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The supply curve slopes upward from left to right because higher prices increase potential profit per unit, which motivates firms to produce more. Free notes.
Drawing and Interpreting Supply Curves is part of Supply in GCSE OCR Economics.
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GCSE OCR
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The supply curve slopes upward from left to right because higher prices increase potential profit per unit, which motivates firms to produce more. Free notes.
Use this lesson to stay inside Supply while connecting the detail back to the wider The role of markets and money topic.
Read the core explanations from Drawing and Interpreting Supply Curves before testing yourself from memory.
In the previous lesson, you learned what supply is and what factors affect it. Now it is time to put that knowledge onto a graph. Supply curves are one of the most important diagrams in GCSE Economics — you will almost certainly need to draw or interpret one in your exam. Getting the diagram right, and understanding the difference between shifts and movements, is essential for picking up marks.
Think of a supply curve as a visual story: it tells you exactly how much of a product firms are willing to sell at every possible price. If you can read this story correctly, you can explain what happens when costs change, when the government intervenes, or when new firms enter a market.
To draw a supply curve, you need a supply schedule — a table showing the quantity a firm is willing to supply at different prices. Let's look at an example for a small UK bakery producing sourdough loaves.
| Price per loaf (£) | Quantity supplied per week |
|---|---|
| 1.00 | 10 |
| 2.00 | 25 |
| 3.00 | 45 |
| 4.00 | 70 |
| 5.00 | 100 |
To construct the supply curve from this data:
Step 1: Draw your axes. The vertical axis (y-axis) shows Price (£). The horizontal axis (x-axis) shows Quantity supplied. Always label both axes clearly — unlabelled axes lose marks.
Step 2: Plot each pair of values from the table as a point on the graph. For example, the first point is (10, £1.00) and the last is (100, £5.00).
Step 3: Join the points with a smooth line or curve. Label it S (for Supply).
Notice how the line slopes upward from left to right. This is characteristic of all supply curves and reflects the law of supply: as price rises, quantity supplied increases.
Worked example: Constructing a Supply Curve from Production Data
A small UK electronics firm produces wireless speakers. Use the data below to plot the firm's supply curve and describe its shape. Price (£): 15, 20, 25, 30, 35 Quantity supplied per month: 50, 80, 120, 170, 230
Method: Plot points from the data and join with a curve
Answer: The supply curve slopes upward from left to right, showing that as the price of wireless speakers increases, the quantity the firm is willing to supply also increases. This is consistent with the law of supply.
The upward slope of a supply curve is not just a rule to memorise — there are clear economic reasons behind it:
Reason 1: Profit motive. When the price of a product rises, the potential profit per unit increases (assuming costs stay the same). This motivates existing firms to produce more. A British farmer will plant more wheat if the market price per tonne doubles because each tonne earns more revenue.
Reason 2: Covering higher marginal costs. As firms try to produce more, they often face rising costs per additional unit (called marginal costs). They may need to pay overtime wages, use less efficient machinery, or source more expensive materials. A higher selling price makes it worthwhile to cover these extra costs and still make a profit.
Reason 3: New firms enter. Higher prices attract new producers into the market. If coffee prices rise, entrepreneurs may open new coffee shops because the market looks more profitable. This adds to total supply.
This is one of the most important distinctions in GCSE Economics, and one of the most common sources of lost marks. You must be able to tell the difference between a movement along the supply curve and a shift of the supply curve.
| Feature | Movement Along | Shift Of |
|---|---|---|
| What causes it? | A change in the PRICE of the good | A change in a NON-PRICE factor (costs, tech, taxes, etc.) |
| What happens to the curve? | The curve stays in the same position | The entire curve moves left or right |
| What do we call it? | Extension (price up) or Contraction (price down) | Increase in supply (shift right) or Decrease in supply (shift left) |
| Example | Price of coffee rises from £3 to £4, so cafes sell more cups | Cost of coffee beans falls, so cafes can supply more at every price |
| On the diagram | Move to a different point ON the same curve | Draw a whole new curve to the left or right of the original |
Memory trick: Think of it this way — if the price changes, you move along the curve (the price is already on the axis, so you just slide along). If anything else changes (costs, technology, tax, subsidy, number of firms, weather), the whole curve shifts.
Rightward shift (increase in supply): The curve moves to the right. At every price, firms now supply a greater quantity. This happens when costs fall, technology improves, subsidies are given, more firms enter, or weather conditions are favourable.
Leftward shift (decrease in supply): The curve moves to the left. At every price, firms now supply a smaller quantity. This happens when costs rise, taxes are imposed, firms leave the market, or bad weather strikes.
| Price (£) | Original Qty Supplied (S1) | After cost fall — Qty Supplied (S2) | After tax imposed — Qty Supplied (S3) |
|---|---|---|---|
| 1.00 | 10 | 20 | 5 |
| 2.00 | 25 | 40 | 15 |
| 3.00 | 45 | 65 | 30 |
| 4.00 | 70 | 95 | 50 |
| 5.00 | 100 | 130 | 75 |
Look at the table above. At every price, S2 (after a cost fall) has a higher quantity than S1 — the supply curve has shifted right. S3 (after a tax) has a lower quantity at every price — the curve has shifted left. This is what we mean by the 'whole curve moving'.
Worked example: Identifying Shifts vs Movements
A UK sandwich shop faces two changes: (a) The price of sandwiches increases from £3.50 to £4.50. (b) The cost of bread (a key ingredient) falls by 20%. For each change, state whether it causes a movement along or a shift of the supply curve, and explain why. [4 marks]
Method: Apply the rule: price change = movement, non-price change = shift
Answer: (a) A movement along the supply curve (extension) because the price of the good has changed. (b) A shift of the supply curve to the right because a non-price factor (cost of an ingredient) has changed, increasing supply at every price.
Worked example: Reading Supply Data and Explaining a Shift
The table below shows the supply of UK-grown tomatoes before and after a government subsidy was introduced. Price per kg: £1, £2, £3, £4 Quantity supplied before subsidy (thousands): 20, 40, 60, 80 Quantity supplied after subsidy (thousands): 35, 55, 75, 95 (a) Describe what has happened to the supply curve. [2 marks] (b) Explain why the subsidy has had this effect on supply. [4 marks]
Method: Compare the data, identify the shift, then explain using economic reasoning
Answer: (a) The supply curve has shifted to the right — at every price, the quantity supplied is higher after the subsidy. (b) The subsidy lowers production costs for tomato growers, making it more profitable to grow tomatoes. This means farmers are willing and able to supply more at every price level, causing the entire supply curve to shift rightward.
Every year, examiners report that students lose marks on supply curve questions because of a few recurring errors. Let's make sure you avoid them.
| Mistake | Why It Is Wrong | What to Do Instead |
|---|---|---|
| Drawing the supply curve sloping downwards | A downward slope shows demand, not supply | Supply curves always slope upward from left to right |
| Saying a price change causes a 'shift' in supply | A price change causes a movement along the curve, not a shift | Only non-price factors (costs, technology, taxes, etc.) cause shifts |
| Not labelling axes or curves | Examiners cannot award marks for an unlabelled diagram | Always label: Price (£) on y-axis, Quantity on x-axis, and label the curve S |
| Using 'increase in supply' for a movement along the curve | 'Increase in supply' means a rightward shift of the whole curve | Use 'extension of supply' for a price rise, 'contraction' for a price fall |
| Confusing supply shifts with demand shifts | A rise in consumer income shifts demand, not supply | Ask yourself: does this factor affect producers (supply) or consumers (demand)? |
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-24.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Marginal cost is the cost of producing an additional unit. As firms produce more, marginal costs often rise because of overtime wages, less efficient machinery or more expensive materials, so a higher price is needed to make extra output worthwhile.
Draw and label the axes, with Price (£) on the vertical axis and Quantity supplied on the horizontal axis. Plot each price-quantity pair from the supply schedule, join the points with a smooth upward-sloping line and label it S.
The supply curve slopes upward from left to right because higher prices increase potential profit per unit, which motivates firms to produce more. A higher price also covers the rising marginal costs of producing extra units, and attracts new firms into the market.
A movement along the supply curve is caused by a change in the price of the good itself: a price rise is an extension and a price fall is a contraction. A shift of the supply curve is caused by a change in a non-price factor, such as costs, technology or taxes, and moves the whole curve left or right.
The supply curve shifts right, an increase in supply, when costs fall, technology improves, subsidies are given, more firms enter the market or weather conditions are favourable. At every price, firms now supply a greater quantity.
An extension of supply is a movement up along the supply curve caused by a rise in the good's own price. An increase in supply means a rightward shift of the whole curve caused by a non-price factor, so more is supplied at every price.
A supply schedule is a table showing the quantity a firm is willing to supply at different prices. For example, a bakery might supply 10 sourdough loaves a week at £1.00 and 100 at £5.00, and each pair of values becomes a point on the supply curve.
Price goes on the vertical axis (y-axis) and quantity supplied goes on the horizontal axis (x-axis). Putting them the other way round is a common mistake, and both axes must be labelled.
A contraction of supply is a movement down the supply curve caused by a fall in the price of the good itself. The curve does not move; firms simply supply a smaller quantity at the lower price.
The supply curve shifts left, a decrease in supply, when costs rise, taxes are imposed, firms leave the market or bad weather strikes. At every price, firms then supply a smaller quantity.
A change in the price of the good itself never shifts the supply curve; it causes a movement along it. Only non-price factors, such as costs, technology, taxes, subsidies, the number of firms or weather, shift the curve.
A rise in consumer income shifts demand, not supply. To decide which curve moves, ask whether the factor affects producers (supply) or consumers (demand).