To compare demand and supply, subtract one from the other, name the result as a surplus or a deficit, and turn it into a percentage of the right base. The skill appears with water, energy, food and minerals, in tables, bar charts and short case texts.
It follows explaining a location choice using evidence, because supply of a resource is often one of the evidence points. It sits in the economic activities and resources module.
What does the comparison show?
Supply is how much of a resource is available. Demand is how much people, farms and industry want to use.
When demand is larger, the area has a deficit and may need to import, ration, store or find a new source. When supply is larger, there is a surplus.
A deficit does not always mean a crisis. The size of the gap compared with supply tells you how serious it is.
How do you compare step by step?
- Add the demand parts if demand is split by user, such as homes, industry and farming.
- Compare totals: demand minus supply gives a positive number for a deficit.
- Name the result as a deficit or a surplus, with units.
- Express it as a percentage of supply (or whatever base is asked): gap ÷ base × 100.
- Write a conclusion that links the figure to its effects.
Worked example
The island of Pulau Lestari is invented for this example. Its water supply is 450 000 m³ a day. Daily demand is shown below.
| User | Demand (m³ per day) |
|---|---|
| Homes | 220 000 |
| Industry | 140 000 |
| Farming | 130 000 |
Total demand. 220 000 + 140 000 + 130 000 = 490 000 m³ per day.
Gap. 490 000 − 450 000 = 40 000 m³ per day. Demand is larger, so this is a deficit.
Percentage of supply. 40 000 ÷ 450 000 = 0.0889, so 8.9% of supply (1 decimal place).
Check. 450 000 × 0.089 = 40 050, close to 40 000, so the percentage is reasonable.
Largest user. Homes use 220 000 ÷ 490 000 = 0.449, so about 44.9% of demand.
Conclusion. “Pulau Lestari has a deficit of 40 000 m³ a day, about 8.9% of supply. Homes are the largest user at about 45% of demand, so reducing household waste could close much of the gap. If demand rises by 10 000 m³ each year, the deficit reaches 70 000 m³ in three years.”
The last line is a simple projection: 40 000 + 3 × 10 000 = 70 000.
What is the mistake to watch for?
A frequent slip is to divide by the wrong base, or to read the table as if low supply alone means shortage.
Mistaken answer: The deficit is 40 000 ÷ 490 000 = 8.2% of supply.
The student divided by demand but called the result a percentage of supply. The figure 8.2% is correct only as a percentage of demand.
The correction is to choose the base first, write its name, and then divide. Also remember that a small supply is not a shortage if demand is smaller still.
Check yourself
1. An invented region has electricity supply of 800 GWh and demand of 920 GWh in a year. Find the deficit as a percentage of supply.
Show answer
920 − 800 = 120 GWh deficit. 120 ÷ 800 = 0.15, so 15% of supply.
2. An invented farming area has food supply of 1 250 tonnes and demand of 1 000 tonnes. State the surplus as a percentage of demand.
Show answer
1 250 − 1 000 = 250 tonnes surplus. 250 ÷ 1 000 = 0.25, so 25% of demand.
3. Why is a deficit of 40 000 m³ a day more serious for a small island than the same figure for a large country?
Show answer
It is a larger share of the island’s supply and the island may have fewer alternative sources. The percentage of supply shows the seriousness better than the raw figure.
Where does this lead next?
The next lesson, tracing a production network without inventing data, uses the same habit of sticking to the evidence. The geography evidence and case-study ledger helps you record the source, place and year of any figure you use. Try the mixed practice set when you are ready.
Percentage questions are where many students lose a mark on a base or a unit. In online one-to-one Geography tuition, a teacher can watch your working and catch these slips as they happen.