An electronic or mercury thermometer is better for measuring basal body temperature 

To measure basal body temperature, both glass (mercury, mercury free) and electronic thermometers are suitable. The first type is more accurate and easy to care for, the second – is more modern, faster and does not require time control. 

What is the proper way to use a thermometer for measuring basal body temperature? What is the minimal amount of minutes needed to measure basal body temperature? And also, the advantages and disadvantages of each device, that you definitely need to know, are described further in this article. 

Is it possible to measure basal body temperature with an ordinary thermometer?

A common glass thermometer is certainly suitable for measuring basal body temperature. 

Important(!): It is necessary to use the same device, since the glass thermometer is very sensitive and each has its own level of error. If, for example, you measure BBT with several mercury thermometers, the results can be very different with a deviation of up to 0.2 °C (0.36 °F) . It is very important to get accurate, undistorted results. 

It is more reliable to measure basal body temperature with a mercury or mercury free thermometer; you should keep it in from 5 to 8 minutes.

Preparation for use: 

  • wash; 
  • dry; 
  • shake to a mark below 35 °C or 95 °F; 
  • prepare near your bed with oil or cream (any oil, baby cream, vaseline petroleum jelly is suitable) – only necessary for rectal measurement. 

Note(!): It is enough to take just a small amount of lubricant and apply up to 2-3 cm on the measuring end of the thermometer. In order not to overdo with insertion – it is better to hold a thermometer at the right distance from the end. Thus, you can protect yourself from unnecessary diving in too deep. 

How long does it take to measure basal body temperature with a mercury thermometer?

To decide how long should a thermometer be kept in, when measuring basal body temperature, you would need to consider the measurement method. If BBT is measured: 

  1. Orally – about 8 minutes;
  2. Rectally – about 5 minutes;
  3. Vaginally – about 5 minutes. 

It is necessary to check the result immediately after the measurement, otherwise the mercury column may drop by 0.05° С (0,9 °F) or more after some time. The thermometer must be washed with ordinary soap or other body wash after each use. 

Pros and Cons of measuring basal body temperature with a glass thermometer 

Pros: high sensitivity and accuracy, easy to wash, cheap (mercury), available at any pharmacy (mercury), safe (mercury free). 

Cons: easy to break, contains dangerous substances (mercury), measurement takes 5-8 minutes and you should check the results immediately. 

Measuring basal body temperature with an electronic thermometer

With the help of this smart device you would quickly measure BBT, but it is mainly used for the oral method of measurement. The result is immediate and you are not bothered by time control and memorization of indicators.

To measure BBT orally – quickly put the thermometer on either side of your mouth, under the tongue, and put your lips together. 

If technical specification allows for a rectal measurement, the thermometer should be inserted into the rectum 1.5 – 2.5 cm deep. 

Tip (!) To maintain the purity of the instrument, when measuring rectally, you can use inexpensive condoms or cut off the fingers of sterile latex gloves. 

Is an electronic thermometer suitable for you?

If you decided to measure your basal body temperature with an electronic thermometer, consider the following:

  • it is an expensive device; 
  • it is not as sensitive as the glass thermometer – therefore, most often the curve on the graph will be less accurate; When measuring basal body temperature with an electronic thermometer, the line is smoother. 
  • the sound signal could be too quiet or vice versa – a loud squeak could wake up your spouse; 
  • you must study the instructions. For some, it is allowed to be used orally and or even rectally, but at the same time, there is a note that the device is not waterproof; 
  • the device is difficult to be cleaned properly – again, you need to study the instructions in advance. 

There are advantages

  • safe to use – no health risk if it breaks; 
  • you do not need to count minutes. A sound signal will notify you that the measurement is complete after about 1 minute (although the manufacturer recommends waiting another 2-3 minutes after the signal) 
  • many electronic thermometers keep a record of your measurements, which is also very convenient. 

I successfully used both glass (mercury,  mercury free) and electronic thermometers for correctly determining the ovulation day. Electronic – orally, glass – rectally (mercury is my favorite!). 

How to calculate the ovulation with BBT you can read in this article on

I hope this article helps you to decide which thermometer is right for you! 

Glass vs electronic thermometer for BBT measuring (+Tips!)