Supply and Demand Shift Combinations: Every Case Solved

When a curve moves in the supply and demand model, the new equilibrium price and equilibrium quantity are decided by which curve shifted and in which direction. If only one curve shifts, both the price and the quantity have a single, definite answer you can read straight off the graph. If both curves shift at the same time, one of the two outcomes always becomes indeterminate, meaning its direction depends on which shift is larger. This guide gives the outcome for all four single shifts, lays out every combination in one nine-case table, then works through each double-shift case so you can predict the new equilibrium for any question.

The four single shifts

When just one curve moves, the other stays put, so the new equilibrium is exactly where the moved curve now crosses the fixed one. Both price and quantity are always determined. Remember that an increase in a curve is a shift to the right and a decrease is a shift to the left.

Demand increases (shifts right). Buyers want more at every price, so the demand curve slides right along a fixed supply curve. The equilibrium price rises and quantity rises. Example: a heat wave raises demand for cold drinks, pushing both the price and the amount sold up.

Demand decreases (shifts left). Buyers want less at every price. The equilibrium price falls and quantity falls. Example: a health scare cuts demand for a food, dropping both its price and the quantity traded.

Supply increases (shifts right). Sellers offer more at every price, so supply slides right along a fixed demand curve. The equilibrium price falls and quantity rises. Example: a bumper harvest raises the supply of coffee, so the price falls but more is sold.

Supply decreases (shifts left). Sellers offer less at every price. The equilibrium price rises and quantity falls. Example: a frost destroys part of the crop, so the price rises but less is sold.

Notice the pattern that makes single shifts easy. A demand shift moves price and quantity in the same direction. A supply shift moves them in opposite directions. You can drag either curve and watch the equilibrium move in the supply and demand sandbox.

The nine-case table

Combine the two curves. Demand can increase, decrease, or stay unchanged, and so can supply, which gives nine possible combinations. This table is the complete answer key. Read the row for demand and the column condition for supply, and take the two outcomes for the equilibrium price and the equilibrium quantity.

Every single-curve row has two definite answers. Every double-shift row has exactly one definite answer and one indeterminate answer. The rule behind the pattern is simple: when both curves push the same variable in the same direction, that variable is determined, and when they push it in opposite directions, it is indeterminate and depends on the relative size of the two shifts.

Both curves shift: how the indeterminate case is decided

When both curves move, picture each shift as two separate votes, one for the direction of price and one for the direction of quantity. If the two shifts vote the same way on a variable, that variable is settled. If they vote opposite ways, the outcome hangs on which shift is bigger, so it cannot be read off a single graph. The magnitude of the shifts breaks the tie.

### Both demand and supply increase

Both shifts push quantity up, so quantity definitely rises. On price they disagree: the demand increase pushes price up, the supply increase pushes price down, so price is indeterminate. If demand shifts more than supply, price rises; if supply shifts more, price falls; if they shift equally, price is unchanged. Intuition: think of concert tickets when fans' income rises (demand right) and cheaper staging technology cuts production costs (supply right). More tickets are sold for certain, but whether the ticket price climbs or falls depends on which force is stronger.

### Both demand and supply decrease

Both shifts push quantity down, so quantity definitely falls. On price they disagree: the demand decrease pushes price down, the supply decrease pushes price up, so price is indeterminate. A larger demand fall lowers price; a larger supply fall raises it. Intuition: a fading fad cuts demand for a gadget while a parts shortage cuts its supply. Fewer units change hands for sure, but the price could go either way depending on which drop dominates.

### Demand increases and supply decreases

Both shifts push price up, so price definitely rises. On quantity they disagree: the demand increase pushes quantity up, the supply decrease pushes quantity down, so quantity is indeterminate. If the demand increase is larger, quantity rises; if the supply decrease is larger, quantity falls. Intuition: gasoline when a holiday raises demand (right) at the same time as a refinery outage cuts supply (left). The price clearly climbs, but the amount sold depends on which shift is bigger.

### Demand decreases and supply increases

Both shifts push price down, so price definitely falls. On quantity they disagree: the demand decrease pushes quantity down, the supply increase pushes quantity up, so quantity is indeterminate. A larger supply increase raises quantity; a larger demand decrease lowers it. Intuition: a laptop model going out of fashion (demand left) just as a factory upgrade floods the market with units (supply right). The price clearly falls, but total sales could rise or fall depending on the relative size of the shifts.

Common exam mistakes

Reading both outcomes off a double-shift graph. This is the mistake that costs the most marks. When both curves shift, you can draw one specific picture in which, say, price happens to rise, but that is only one of several possible drawings. Because the indeterminate variable depends on the relative size of the shifts, the correct answer is literally the word indeterminate, not whatever your particular sketch shows. If you draw both shifts and confidently report both a price and a quantity direction, you have almost certainly reported the indeterminate one wrong.

Forgetting that increase in supply means a rightward shift. An increase in supply lowers the equilibrium price, which feels backward until you remember that increase means the whole curve moves right, so sellers offer more at every price. Students who read increase in supply as a higher price have reversed the shift. The same trap catches an increase in demand, which raises price. Say it deliberately: increase equals rightward shift, and only then trace the new crossing point.

Confusing a shift with a movement along the curve. A change in the good's own price moves you along a fixed curve and does not shift anything. Only the determinants of demand or the determinants of supply, such as income, input costs, or the number of sellers, shift a whole curve. If the price changed because a curve moved, do not also treat that price change as a separate shift.

Assuming price and quantity always move together. They move together only for demand shifts. For supply shifts they move in opposite directions, and for double shifts one of them may be indeterminate. Work out the direction of each variable separately every time rather than assuming they travel as a pair.

Practice and connect

Every case here builds on a solid grasp of a single equilibrium, so make sure the market equilibrium model is automatic first, and that you can tell a shortage from a surplus when the price is off target. Then drill the shifts by dragging both curves in the supply and demand sandbox, and step through fully worked shift scenarios in the graph walkthroughs. Reinforce the underlying terms, the equilibrium price, quantity demanded, and quantity supplied, in the supply and demand module until you can name both outcomes, or correctly call one indeterminate, for any combination of shifts.