Compost bin size: How big do you need it?

Producing compost significantly enhances the health of your garden. The construction and dimensioning of compost bins consider a variety of factors.

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, consider the amount of humus you need and the waste you can provide.

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 Compost created by gardeners. They come in different forms, whether 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 lawns to sawdust and ash. All this will be stored in the bin and 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 decomposing rate will be slower.

If your compost bin is too big, heat will be quickly dispersed. Composting is slower without heat 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 of 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 following:

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

When you multiply all the dimensions above, you will get the needed amount of Compost. To get the required dimensions, you will need a measuring tape.

Let us consider a garden with a length of 20 feet and a width of 15 feet. The garden owner 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 and then convert every other unit to be the same as the unit of selection.

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 their compost bin should be 250 cubic feet. How tall, wide, or long are 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 your compost bin’s average length, width, and height.

6.3 feet (i.e., approximately 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 your answer’s square root ( ), or find the factors.

We will include a hypothetical situation and the computation for the compost bin in the next section to aid you.

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 seen in our table above). Using factors of the number (requires extra calculations or searching online) gives you various numbers for the length and width. Selecting the method to get your compost bin dimensions is yours.

Using the calculations above (I prefer using square root), our hypothetical garden will be 5 feet tall, 7.1 feet long, and 7.1 feet 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 precisely 250 because we calculated with approximated results (i.e., = 7.07107, not 7.1). For a more accurate result, do not 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 course of composting in two ways:

  • Availability of Oxygen
  • Regulation of temperature

The microbes that decompose your Compost into healthy soil rely on oxygen and temperature. When oxygen levels 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 possible.

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 must regularly turn and mix your compost pile. 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 combined with parts with a high population of bacteria.
  • Turning the Compost helps the complex parts on the pile surface go down and decompose quickly.
  • When you mix the Compost, you expose the oxygen-depleted parts of the compost pile to oxygen.

Turning and mixing the Compost is essential, but your pile may not generate as much heat as it needs if you do it too much. Do not turn and mix the pile daily to give your pile the required heat. 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, mulch, and fertilizer to garden beds. Composting has many more benefits covered in great detail in this article I wrote on composting for beginners, which is linked here.

But to push on, we will discuss composting vital components, such as how long it takes to compost and the processes within.

One of the best ways to understand how Compost works is to watch the video below, where I briefly summarize the entire process. I explain what and why you need certain things to prevent them 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 additional heat source is not added, your Compost may slowly decompose 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. You need to turn and mix the Compost as required according to the size of your pile. 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 must 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 decomposition process, chop the things 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. Every method is acceptable in composting, but different methods have different composting durations 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 Compost is lower than that of closed Compost.
  • Flies, lizards, and other pests will disturb your garden since they feed on kitchen waste.

It is your choice to select between an open and a closed compost bin; hopefully, the list above will 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 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 a population of anaerobic bacteria (i.e., they do not require oxygen) will grow without oxygen.

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 to 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 or 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 water drainage. 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 (from green leaves and protein-rich kitchen waste) is the element the plants in your garden need to grow well.

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

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 kilograms 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

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

FAQs on Compost bin size: How big do you need it?

Compost Bins Should Be Made of What Material?
Some people use wood; others use plastics, metals, etc., as the materials for their compost bins. Wood or metal is not a great idea because your pile’s moisture and microbial activity can rust metal and decompose wood. The best material to make compost bins is plastic.

What kind of waste can be composted in the compost bin?
Everything organic that is not yet decomposed can be composted. You can compost grass clippings, kitchen wastes, leaves, etc. However, do not compost meat, milk, bone, or other organic materials from animals since they need longer decompose and contain harmful bacteria.

Why Does a Compost Pile Emit Steam?
Your compost pile emits steam when it is decomposing. This is because some bacteria that decompose the particles in your compost pile are thermophilic (i.e., they love high temperatures). The decomposition process produces heat that, in turn, vaporizes the moisture of the compost pile.

How Can You Tell If the Pile is Fully Decomposed?
You can tell that your compost pile has fully decomposed when you no longer see steam when you turn and mix it. It will also have a stable temperature and pH. When felt, finished Compost will smell like rich earth instead of rotting material and have a loose and crumbly texture.

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 relatively easy, and the room for DIYing is large, making it inexpensive.

Just make sure to use the right side of the compost bin and suitable 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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