Why does a 2% supply shortfall move the price 40%?

Equity markets move on stories about the future. Physical commodity markets move on a much cruder fact: today, somebody has to deliver an actual barrel, and somebody has to physically take it. Neither side can change its mind quickly — and that single constraint is why commodities are the most violent thing you will study in this academy.

Elasticity in one line

Elasticity measures how much quantity responds to price:

elasticity = (% change in quantity) ÷ (% change in price)

A value near −1 describes a responsive market: price up 10%, quantity down 10%. A value near −0.05 describes a nearly frozen one: price up 10%, quantity down half a percent.

Now rearrange it, because the rearranged version is the whole course in one line:

% change in price = (% change in quantity) ÷ elasticity

Dividing by a small number produces a large number. That is not a metaphor for commodity volatility — it is commodity volatility.

Why supply cannot answer quickly

In most markets, a high price summons more supply within weeks. In commodities, the supply curve near full capacity is close to vertical, for physical reasons:

• Wells, mines, smelters and refineries have a maximum flow rate. Above it, no price helps.
• Building new capacity takes years (a copper mine) or at best months (a shale well) or a whole season (a crop).
• Idle capacity that can be switched on quickly — spare capacity — exists in only a few places and only for a few commodities.

So in the short run the world produces roughly what it produced last month, whatever the price does.

A worked example

Rounded, illustrative numbers. The world consumes about 100 million barrels of oil a day. An outage removes 2 million barrels a day — a 2% shortfall.

Those barrels do not exist. Storage aside, somebody has to consume 2% less. How much must the price rise to make that happen?

• Short-run demand elasticity for oil is small. Take −0.05 (published estimates cluster in a −0.02 to −0.1 range).
• Required price change = 2% ÷ 0.05 = 40%.

Change one assumption and watch the answer swing:

• Elasticity −0.10 → 2% ÷ 0.10 = 20%.
• Elasticity −0.30 (a plausible long-run figure, once people can buy a different car or a different boiler) → ≈ 7%.

Same missing barrels. A 40% move today, a 7% move over years. The commodity is not more "risky" in some mystical sense — the market is simply being asked to clear an imbalance with the only tool it has left.

Try it now

1. Five years of crude and five years of broad equity are below. Measure the largest peak-to-trough fall you can find inside a single calendar year on each, and write the two percentages side by side. The gap between them is elasticity, made visible.

2. Do the arithmetic yourself: with elasticity −0.05, what price move does a 1% shortfall imply? With −0.2?
3. Write down the sentence you will reuse all course: small physical imbalances become large prices when neither side can move.

A note on what we do here. EODHD Academy teaches how markets work. Nothing here is a recommendation to buy, sell, hold or avoid any commodity, and nothing here forecasts a price. We describe mechanisms and read data; we do not predict or prescribe.