Fundamentals of Business Economics and Management · The Fundamentals of Economics

Supply and Law of Supply for CMA Foundation

Updated 10 October 2026 · Fact-checked

Supply is the quantity of a good a seller is willing and able to offer for sale at a given price in a given period. The law of supply says that, other things remaining equal, a higher price raises quantity supplied and a lower price reduces it. The supply curve slopes upward.

Understand Supply and Law of Supply

Supply is not the same as stock. Stock is what a seller holds. Supply is the part the seller is willing and able to sell at a particular price, in a particular time period and market.

The law of supply says: other things remaining the same, as the price of a good rises, its quantity supplied rises, and as the price falls, quantity supplied falls. Price and quantity supplied move in the same direction. This happens because a higher price means higher profit per unit, so sellers want to produce and sell more. Also, producing more usually raises cost per unit, and only a higher price makes that worthwhile.

The law holds only under assumptions: the prices of other goods, input costs, technology, taxes and subsidies, and the seller's goals stay unchanged. This is the meaning of ceteris paribus (other things equal). If any of these changes, the law does not describe what happens.

A supply schedule is a table of prices and quantities supplied. A supply curve is its graph, with price on the vertical axis and quantity on the horizontal axis. It slopes upward from left to right. An individual supply curve is for one seller. The market supply curve adds up the quantities of all sellers at each price.

The key exam idea is the difference between two kinds of movement. A change in the good's own price causes a movement along the same curve: extension (price up, quantity up) or contraction (price down, quantity down). A change in any other determinant shifts the whole curve: increase in supply (shift right) or decrease in supply (shift left).

Key formulas to remember

Law of supply: Price ↑ ⇒ Quantity supplied ↑; Price ↓ ⇒ Quantity supplied ↓; Holds only when other determinants stay constant. Supply curve slopes upward.
Supply function: Sx = f(Px, Pr, Pf, T, G, Tx, O); Px = own price, Pr = price of related goods, Pf = price of inputs, T = technology, G = firm's goal, Tx = taxes/subsidies, O = other factors such as weather and number of sellers.
Movement along the curve: Change in own price ⇒ extension or contraction of supply; Same curve, different point.
Shift of the curve: Change in any other determinant ⇒ increase (right shift) or decrease (left shift) in supply; A new curve is formed.
Market supply: Market supply at a price = sum of all sellers' quantities at that price; Add horizontally across sellers.

How to solve Supply and Law of Supply questions

Most supply questions test whether a change is a movement along the curve or a shift of the curve. Use this method.

1Read the question and find what changed: the good's own price, or something else.
2If the own price changed, the answer is extension (price rises) or contraction (price falls). The curve does not shift.
3If another factor changed, such as input cost, technology, tax, subsidy or number of sellers, the curve shifts.
4Decide the direction of the shift: anything that raises profit or lowers cost increases supply (right); anything that raises cost or lowers profit decreases supply (left).
5For schedule questions, add the sellers' quantities at each price to get market supply.
6For theory questions on the law, state the rule, then the assumption (other things equal), then the reason, then the exceptions if asked.
7Check the option wording: 'extension/contraction' means movement; 'increase/decrease' means shift.

Quickest way: Own price or something else?

When to use it: Use this for any MCQ asking which term describes a change in supply.

Ask: did the good's own price change? If yes, pick extension or contraction.
If no, the answer is increase or decrease in supply.
For direction, ask: does this make production cheaper or more profitable? If yes, right shift (increase). If no, left shift (decrease).
Remember a tax on the good raises cost and reduces supply; a subsidy lowers cost and increases supply.
Eliminate options that mix the two ideas, such as 'extension due to fall in input cost'.

Common mistakes in Supply and Law of Supply

Calling a price-led change an 'increase in supply'.
In daily speech, 'supply increased' covers any rise in quantity.
Fix: If the cause is the good's own price, the term is extension. Use 'increase' only for a rightward shift caused by other factors.
Treating supply as the same as stock.
Both words sound like goods available.
Fix: Supply is the quantity offered for sale at a price. Stock is the total held. Supply cannot exceed stock but can be much less.
Thinking higher input costs increase supply.
Students link 'higher' with 'more'.
Fix: Higher input costs cut profit, so the curve shifts left: a decrease in supply.
Forgetting the 'other things equal' assumption in theory answers.
Students memorise only 'price up, supply up'.
Fix: Always write the law with ceteris paribus and list what is assumed constant.
Showing a subsidy as a left shift.
Students confuse subsidy with tax.
Fix: A subsidy lowers cost per unit, so supply increases and the curve shifts right. A tax does the opposite.
Assuming the supply curve is always upward sloping.
The law is learnt as absolute.
Fix: Know the exceptions: agricultural goods in the short run, rare items, backward-bending labour supply, and expected future price rises.

Worked examples

Example 1

The price of wheat flour rises from ₹30 to ₹40 per kg and a mill raises its quantity supplied from 500 kg to 700 kg. Which term describes this, and does the curve shift?

Show the solution

The change was caused by the good's own price, which rose.
So this is a movement along the same supply curve.
Quantity supplied rose with price, so it is an extension of supply.
No other determinant changed, so the curve does not shift.

Answer: Extension of supply; there is a movement along the same curve, with no shift.

Example 2

At a price of ₹20, Seller A supplies 40 units and Seller B supplies 60 units. At ₹30, A supplies 70 units and B supplies 90 units. Find market supply at each price. Later, a new technology lowers cost and each seller supplies 20 more units at every price. What is the new market supply at ₹30, and what is this change called?

Show the solution

Market supply at ₹20 = 40 + 60 = 100 units.
Market supply at ₹30 = 70 + 90 = 160 units.
New technology lowers cost, which is a non-price factor, so the curve shifts.
Each of the 2 sellers adds 20 units at every price, so the total adds 2 × 20 = 40 units.
New market supply at ₹30 = 160 + 40 = 200 units.
Supply rises at the same price, so the curve shifts right.

Answer: Market supply is 100 units at ₹20 and 160 units at ₹30. After the technology change it is 200 units at ₹30, an increase in supply (rightward shift).

Exam tips

Expect MCQs that give a situation and ask you to choose between extension, contraction, increase and decrease. Check the cause first.
Learn the list of determinants with direction: input cost up means supply down, technology better means supply up, tax up means supply down, subsidy means supply up.
Remember that the law of supply needs ceteris paribus; options that omit it as a condition are often wrong.
Know the exceptions to the law: agricultural goods, rare and perishable items, backward-bending labour supply, and sellers expecting higher future prices.
There is no negative marking, so attempt every question and use elimination when unsure.

Practice questions from The Fundamentals of Economics

Supply and Law of Supply in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Supply and Law of Supply: frequently asked questions

What are the assumptions of the law of supply?

The law assumes other things remain equal. Input prices, technology, taxes and subsidies, prices of related goods, the seller's goal and the number of sellers do not change. Only the good's own price changes.

What is the difference between extension and increase in supply?

Extension is a rise in quantity supplied caused only by a rise in the good's own price, shown as a movement up along the same curve. Increase in supply is a rise caused by other factors, such as better technology, and the whole curve shifts right.

What are the main determinants of supply?

They are the price of the good, prices of related goods, prices of inputs, state of technology, taxes and subsidies, the firm's goals, the number of sellers and natural factors such as weather. Except the own price, all of them shift the curve.

What are the exceptions to the law of supply?

The supply curve may not slope upward for agricultural goods in the short run, rare or unique items, and labour at very high wages, where it bends backward. It can also differ when sellers expect prices to rise further and hold back stock.