How many mobile phones are there in the world?

1. CLARIFY:
   1. Is it OK to focus on active phones (i.e. ignore phones in the trash, at home but no longer used, storage at phone providers or phone manufacturers, etc.)? Yes.
   2. Should I include both smart phones and feature phones? You choose. (I will include both.)
   3. Is it OK if I assume phone ownership likelihood based on income / ignore possibility of stolen phones, etc? Yes.
2. EQUATION: Total active mobile phones = Active smart phones + active feature phones 
3. BREAKDOWN UNKNOWNS:
   1. Eligible Phone Users (i.e. Proper Age): 
      1. Total World Population: ~8B
      2. Average Lifespan: 0-80
      3. Average Population / Age Year: 8B / 81 age years = ~99M people in each age year (assuming even distribution across ages)
      4. Average Age of Phone User: 15-80 (i.e. 65 years)
      5. Total Potential Phone User Population: 99M people * 65 years = 6.4B
   2. Population Breakdown by Country Income: 
      1. Let's classify countries by 1st - 3rd world. Let's estimate the total population of countries in each group by using the population of presumably the biggest country in that bucket.  
      2. Let's breakdown the population of potential phone users by 1st - 3rd world countries. 

         1. Country Classification	Example Country	Total World Population	% of World Population	Total Potential Phone User
            1st World	USA (Population 330M)	~1B	1B / 8B = 12.5%	.125 * 6.4B = 800M
            2nd World	China (Population 1.4B)	3.5B	3.5B / 8B = ~ 43.75%	.4375 * 6.4B = 2.8B
            3rd World	India (Population 1.4B)	3.5B	3.5B / 8B = ~ 43.75%	.4375 * 6.4B = 2.8B

   3. Breakdown of Income by Country Type: Later, we will use the income breakdown by country to estimate phone users.  %s are assumptions. Essentially, the greater the poverty level of a country, the higher the percentage of the population in the lowest income bucket.

      1. Country Classification	$	$$	$$$
         1st World	25%	60%	15%
         2nd World	60%	30%	10%
         3rd World	75%	15%	10%

   4. Smart Phone Users: 
      1. Connected to Internet: Assume that anyone who uses a smartphone must be connected to the Internet. Internet connection is assumption based on knowledge of average of 60% being connected to internet. 

         1. Country Classification	Total Potential Phone User Population	% Internet Connection	Total Potential Phone User Population with Internet
            1st World	800M	75%	.75 * 800M = 600M
            2nd World	2.8B	60%	.6 * 2.8B = 1.68B
            3rd World	2.8B	45%	.45 * 2.8B = 1.26B

      2. Likelihood of Phone: Let's assume that the likelihood an individual has a phone varies by country type.  Let's assume that the likelihood that an individual connected to the internet has a phone varies by country.

         1. Country Classification	Total Potential Phone User Population with Internet	Phone by Individual	Likelihood of phone population
            1st World	600M	1 phone / individual	1 * 600M = 600M
            2nd World	1.68B	1 phone / average family of 4	.25 * 1.68B = 420M
            3rd World	1.26B	1 phone / average extended family of 12 (3 average families of 4)	(1/12) * 1.26B = 105M

         2. Likelihood of Smart Phone: Given that smart phones are expensive, let's assume that in 1st world countries, there is a high presence of smart phones (based on personal observations), but in 2nd and 3rd world countries, only the middle / upper class families would have a phone. (% of country by $ listed above.)

            1. Country Classification	Likelihood of Phone Population	Likelihood of Smart Phone	Total Smart Phone Population
               1st World	600M	70% across all classes	600M * .7 = 420M
               2nd World	420M	50% across $$ and $$$ classes	(420M * .3 + 420M *.1) * .5 = 84M
               3rd World	105M	30% across $$ and $$$ classes	(105M*.15) + (105M *.1) * .3 = 7.9M

            2. Total Smart Phone Population: 420M from 1st world + 84M from 2nd world and 7.9M from 3rd world = ~512M
   5. Feature Phone Users: We can use similar calculations we did with smart phone users to help us find the population of those with feature phones.  
      1. Likelihood of Phone: With smart phones, we assumed that anyone who had one must be connected to the internet. Feature phones offer very basic funcitonality, so individuals with feature phones do not necessarily need internet connection. Let's assume the likelihood of someone having a phone though stays the same by country. 

         1. Country Classification	Total Potential Phone User Population	Likelihood of any phone	Likelihood of any phone Population	Likelihood of any phone Population - smart phone population
            1st World	800M	1 phone / individual	800M	800M - 420M = 380M
            2nd World	2.8B	1 phone / average family of 4	.25 * 2.8B = 700M	700M - 84M = 616M
            3rd World	2.8B	1 phone / average extended family of 12	(1/12) * 2.8B = 233M	233M - 7.9M = 225M

      2. # of Feature Phones by Country Classification: Feature phones are less expensive than smart phones, so let's assume the likelihood that any class ($ - $$$) could have a feature phone. Let's assume the likelihood that someone has a feature phone is higher in 1st world countries and lessens as the country poverty level increases. If you don't have a smart phone in a first world country, there is a high chance you have a feature phone.

         1. Country Classification	Likelihood of any phone Population - smart phone population	Likelihood of feature phone	Population with feature phones
            1st World	380M	90%	.9 * 380M = 342M
            2nd World	616M	50%	.5 * 616M = 308M
            3rd World	225M	30%	.3 * 225M = 67.5

      3. Total Feature Phones: 342M in first world + 308M in second world + 67.5M in third world = 717.5M
4. TOTAL MOBILE PHONES: 512M smart phones + 717.5M feature phones = 1.22B mobile phones
5. CAVEATS:
   1. Certain individuals may have second phone for business. Not included in this country. 

Assumptions:

1. We are referring to regular mobile phones only and not highly sophisticated devices such as Satellite phone

2. You need to be at least 15 years of age to own a phone

3. People who are poor cannot afford a mobile phone

4. Assume mobile phones owned by people only, i.e. we will ignore mobile phones that are unsold

Steps to Calculate: 

1. World population = 8Bn

2. Percentage of people that are above 15 years of age = 80%

3. Total number of people eligible to use a phone = 8 Bn * 80% = 6.4 Bn

4. Assume the following categories of people: Affluent, Higher Income, Moderate Income, Poor

5. Assume the demographic distribution by income as follows:

• Affluent: 10% = .64Bn
• Middle Class: 25% = 1.6 Bn
• Lower Income: 50% = 3.2 Bn
• Poor: 15% = .96

6. Assume that the adoption of mobile by each demographic is as follows:

• Affluent: 90% = 600 Mn
• Middle Class: 70% = 1120 Mn
• Lower Income: 30% = 960 Mn
• Poor: Negligible

• Affluent: 70% = 600 Mn + 420 Mn = 1020 Mn
• Middle Class: 5% = 1120 Mn + 56 Mn = 1176 Mn
• Lower Income: 0% = 960 Mn

1. Clarify the question
When you say "mobile phones", I assume there are generally two types: typical smartphones and feature phones, not including satellite phones that are used for special purposes, i.e., military or scientific research. I also assume that antique collections of phones are also not included in the scope of discussion.
 

2. Equation
Generally speaking,
Total Number of Phones = Total Number of Smartphones + Total Number of Feature Phones
Total Number of Smartphones in use = Population of the world * Penetration of smartphones
Total Number of Feature Phones in use = Population of the world * Penetration of feature phones

3. Assumptions
People usually get a new phone every two years in the developed countries, I assume people in developing countries, and the third world tend to keep their phones a bit longer; I am guessing four years. Unused phones rarely get recycled because, in my experience, I have never seen anyone recycle their old and unused phones, rather people leave them somewhere in the house. Most people have only one phone that they carry every day, only a few people have a secondary phone for work. Now, a given person that uses a smartphone probably has 2-6 old smartphones, given the fact that the first iPhone was introduced in 2007. (2019-2007) / 2 = 6. I assume the average is 3. Again a given person that uses a phone probably has few feature phones lying around, and I am guessing two because I started seeing people using mobile phones in about 2003. (2007-2003) / 2 = 2. Here I assume that people switch to smartphones around 2007. Moreover, of course, some people have never used a smartphone before, and start using mobile phones only recently. I am guessing these people may have just one feature phone.

4. Calculations
Now, by factoring in these assumptions, the equations expands to,

Total Number of Smartphones = # of Smartphones In use + # of old smartphones = 4* smartphones in use = 4* (Population of the world * Penetration of smartphones )

Total Number of feature phones = # of feature phone in use + # of old feature phones = Population of the world * Penetration of feature phones + 2*(Population of the world * Penetration of smartphones )

We know the Population of the world is roughly 7bn, and penetration of phones in total is about 70%, where I am guessing 40% are smartphones, and 30% are feature phones. Plugging in these numbers, we have:

Total Number of Smartphones = 4 *(7bn*0.4) = 11.2bn
Total Number of feature phones = 7bn*0.3+ 2*(7bn*0.4) = 2.1+ 5.6 = 7.7bn
Total Number of phones = 11.2 + 7.7 = 18.9 bn

5. Summary and Review
By introducing the age group, we may get even more accurate results; for example, young children do not have phones. However, it would be too complex to calculate.