenth of a millimetre thick. Both faces together are 2 times 215.9 times 279.4, which is 120,644 square millimetres. The edges are the perimeter times the thickness: 2 times 495.3 times 0.10, which is 99 square millimetres.

So before you do anything, 99.92 percent of that sheet's surface is already in contact with whatever it is sitting in. Now shred it. A strip cut into 6 millimetre ribbons makes 35 cuts, and each cut exposes two new strips of edge 279.4 long and 0.10 thick, so 55.88 square millimetres a cut, so 1,956 in total. That is an increase of 1.62 percent. A cross cut into 4 by 40 millimetre particles makes 53 lengthwise cuts and 6 crosswise ones, for 3,221 square millimetres, which is 2.67 percent.

Two and a half percent, for a machine, a power cord and ten minutes. If surface area were the mechanism, shredding would be one of the least effective things anyone does in a garden.

Why the rule works for everything except sheets

The folklore is not wrong in general, which is why it survives. Take a solid 50 millimetre cube: 15,000 square millimetres of surface, 125,000 cubic millimetres of volume. Cut it into 10 millimetre cubes and you have 125 pieces of 600 square millimetres each, which is 75,000. Exactly five times the surface, for a fivefold reduction in size, because surface area goes as one over particle size.

And there is the whole distinction. That law applies in the dimensions you cut. A cube has three available and cutting it in all three gives you the full factor. A sheet has two available and the third one, the thickness, is already a tenth of a millimetre and no shredder touches it. Whatever you do to the other two dimensions is spent creating edge, and edge is 0.082 percent of the object.

Our own beginners' guide to composting at home says "Chopping materials smaller speeds things up" and that "Woody material breaks down faster if chopped or shredded small". Both are correct, and both are correct specifically because wood is three dimensional. The same page also already contains the right answer for paper and does not connect the two: browns, it says, "provide carbon, structure and air pockets, and they stop the pile turning into a wet, smelly sludge". Structure and air pockets is the porosity mechanism, stated plainly. The only thing missing is that for sheet material it is the entire mechanism and the chopping rule contributes essentially nothing.

The quantity that moves by a factor of forty

A sheet of paper has a material density of about 780 kilograms per cubic metre. A ream of 500 sheets, pressed flat, measures 753, so the gas-conducting space between the sheets is around 3.5 percent, and in a real heap with curl and roughness call it 6. Loose shredded paper has a bulk density of 40 to 80 kilograms per cubic metre. Take 60. Against 780 that is a void fraction of 92.3 percent.

Oxygen gets into the inside of a pile by diffusing through those voids, and the standard relation for a dry porous medium is that effective diffusivity goes as the void fraction to the power four thirds. For 6 percent void that is 0.0235 of free air diffusivity. For 92.3 percent it is 0.8987. The ratio is 38, and if the sheets lie as flat as a pressed ream it is 78.

Now convert it into time, because that is what decides whether a pile works. A VEVOR 220 gallon expandable compost bin is roughly a 0.94 metre cube, so oxygen has to travel 0.47 metres to reach the centre. Diffusion time is the distance squared over the diffusivity. Through shreds that is 12,290 seconds, which is 3.4 hours. Through flat sheets it is 470,000 seconds, which is 5.4 days.

An active aerobic pile consumes the oxygen held in its own pores in a small number of hours. So a centre that is resupplied every 3.4 hours stays aerobic and a centre that is resupplied every 5.4 days does not. It is not a slower version of the same process. It is a different process, and its product is the flat grey readable mat of newspaper that people dig out of a bin two years later, which is anaerobic preservation rather than slow composting.

Our own bin record says the right thing about the wrong surface

That bin's record is strong on the thing that matters and makes one revealing slip. It correctly leads on airflow: the walls "are covered in holes that provide excellent air circulation", the HDPE is described as corrosion-proof and weather-resistant, and its own drawbacks are unusually honest, noting that it is "Lightweight, so it must be anchored down or wind moves it" and that "Composting still needs green-brown balance, moisture and turning".

The slip is the next clause: the holes, it says, "increase surface contact with the air and improve fermentation". A perforated wall is a boundary condition, not a bulk property. It sets what happens at the skin of the pile, and the arithmetic above is about the 0.47 metres between that skin and the middle. No number of holes in a wall can change the material's diffusivity, which is exactly why the shredding question matters and exactly why a well ventilated bin full of flat sheets still goes sour in the centre.

And there is a unit problem worth flagging because it is 20 percent. The record publishes the capacity as "220 gallons or 1,000 litres" as though those were the same quantity. They are not: 220 US gallons is 832.8 litres, and 1,000 litres is 264 US gallons. The 220 is an Imperial gallon count, since 220 times 4.54609 is 1,000.1, sold with a US gallon label. Everything above uses the 833 litre reading, which is the conservative one. Our VEVOR compost bin review has the assembly detail.

A film is a sheet with the problem turned up

The same audit run on something thinner makes the point unarguable. GreeneWrap compostable snack bags at 2 mil are one of the few items in our catalogue that publishes its own thickness, and it is the number this page needs. Two mil is 0.0508 millimetres, and the bag is 6.5 by 3.7 inches, so 165.1 by 94.0. Faces: 31,039 square millimetres. Edges: 2 times 259.1 times 0.0508, which is 26.3. The edges are 0.085 percent of the bag, which is almost exactly the same fraction as a sheet of office paper. Halving the thickness halves the edge area and halves the face area along with it, so the ratio barely moves. There is no thickness at which cutting a film up gives you meaningful surface area.

Which means the whole fate of a compostable bag in a home heap is a porosity question, and the record is appropriately careful: the certifications are real and specific, "BPI, OK Compost Home and Industrial by TUV Austria, plus ASTM D6400 and EN 13432", the "Breaks down in 180 days per maker" claim is attributed, and the drawbacks state that "Composting only completes in proper home or industrial conditions, not general trash". Those standards are run on comminuted material at controlled temperature and moisture. A whole flat bag laid in a cold heap is a gas barrier of its own, so it will be the last thing in the bin to go, and tearing it into strips before it goes in is worth more than any certificate on the box. Our page on choosing compostable snack bags that actually break down has the certification detail, and our GreeneWrap snack bag review has the rest.

The opposite error, which is grinding too fine

Since the mechanism is porosity rather than surface area, it follows that you can overdo it, and there is a machine in our catalogue that does. An Ouaken 4L electric kitchen composter works by "High-temperature drying, grinding and cooling", and its output is, in its own words, "a small amount of dry, ground material".

Grinding is the three dimensional case, so here the surface area argument is genuinely true and the gains are large. But permeability of a packed bed goes as the square of the particle size, so a fine dry powder has a high surface area and a low permeability at the same time, and a layer of it will shed water rather than absorb it and will pack rather than breathe. This is why the record's central honest statement is the correct one and not a hedge: "The dry, ground material is not finished compost. It is a dehydrated pre-compost or soil amendment. Mix it into garden soil or add it to a compost pile to mature." Mixing is not an optional finishing step. It is the step that restores the porosity the grinder removed. Our Ouaken electric composter review covers the filter consumable, and our comparison of electric composters against bins and traditional heaps gets the pre-compost point right too.

So there is an optimum rather than a direction, and the two bounds are easy to state. Too large and flat, and nothing breathes. Too fine and dry, and nothing breathes either, for the opposite reason. A stainless soil sifter sized for a 5 gallon bucket is a good instrument for finding out where you landed, because its mesh is published: 3 by 8 of an inch, which is 9.5 millimetres. Anything that comes out on top of that sieve has not broken down, and the diagnostic value is in what shape it is. Sticks on the sieve mean the chopping rule applies and you did not chop. Flat intact sheets on the sieve mean the porosity rule applies and the pile went anaerobic around them. Its drawbacks note it is "3/8-inch coarse sift, not for fine seed-starting soil", which is the right expectation. Our soil sifter review has the build detail.

The honest limit, and the free technique that beats the shredder

The limit first, and it is the sort that changes the answer rather than decorating it. Nothing on this page is a chemistry argument, and the reason your paper is not breaking down may be chemistry. Paper is roughly 43 percent carbon with a carbon to nitrogen ratio somewhere near 200 to 1, and decomposition wants about 30 to 1. So a kilogram of paper carries about 430 grams of carbon and about 2.15 grams of nitrogen, when it needs 14.3 grams to balance. The missing 12.2 grams has to arrive with something else: fresh grass clippings are about 2.4 percent nitrogen on a dry basis and about 80 percent water, so you need roughly 2.5 kilograms of fresh clippings per kilogram of paper. No amount of shredding changes that figure by anything at all. Shredding is a physics intervention on a pile that may have a chemistry problem.

The second limit is that no listing we carry publishes a bulk density, a void fraction or a particle size for any of this, so the 60 kilograms per cubic metre for shreds and the 6 percent for a stack are both worked figures. The thickness on the compostable bag is the one geometric number in the whole article that came off a record, which is why it is in it.

And then the free technique, which follows directly from the arithmetic and makes the shredder more or less pointless for this purpose. Crumple the paper instead of shredding it. A loosely crumpled sheet of Letter makes a ball about 60 millimetres across, which is 113 cubic centimetres holding 4.54 grams, so 40 kilograms per cubic metre. That is at the loose end of the shredded range, achieved in one second per sheet with no machine, and it delivers the entire porosity gain while adding precisely zero surface area. Which is the clean experimental proof that porosity was the mechanism all along: crumpling does nothing the folklore credits and everything the physics requires.

Two more that are free. Tear rather than cut, because a torn edge is ragged and a ragged edge cannot lie flat against the next piece, so torn paper packs looser than guillotined paper of the same size. And never add a layer of flat sheets, of any size, as a layer. The failure mode is continuity rather than area: a 2 centimetre bed of flat paper is a 2 centimetre gas barrier spanning the whole bin, whereas the identical paper distributed through 20 centimetres of greens has no continuous barrier anywhere. Mixing is worth more than cutting, and it is the one thing the arithmetic says your effort belongs in.