Birth, death and fertility rates leave a fingerprint on a population: how many people there are of each age. A population pyramid draws that fingerprint.
Key idea
A population pyramid is two bar graphs back to back: males on the left, females on the right. Each bar is one age group (0–4, 5–9, …), youngest at the bottom. The length of a bar is the share of the whole population (or the number of people) in that group today.
Why it works
Everyone in the 40–44 bar was born 40 to 44 years ago, so each bar records a past group of births, minus the people who have since died or moved away, plus those who moved in. Read upward and you are reading backward in time. A wide bar at 20–24 above narrower bars below means births have fallen in the last twenty years.
Worked examples
Worked example: Reading a bar
On a pyramid, the 10–14 bar shows 6% male and 5.8% female. What share of the population is 10 to 14 years old?
- Add the two sides: 6 + 5.8 = 11.8.
- Each side is a share of the whole population, so the total is 11.8%.
Answer: 11.8% of the population.
Worked example: Under 15
The bars for 0–4, 5–9 and 10–14 total 8%, 7.5% and 7% (both sexes). What share is under 15?
- Under 15 means the three youngest groups: 0–4, 5–9 and 10–14.
- 8 + 7.5 + 7 = 22.5.
Answer: 22.5%.
Common mistake
Students read the bars as births each year, or as the number of people who died. Each bar is how many people of that age are alive now. A narrow base means few young children today, which usually means fewer births in recent years. Check: the bars of a percentage pyramid add up to 100%.