In this guide we’ll take a look at the best compost thermometers for the UK market.
I’ve compared accuracy, lens quality, build quality and cost
to give you my top recommendations.
In this guide we’ll take a look at the best compost thermometers for the UK market.
What is the Best Compost Thermometer?
In a rush? Here's my top choice...
Everything I Recommend
More Detailed Compost Thermometer Reviews
Green Wash Compost Thermometer Review
Taking the guesswork out of composting, this Green Wash Compost Thermometer gives an accurate temperature reading making it easy to know when to turn the compost.
Whereas some compost thermometers can give ‘approximated’ readings, this one seems accurate and reliable as long as it is well looked after. Leaving it out in the elements can cause condensation to build up, which may affect the reading, so it needs to be kept dry.
However, you don’t need to worry about the stainless-steel body suffering from water damage – its rust-resistant properties protect it from any moisture in the heap, meaning the 40 cm probe remains rust free even if it is frequently exposed to humidity.
If you’re nervous about letting heat escape from your compost, the probe is long enough to stick through any vents in your compost bin, meaning you can take a reading without having to open it up. Otherwise, it’s a good length, and strong enough, to easily push into any compost pile, clearly displaying the reading on its 50 mm dial.
This dial is large enough to read without having to remove the thermometer from the compost, and the black writing contrasts against the silver steel face making it easy to read. It shows the temperature in Celsius.
As mentioned, the one primary flaw of this thermometer is its susceptibility to condensation. This can make the display difficult to read, so the importance of keeping it dry can’t be overstated. It also only shows the temperature in Celsius, so you might find this isn’t the best compost thermometer for you if you prefer using Fahrenheit.
- The large dial allows you to read the temperature without removing it from the soil
- The long probe is suitable for most compost bins and can be inserted through vents if needed
- Reacts quickly to temperature changes
- The stainless steel build is good quality and, provided it is stored correctly, should last a very long time
- Any condensation inside will make this thermometer difficult to read
- Not waterproof so shouldn’t be left in exposed areas
- The dial only shows Celsius and not Fahrenheit
ETI Ltd Stainless Steel Compost Thermometer Review
Some compost thermometers can be surprisingly expensive, so if you’re looking for a reasonably ‘budget’ option that will still give a good reading, this ETI Ltd Stainless Steel Compost Thermometer is cheap yet sufficiently accurate.
Its 50 mm dial displays three coloured zones (representing ‘warm’, ‘active’ and ‘hot’) which make it easy to tell, at a glance, how the compost is performing. Alongside this, it has a standard temperature scale of 0 – 100°C.
When it comes to temperature accuracy, this thermometer can be out by around +/- 2°C; however, for anyone who is just starting out with hot composting, this will likely be as accurate a reading as you need. Plus, for the price, this isn’t too surprising or disappointing.
It can take a little while to display a reading, so the 50 cm probe needs to be left in the compost for some time in order to show the right temperature; however, like most compost thermometers, despite its stainless-steel construction, the display itself is not completely water-resistant, so it shouldn’t be left in the compost permanently.
Condensation can get inside the dial if the thermometer is exposed to water, like rain or too much humidity.
The temperature is shown in Celsius only, and the display is generally easy to read. That said, the 1°C increments are small, and likely difficult to see accurately from a distance. The colour zones do make it simple to quickly understand how your compost is fairing, though, without needing to see specific temperatures.
- The 50 cm long probe means this thermometer can get right into the middle of most compost piles
- It’s easy to see at a glance whether your compost is in the warm, active or hot zone
- The accuracy is sufficient for measuring compost - at about +/- 2°C
- Sturdy and durable stainless steel build
- The inside of the dial may fog up with condensation which makes it difficult to read accurately
- Accurately reading the dial may be tricky for some users as the writing is quite small
- The probe is relatively easy to scratch
- It only displays the temperature in Celsius
Things to Know Before Buying a Compost Thermometer
Monitoring the temperature of your compost is the best way to ensure efficient composting and quality compost.
By knowing when your compost is heating and cooling, you’ll be able to turn it at the appropriate time during its heating cycle to achieve optimum results.
Compost thermometers also indicate when your compost has reached certain stages of its development, so you’ll know the exact moment when your compost is ready to use.
Here you’ll find more information about thermometers and composting in general, to help you find the best compost thermometer and get the most out of your compost:
You might think that everything is going well with your compost – you’ve prepared the heap, got loads of material on it, and are patiently waiting for that quality compost yield to come.
The problem is, without using a thermometer, you can’t really know what’s going on with your compost and if it’s working effectively.
The optimum temperature for compost, when hot composting, is between 55-72°C (131-161°F). If you don’t add any more organic matter, the compost can remain at this temperature for up to several weeks. This is ideal for efficient composting.
However, sometimes the compost might be colder than this, which can be checked using a thermometer.
If you’ve identified that your compost is not heating up, these steps should help you get it warm again:
- Compost can fail to heat up if the compost pile isn’t large enough. If your compost thermometer shows that the core of the compost it too cool, consider the size of your compost heap. Hot-composting heaps need a minimum biomass of 3 m³ – this can be achieved in a composting area that is 1 x 1 x 1 m. The aerobic bacteria in the compost heap will get to work and heat up the contents, but, without enough surrounding biomass for insulation, this heat it will get lost quickly.
- Another thing to consider after getting a low temperature reading on your compost thermometer is whether you are turning the heap frequently enough (or too frequently!). The heat is in the core of the heap, so it’s important to flip it regularly to make sure the ‘shell’ (outside) of the heap gets some time in the centre. Plus, turning the pile aerates it as well. This should be done approximately every 3 days.
- Your compost pile might be too dry, which will result in it being too cool to work properly. This might be something you can’t tell without a getting a ‘cold’ temperature reading from a compost thermometer. Moisture is needed to help microbes survive, and they’re what create the heat. Compost should feel damp, not dry, with a moisture level of 40-60%. You can add water to your compost heap if it’s too dry, but avoid adding too much. A balance needs to be found between the compost having access to enough oxygen and enough water (so you want to avoid filling all of the airways with water).
You can also use a compost thermometer to make sure your compost is correctly following through the different stages of composting. Which leads us onto the next section:
It’s really important to know what stage of composting your compost heap is at. This can dictate what action you need to take with it, and also let you know when it’s ready to use.
These are the different stages:
If all of the correct conditions are met (see information above in “Checking if Your Compost is Too Cold”), compost should start warming up within a couple of days. Different microbes get to work at different temperatures, so at each heat stage different activity will be taking place in the pile.
When the pile reaches a temperature of 55-72°C, this is the optimum temperature. It will stay at this temperature for a couple of days whilst the microbes get to work, then, when this ‘hot temperature’ bacteria dies off, the compost will start to cool.
Regularly checking the temperature of your compost will allow you to keep an eye on when it reaches its maximum temperature, and when the temperature starts to drop.
Turning the compost every three days whilst it’s in this ‘optimum heat’ phase should help it maintain this temperature for around 10 – 15 days. If, in this time, the temperature drops below 55°C or rises above 72°C, this is another indication that the heap should be turned.
Eventually there comes a time when most the bacteria that works at the hottest temperature dies off completely, and the pile will cool down.
When it cools down enough, if there is more composting to be done, the bacteria which work at lower temperatures will come back and kick start the cycle again.
Identifying When Compost is ‘Ready’
When compost has gone through all of the above stages and is no longer producing heat, it’s likely ready to use. Finished compost has an ‘earthy’ ‘dirt’ smell, and there won’t be any of the original organic matter visible. The pile will generally be half the size that it started out as, and the ‘compost’ will be dark and crumbly.
Compost thermometers are designed to be used in warm and moist environments, so it’s important that they are durable and, most importantly, rust proof.
Most thermometers are made from stainless steel; this is good because the metal won’t rust. A plastic thermometer won’t rust either, but it may get brittle or break when inserted into more compacted compost.
A common problem with compost thermometers is condensation getting into the dial. A sealed dial can prevent water from seeping in, although the best compost thermometers will have an IP55 rating.
A rating of IP55 means the thermometer is tightly sealed enough to not allow dust in, and is also water resistant (it’s just not resistant to immersion in water).
Thermometer Probe Length
The length of the probe is important, as you’ll need to take the temperature from as close to the middle of the pile as possible. While your measurements don’t need to be right at the centre, taking the temperature too close to the surface won’t give a good reading of what’s really going on at the core.
A probe of around 50 cm long is suitable for large piles of compost; some thermometers have a probe of 40 cm or less, but it’s advisable to check the size of your compost heap before buying a smaller thermometer. A 50 cm or 40 cm probe is generally optimal for taking readings from the middle of the pile.
Displayed Temperature Range
How the temperature is displayed on the thermometer can vary between models. Most thermometers measure in 1 or 2 degree increments and the best models will have a range of 0 – 100°C. Cheaper models may have a smaller temperature range.
Some compost thermometers come with colour coded sections, so you can see whether the pile is ‘warm’, ‘active’ or ‘hot’. This makes the thermometer easy to read quickly, as you’ll be able to see at a glance where your compost is in its heat cycle.
You may have a preference when it comes to reading temperatures in Celsius or Fahrenheit. Some thermometers are exclusively in Celsius or Fahrenheit, whilst others display both units. If you’re looking at a thermometer which shows both Celsius and Fahrenheit, make sure it’s clear to read and understand. Some thermometers that display both still seem to favour one unit over the other and can be hard to interpret quickly.
A Clear, Easy-Read Dial
Trying to find a dial that doesn’t mist up when in contact with humid compost can be difficult.
Some have an anti-fog coating which helps stop the dial misting up on the outside.
Condensation on the inside can also cause problems and make the dial hard to read. The best way to avoid this is by keeping the thermometer as dry as possible and not leaving it outside. Purchasing a thermometer with a ‘water resistance rating’ (e.g., IP55) should also reduce the amount of moisture that is able to creep in.
When it comes to the dial itself, a dial which is 5 cm in diameter is generally easy to read from a distance. If the numbers are too small, or of a lighter colour, they can be hard to read, so large numbers that contrast with the background colour are optimal.
The coloured ‘zones’ that some thermometers have to show ‘warm’, ‘active’ and ‘hot’ are also very simple to read without needing to see specific numbers.
Compost Thermometer FAQs
Simply insert the compost thermometer into the pile as far as it will go. Wait until the hand on the dial stops moving and make a note of the temperature. It is advisable to check the reading in several areas of the compost heap as there is not always a uniform temperature across the pile and you may occasionally find cold or hot spots.
Ideally, for a hot compost pile, once the temperature reaches between 55°C and 72°C you should turn the pile every three days. It should maintain this temperature for around two weeks. If the temperature drops below 55°C in this time, or goes above 72°C the pile should also be turned.
After this couple of weeks the temperature will drop and the process will start from the beginning. You will need to continually check the temperature of the compost until it starts to rise again.
Compost that is ready to use looks dark and has a crumbly texture. It smells ‘earthy’ and has no residue of any of the original organic matter that you fed to it.
Whilst finished compost can still retain a little heat, the pile won’t be particularly warm. The temperature of ‘ready’ compost will be approximately 21°C.