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The doubling time is the period of time required for a quantity to double in size or value. It is most often applied to population growth although it can be used for any increasing quantity, such as price inflation or the volume of malignant tumours. When the growth rate is constant, the quantity undergoes exponential growth (also known as geometric growth) and has a fixed doubling time which can be calculated directly from the growth rate. This time can be derived by dividing the natural logarithm of 2 by the exponent of growth, or approximated by dividing 70 by the percentage growth rate. For example, given Canada's net population growth of 0.9% in the year 2006, dividing 70 by .9 gives an approximate doubling time of 77.7 years. Thus if the growth rate remains constant, Canada's population will double from its current 33 million to 66 million by 2083. Examining the doubling time can give a more intuitive sense of the long-term impact of growth than simply viewing the percentage growth rate.
For a constant growth rate of r%, the formula for the doubling time Td is given by
Some doubling times calculated with this formula are shown in this table.
Doubling times Td given constant r% growth
| r% |
Td |
| 0.1 |
693.49 |
| 0.2 |
346.92 |
| 0.3 |
231.40 |
| 0.4 |
173.63 |
| 0.5 |
138.98 |
| 0.6 |
115.87 |
| 0.7 |
99.36 |
| 0.8 |
86.99 |
| 0.9 |
77.36 |
| 1.0 |
69.66 |
|
| r% |
Td |
| 1.1 |
63.36 |
| 1.2 |
58.11 |
| 1.3 |
53.66 |
| 1.4 |
49.86 |
| 1.5 |
46.56 |
| 1.6 |
43.67 |
| 1.7 |
41.12 |
| 1.8 |
38.85 |
| 1.9 |
36.83 |
| 2.0 |
35.00 |
|
| r% |
Td |
| 2.1 |
33.35 |
| 2.2 |
31.85 |
| 2.3 |
30.48 |
| 2.4 |
29.23 |
| 2.5 |
28.07 |
| 2.6 |
27.00 |
| 2.7 |
26.02 |
| 2.8 |
25.10 |
| 2.9 |
24.25 |
| 3.0 |
23.45 |
|
| r% |
Td |
| 3.1 |
22.70 |
| 3.2 |
22.01 |
| 3.3 |
21.35 |
| 3.4 |
20.73 |
| 3.5 |
20.15 |
| 3.6 |
19.60 |
| 3.7 |
19.08 |
| 3.8 |
18.59 |
| 3.9 |
18.12 |
| 4.0 |
17.67 |
|
| r% |
Td |
| 4.1 |
17.25 |
| 4.2 |
16.85 |
| 4.3 |
16.46 |
| 4.4 |
16.10 |
| 4.5 |
15.75 |
| 4.6 |
15.41 |
| 4.7 |
15.09 |
| 4.8 |
14.78 |
| 4.9 |
14.49 |
| 5.0 |
14.21 |
|
| r% |
Td |
| 5.5 |
12.95 |
| 6.0 |
11.90 |
| 6.5 |
11.01 |
| 7.0 |
10.24 |
| 7.5 |
9.58 |
| 8.0 |
9.01 |
| 8.5 |
8.50 |
| 9.0 |
8.04 |
| 9.5 |
7.64 |
| 10.0 |
7.27 |
|
| r% |
Td |
| 11.0 |
6.64 |
| 12.0 |
6.12 |
| 13.0 |
5.67 |
| 14.0 |
5.29 |
| 15.0 |
4.96 |
| 16.0 |
4.67 |
| 17.0 |
4.41 |
| 18.0 |
4.19 |
| 19.0 |
3.98 |
| 20.0 |
3.80 |
|
For example with an annual growth rate of 4.8% the doubling time is 14.78 years, and a doubling time of 10 years corresponds to a growth rate between 7% and 7.5%.
When applied to the constant growth in consumption of a resource, the total amount consumed in one doubling period equals the total amount consumed in all previous periods. This enabled US President Jimmy Carter to note in a speech in 1977 that in each of the previous two decades the world had used more oil than in all of previous history, because of the roughly exponential growth in world oil consumption between 1950 and 1970 with a doubling period of under a decade.
Given two measurements of a growing quantity, q1 at time t1 and q2 at time t2, and assuming a constant growth rate, you can calculate the doubling time as:
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