Enter the lowest price at which the first unit would be supplied (the supply curve’s intercept), the market price, and the quantity sold to size the welfare gain that flows to producers under a linear supply curve.
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What it calculates: Producer Surplus, Price Gap, Total Revenue.
The first unit earns $20 of surplus, the fortieth earns nothing
A supply curve is a ranking of units by how cheaply they can be made. Enter a minimum supply price of $10, a market price of $30 and a quantity of 40, and you have described a market whose first unit comes from the lowest-cost source available — the most efficient producer, the easiest deposit to extract, the machine that was sitting idle — and whose fortieth unit comes from whoever could only be coaxed into producing once the price reached $30. Because a competitive market pays one price for every unit, that first seller banks the whole $20 gap between the $30 they receive and the $10 they would have accepted. The fortieth seller banks nothing at all: for them the price and the cost are the same number.
Every unit in between sits somewhere on that ladder, and with a straight-line supply curve the margin shrinks evenly from $20 down to zero. Stack those shrinking margins side by side and they form a right triangle — bounded above by the $30 price line, below by the supply curve, and on the left by the price axis. Its base is the 40 units sold and its height is the $20 price gap, so its area is ½ × $20 × 40 = $400. That is the producer surplus reported here, shown alongside the $20 price gap and the $1,200 of total revenue ($30 × 40) that it sits inside.
The number means something concrete: sellers as a group received $400 more than the minimum they would have accepted to bring 40 units to market. Only the marginal unit is priced at its true cost. Every inframarginal unit — every unit before the last one — is sold for more than it cost to make, and the sum of those gaps is the surplus. No cash changes hands to represent it, which is why the figure never appears on an income statement, but it is real economic benefit rather than an accounting artefact.
The minimum supply price is the cheapest unit’s cost, not your average cost
The input most often mistyped is the minimum supply price. It is the supply curve’s vertical intercept: the price at which the very first, lowest-cost unit would be offered. Below that price no producer finds selling worthwhile, so quantity supplied is zero. It is not the average cost across your output, and it is not the price you would need to break even once overheads are covered.
The difference is not cosmetic, because average cost sits above the intercept whenever the supply curve slopes up at all. Feed the calculator an average cost in place of the intercept and the price gap shrinks, so the surplus comes out too small. In the worked figures, typing $20 instead of $10 halves the gap to $10 and halves the surplus to ½ × $10 × 40 = $200 — a $200 error produced by one wrong input, with nothing on screen to flag it.
The same care applies to the other two fields. The model assumes a single clearing price received by every seller for every unit, so if some units went out at a discount, on a long-term contract, or through a different channel, the triangle has nothing to say about them. It also assumes the quantity you enter is the quantity actually supplied and sold at that price, not a production forecast and not a capacity figure. Match all three inputs to one market at one moment and the triangle describes that market; mix figures from different periods or price tiers and it describes nothing in particular.
Why $400 of surplus is not $400 of profit
This is the distinction that trips up nearly everyone meeting the diagram for the first time. Producer surplus is the market price minus variable, or marginal, cost, summed over every unit sold. It ignores fixed costs entirely. Economic profit subtracts those as well. The two numbers coincide only when fixed costs are zero, which is almost never.
Read it through the worked figures. Forty units at $30 bring in $1,200 of revenue, and the surplus is $400, which means the variable cost of producing all 40 units was $1,200 − $400 = $800. Nothing in that arithmetic has paid for the factory, the lease, the licence or the machine. If fixed costs come to less than $400 the firm is ahead; if they come to more, the firm is losing money in a market whose surplus triangle still looks perfectly healthy.
That gap is exactly why the two measures are kept separate. In the short run, with the plant already built and the money already spent, a firm should keep producing as long as producer surplus is positive, because the surplus contributes toward fixed costs that are sunk either way. In the long run, when those costs become avoidable, only economic profit answers whether staying in the market is worth it. Quoting the surplus as take-home earnings overstates performance by the whole of the fixed-cost base, and it does so silently, because fixed costs appear nowhere on the diagram.
Double the price gap and surplus doubles; double the quantity too and it quadruples
Because the surplus is a triangle, it scales with the product of two things rather than one. Hold output at 40 units and lift the market price from $30 to $50: the gap over the $10 intercept doubles from $20 to $40, and surplus doubles to ½ × $40 × 40 = $800. Now let that higher price also draw out twice the output, 80 units, and the surplus becomes ½ × $40 × 80 = $1,600 — four times the original $400. A price rise that also expands quantity compounds, which is why sellers care about both at once.
This is also the reason producer and consumer surplus pull in opposite directions. Producer surplus rises with price; consumer surplus falls with it. The same price movement that fattens one triangle thins the other, so most arguments about pricing are fights over how the total is split rather than over how big it is.
Price is only half the story. Moving along the supply curve, which is what a price change does, is a different event from shifting the curve itself. A fall in input costs, a productivity gain, or cheaper energy lowers the whole curve, dropping the intercept and widening the margin on every unit even when the market price never moves. In this calculator that shows up as a lower minimum supply price, not a higher equilibrium price, and the surplus grows for an entirely different reason. Telling a movement along the curve apart from a shift of the curve is the difference between predicting where the triangle goes and guessing.
A price floor lifts the triangle only for the units that still find a buyer
Producer surplus is the seller-side score in policy arguments, and the price floor is its signature case. An agricultural support price or a statutory minimum price is defended on precisely these grounds: hold the price above the market-clearing level and every unit sold earns a wider margin over the supply curve, so the triangle grows taller.
The catch is in the quantity. A floor set above equilibrium pulls out more supply than buyers will absorb at that price, and the extra output goes unsold. Unsold units earn no surplus at all. Whether producers finish ahead depends on whether the wider margin on the units that do sell outweighs the units that do not, and this calculator cannot settle that for you: it takes the quantity sold as an input. To model a floor honestly, enter the quantity buyers will actually take at the higher price, not the quantity sellers would like to produce.
The same reasoning drives the politics of trade. Opening a market to imports pushes the domestic price down, narrowing the gap above the domestic supply curve and shrinking home producers’ surplus, while buyers’ surplus expands — which is why tariff debates are so reliably bitter. Subsidies, quotas, licensing rules and marketing boards are all scored the same way, by how much producer surplus they create, transfer or destroy. The concept travels far past the textbook triangle because it is the only measure that puts a number on what sellers gain from a market, as opposed to what they bill.
Stacking the seller’s triangle on the buyer’s to get the gains from trade
Producer surplus is one half of the welfare picture; consumer surplus is the other. Add them and you have total surplus, the standard yardstick for whether a market is doing its job. The producer triangle sits below the price line and above the supply curve. The consumer triangle sits above the price line and below the demand curve. Together they fill the region between the two curves out to the equilibrium quantity — the full value the market creates above the cost of creating it.
The paired worked figures make the symmetry easy to see. A $10 supply intercept, a $30 market price and 40 units give producers ½ × $20 × 40 = $400. A $50 demand choke price at the same $30 price and quantity gives buyers ½ × $20 × 40 = $400 as well, for $800 of total surplus. The even split is a coincidence of these particular numbers, not a rule: whichever curve is flatter claims the larger share, which is the geometric face of relative elasticity.
From here the natural next steps are to impose a tax or a price control and watch a wedge get carved out of both triangles — that wedge is deadweight loss — and to connect the slope of the supply curve to the price elasticity of supply, which governs how steeply marginal cost climbs as output expands. A steep supply curve means costs rise fast, quantity barely responds, and the producer triangle stays thin. A flat one means the opposite.
Capacity ceilings, entry barriers and curved supply all fall outside the triangle
The model behind this calculator is deliberately thin, and it is worth stating plainly what it leaves out. It assumes the supply curve is a straight line rising from the minimum supply price to the equilibrium point, that every seller receives the same single market price for every unit, and that the intercept reflects the true marginal cost of the first unit. Actual supply curves are seldom perfectly linear, so the triangle only approximates the real area between the price line and the curve.
The half-base-times-height shortcut is exact for straight-line supply and for nothing else. With a curved supply schedule you would measure the actual area between the price line and the curve, and the result generally differs — sometimes materially, if the curve bends sharply near the equilibrium quantity. When your supply data is a schedule of observed points rather than a line, treat the triangle as a first approximation and compute the area properly before any decision rests on it.
Beyond curvature, the model treats marginal cost as the only thing that matters. Fixed costs, capacity ceilings, minimum efficient scale, entry barriers and the lead time needed to bring new capacity online are all invisible to it. So is any market where sellers receive different prices: auctions, negotiated contracts, tiered wholesale pricing, or price discrimination of any kind. Nothing here measures a particular firm’s profit, and none of it is a precise welfare figure for a real pricing or policy decision. It is a teaching and estimating device — use it to see how the pieces move, then test the assumptions against the market in front of you.
Sources & References
Figures on this page are checked against primary, authoritative sources. Links open in a new tab.
Published the producer surplus calculator: the surplus triangle, price gap, and total revenue from the minimum supply price, equilibrium price, and quantity sold.
Added visual result charts.
Reviewed the formula and assumptions for accuracy.
What the calculation assumes and where it stops now show on the page.
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