Compost bin size: How big do you need it?


It is a great idea to make compost for the health of your garden. To make compost bins and their sizing, a lot of factors are put into consideration.

The size of your compost bin should be two times the size of your compost pile. The minimum size of a compost bin should be 50 cubic feet. When building a compost bin, you must consider the amount of humus that you need and the waste you can provide at a time.

The sizing of the compost bin determines how fast you can get humus or finished compost. More on the sizing correlation and just how fast it aids the decomposition process will be covered in this article.

What is a compost bin?

Compost bins contain the created compost of gardeners. They come in different forms, whether they may be storebought, DIY made, or a lump of the ground covered in a tarp.

Nature demands a gift for everything that it gives, so what we have to keep doing, is returning [leaves & compost materials] back to the soil, then we’re continuously giving the gifts to nature, because we have a return cycle.

Geoff Lawton

They contain organic and inorganic materials ranging from food scraps to grass clippings from the lawn to even sawdust and ash. All this will be stored in the bin that can be later utilized as fertilizer to boost plant growth and improve soil quality.

How Big Should A Compost Bin Be?

A compost bin should be big enough to have a high oxygen concentration and small enough to retain heat.

If a compost bin is too small, your compost pile will not get enough oxygen to decompose, so the rate of decomposing will be slower.

If your compost bin is too big, heat will be easily dispersed. In the absence of heat, the process of composting is slower because the bacteria that act on your pile require heat.

To balance the oxygen and heat requirement of a compost pile, your compost bin should be two times the size of your pile. How can you calculate how much compost pile you need? Learn the easy equation below.

How Much Compost Do You Need?

The amount of compost you need depends on the size of your garden (i.e., the location in which apply the compost) and the desired depth of compost you wish to apply.

In simpler terms, to calculate how much compost you need, you have to know:

  • The width of your garden
  • The length of your garden
  • The desired depth of compost to be applied.

When you multiply all the dimensions stated above, you will get your needed amount of compost. To get the needed dimensions, you will need a measuring tape.

Let us consider a garden that has a length of 20 feet and a width of 15 feet. The owner of the garden wants to apply compost with a depth of 5 inches. Multiplying 20 feet, 15 feet, and 3 inches, we have 125 ft3 (cubic feet). The owner of the hypothetical garden needs 125 cubic feet of compost.

It is important to note that when calculating with multiple units like feet and inches, select a unit of choice then convert every other unit so that they will be the same with the unit of choice.

Computing for the size of the compost bin

Since you know how much compost you need, multiply that amount by 2. The answer is the size of your compost bin.

Considering our hypothetical garden once again, the gardener needs 125 cubic feet of compost, meaning that their compost bin should be 250 cubic feet. How tall, wide, or long is 250 cubic feet? To convert feet from cubic feet, you need the cubic feet’ cube root (3√ ).

The cube root of 250 (i.e. ) is 6.2996. The number 6.2996 is the average of the length, width, and height of your compost bin.

6.3 feet (i.e.,, approximate 6.2996) is too high, so we will reduce the height, then increase other dimensions. The table below shows the length and width of different compost bin volumes with a height of 2 feet, 3 feet, and 4 feet.

The volume of Compost BinCompost Bin Length and Width (height of 3 feet)Compost Bin Length and Width (height of 4 feet)Compost Bin Length and Width (height of 5 feet)
50 cubic feet4.1 feet each3.5 feet each3.2 feet each
100 cubic feet5.8 feet each5 feet each4.8 feet each
150 cubic feet7.1 feet each6.1 feet each5.5 feet each
200 cubic feet8.2 feet each7.1 feet each6.3 feet each
250 cubic feet9.1 feet each7.9 feet each7.1 feet each
300 cubic feet10 feet each8.7 feet each7.7 feet each
350 cubic feet10.8 feet each9.35 feet each8.4 feet each

Getting the dimensions of your compost bin is very easy.

When you have gotten the volume of your compost bin (check the equation above), divide it by your desired height, and then get the root (√), or the factors of your answer. In simpler terms:

  • Get the needed volume of your compost bin
  • Divide the volume by your desired height
  • Get the square root ( ) of your answer, or find the factors.

To aid you, we will include a hypothetical situation and the computation for the needed compost bin in the next section.

Example computation for computing of compost bin

For example, we need to build a compost bin of 250 cubic feet for our hypothetical garden, and we want a height of 5 feet:

1.Needed volume of compost bin: 250 cubic feet

2. Divide the needed volume by desired height: 250 / 5 = 50 ft2 (square feet)

3. The square root of the answer:  = 7.1 feet each OR

4. Factors of answer (50 ft2): 1ft by 50ft, 2ft by 25ft, and 5ft by 10ft. 

Please remember that if you use a square root to get your final answer, the length and width of your compost bin will be identical (as you can see in our table above). Using factors of the number (requires extra calculations or searching online) give you various numbers for the length and width. The choice of selecting the method to get your compost bin dimensions is yours to make.

By utilizing the calculations above (I prefer using square root), our hypothetical garden will be 5 feet tall, 7.1 feet long, and 7.1 wide.

Let us confirm the answer:

5 feet x  7.1 feet x  7.1 feet = 252.05 ft3

The answer 252.05 is not exactly 250 because we calculated with approximated results (i.e., = 7.07107, not 7.1). For a more accurate result, do not approximate, or approximate two, three, or four significant figures instead of one significant figure.

Remember that the first factor to consider when deciding on how big a compost bin should be is the needed amount of compost.

How the Size of Compost Bins Affects the Duration of Composting

The duration of composting depends on several factors,, including the size of compost bins used. The size of a compost bin affects the duration of composting in two ways:

  • Availability of Oxygen
  • Regulation of temperature

The microbes that decompose your compost into a healthy soil amendment rely on oxygen and temperature. When the levels of oxygen and temperature are high, your compost decomposes faster. Let us discuss how compost bin sizes affect oxygen and temperature.

How the Size of Compost Bins Affects the Availability of Oxygen

A compost bin houses decomposing compost. The decomposing bacteria are aerobic (i.e., they require oxygen), so they need as much oxygen as they can get.

If the decomposing bacteria do not get as much oxygen, their population will be unable to grow, and they will not decompose as fast as others in a compost bin full of oxygen.

If oxygen is the only factor to be considered when building a compost bin, compost bins would have been constructed to be very large.

How the Size of Compost Bins Affects Temperature

Go back to the room in your imagination. Will you feel hot if the room is large? Nope. When a room is smaller, the temperature in the room increases. Likewise, small compost bins retain heat better than big compost bins.

The friendly bacteria that decompose your compost pile require 130°-140°F to decompose your pile. When exposed to lower temperatures, the bacteria do not decompose as fast as they should.

If the temperature is just what we consider when building a compost bin, our compost bins should be as small as possible.

Balancing the Oxygen and Temperature Requirements of Compost

What size of the compost bin balances the oxygen and temperature requirement of composting? As stated above, your compost bin should be two times the size of what you are composting.

The extra space is just enough for oxygen and is not too much for high temperatures.

How to Provide Extra Oxygen While the Compost Pile Remains Hot

To provide extra oxygen, you have to turn and mix your compost pile regularly. Here are some reasons why you have to turn and mix your compost pile:

  • Mixing the compost pile evenly distributes the microbes so parts of the pile without bacteria will be mixed with parts of the pile with a high population of bacteria.
  • Turning the compost helps the cold parts which were previously on the surface of the pile to go down and decompose quickly.
  • When you mix the compost, you are exposing the oxygen-depleted parts of the compost pile to oxygen.

Turning and mixing the compost is important, but your pile may not generate as much heat as it needs if you do it too much. To give your pile the needed heat, do not turn and mix the pile daily. Turn and mix your compost once or twice weekly.

Composting – a rundown

Composting is beneficial in your garden and can be utilized for many functions, such as adding it to potting soil, using it as mulch, and adding fertilizer to garden beds. Composting has many more benefits covered in great detail in this article that I wrote on composting for beginners, which is linked through here.

But to push on, we will be talking about composting vital components, such as just how long it takes to compost and the processes that happen within.

One of the best ways to understand how compost works are to watch the video below where I break down the entire process in detail, I explain what and why you need certain things to prevent it from turning into a smelly mess or not breaking down at all.

How Long Will it Take for the Compost to Be Ready?

The duration of composting varies from person to person and from method to method. Here are factors (asides from the size of your composting bin) that affect the duration of composting:

The availability of oxygen inside the compost bin

When your compost pile has sufficient oxygen, it will decompose quickly. The availability of oxygen determines the size of the population of decomposing bacteria.

The average climatic temperature in your garden where the compost bin is located

Compost decomposes quickly in warm temperatures. If an extra source of heat is not added, your compost may decompose very slowly in winter. The time of the year determines how quickly your pile will decompose.

How Active You Are in Turning and Mixing your compost inside the compost bin

When you do not turn and mix your compost pile as needed, the pile will decompose slowly. If you turn and mix it too much, it will decompose slowly as well. According to the size of your pile, you need to turn and mix the compost as required. I suggest once or twice per week.

The Size of Your Compost Pile inside the compost

The larger your compost pile is, the longer it will take to decompose. If you want a finished compost quickly, you have to decompose a smaller pile.

The Size of Individual Particles in Your Compost Pile in the compost bin

A small item decomposes quicker than a bin item. To speed up the process of decomposition, chop the items you are composting into smaller particles. Some people use lawnmowers to cut the grass they wish to compost before adding it to the compost pile.

Covering or Exposing Compost in the compost bin

You must have seen many gardeners who make compost bins with materials that allow constant air to reach the pile like mesh, wood, etc. In composting, every method is acceptable, but different methods have different duration of composting and results.

Here are some effects of composting in open compost bins:

  • Open compost receives more oxygen
  • Moisture in open compost evaporates quickly
  • Open compost is exposed to various kinds of microbes
  • The temperature of open composts are lower than that of closed compost
  • Flies, lizards, and other pests will disturb your garden since they feed on kitchen wastes.

It is your choice to select between an open and a closed compost bin, and hopefully, the list above with aid you in your decision.

Potential Causes of Odour of compost in the compost bin

At times, the compost shall start to have a rather foul scent, and the reason as to why may be found below.

The compost Pile is Oxygen-depleted inside the compost bin

When properly aerated (i.e., mixed and turned), your compost will not emit any foul smell. Oxygen-depleted compost emits a foul smell because, in the absence of oxygen, a population of anaerobic bacteria (i.e., they do not require oxygen) will grow.

Anaerobic bacteria produce methane and hydrogen sulfide, etc. Methane is odorless, but hydrogen sulfide makes your pile stink. To prevent the anaerobic bacteria from growing in your pile, turn and mix the pile regularly.

Too Much Moisture in the compost Pile within the compost bin

Another reason why your compost pile may stink is because of too much moisture. Covered compost bins retain much more moisture than compost bins that expose compost piles to air. When composting with a covered compost bin, do the following:

  • Add less water into your compost
  • Poke holes on the sides and bottom of the compost bin.
  • Add dry (brown) leaves, newspapers, etc. into your compost pile.

Even though your compost pile needs to be moist, it should not be damp nor waterlogged. Anaerobic bacteria love nutrient-rich water and will make your compost pile stink.

Holes on the sides and bottom of the compost bin allow drainage of water. The holes also allow the composting bacteria to breathe since oxygen can pass through them into the compost bin.

Carbon to Nitrogen Ratio of the compost pile within the compost bin

Carbon is the source of energy for microbes. Without carbon, they will not be able to decompose the materials. Nitrogen (which is gotten from green leaves and protein-rich kitchen waste) is the element needed by the plants in your garden to grow well.

Since carbon is used to decompose your compost pile to get nitrogen, we need to add more carbon into the pile.

The recommended carbon to nitrogen ratio is 4:1; 4:1 means 25 part carbon to 1 part Nitrogen, so for every 1 kg of a nitrogen-rich substance you add to your pile, you must add 25 kg of carbon.

The table below shows the carbon/nitrogen ratio in various compostable items.

Decomposable ItemCarbon/Nitrogen Ratio
Ash25: 1
Hair/Fur10: 1
Sawdust325: 1
Seaweed19: 1
Tree Bark131: 1
Cow dung20: 1
Peat moss58: 1
Newspaper100: 1
Office paper129: 1
Fresh weeds20: 1
Tissue paper70: 1
Grass clipping17-20: 1
Coffee grounds14: 1
Kitchen/Food waste15: 1
Fresh Vegetable Waste11: 1

There are many options specified in the list, so choose a material you have an excess of and add it into your compost bin.

FAQs

Conclusion on compost bin size and how big you need it to be

Composting is a cycle of nature that uses materials that we have in excess to give back and make it flourish more. The process is rather easy, and the room for DIYing is large, which also makes it inexpensive.

Just make sure to use the right size of the compost bin and the right materials when composting to maximize the process. Hopefully, the step-by-step computation and the information above will aid you in having a successful composting journey.

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Tony O'Neill

I am Tony O'Neill, A full-time firefighter, and professional gardener. I have spent most of my life gardening. From the age of 7 until the present day at 46. My goal is to use my love and knowledge of gardening to support you and to simplify the gardening process so you are more productive

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