The 20%-off trap: when a promo lifts volume and loses money

1. Before you start

Price elasticity of demand is one number: the percentage change in units sold divided by the percentage change in price. Cut price 10% and sell 25% more, and the elasticity is 25 ÷ 10 = 2.5 — every 1% off buys about 2.5% more volume. Promo lift is that extra volume a temporary discount produces. The catch this course is about: a promo can lift volume and still lose money, because a discount also shrinks the margin on every unit — including the ones you would have sold anyway.

Three honest statements before you start:

This is a Decide course. You read a situation, learn the idea, and make a call. You do not write or run any code.
The companies below, Nordwell Snacks and Vireo Sparkling, are composite — invented firms built from ordinary figures so the arithmetic is clean. No number is a claim about any real company.
This is not a certification. It proves, to you, that you can defend a promo call.

You need only arithmetic and percentages. The hard part is judgment, not maths.

2. The Situation

Nordwell Snacks sells a bag of kettle popcorn at $4.00. A retail buyer proposes a four-week 20%-off promotion — $3.20 a bag — and the brand manager is keen: “our last cut moved a ton of volume, and this one should sell 50-60% more.” The finance partner is uneasy: “we lift volume every time, and margin still ends up worse.” You have to make the call, and volume alone will not tell you who is right.

The promo can look like a triumph on the sell-through report and quietly destroy contribution — and which one it does turns on how much extra volume the discount actually buys, not on how big the pile of extra bags looks.

3. What you’ll be able to do

After this course you will be able to:

Compute price elasticity of demand from a price change and the volume response, and read what it says about a discount.
Decide whether a promotion adds or destroys contribution, and name the break-even lift a given discount needs before a single extra bag helps.
Separate incremental volume from pull-forward / cannibalized volume, and say why counting the wrong one flatters a promo.
Find the deepest discount still worth running at a given elasticity.

4. Prerequisites & time box

Prerequisites: arithmetic and percentages. Helpful but not required: the idea of contribution margin (price minus variable cost), which section 6 rebuilds from scratch. No spreadsheet, no marketing background. This is the Decide hall — read and decide in your browser; there is no setup.

Time box: about 19 minutes of reading (measured), plus your own thinking time on the call. That is under the 25-minute cap for a concept course.

Difficulty: 5 / 8 — a manager-level decision: several factors move at once and the naive reading points the wrong way, so you have to reason past it.

Free-tier honesty: no signup, no paid tools, requires_gpu: false.

5. The case & where the numbers come from

Nordwell Snacks is a composite consumer-goods brand: its cost structure is built from ordinary figures a packaged-snack maker would recognise, chosen for clean arithmetic and not drawn from or claimed about any real firm. The definitions — price elasticity, contribution margin, promo lift — are standard and cited in section 11. Every figure below is an in-course assumption; every later number is computed from these.

Item	Figure
List price per bag	$4.00
Variable cost per bag (corn, oil, bag, co-pack labour)	$2.50
Baseline weekly volume at list	10,000 bags
Proposed promotion	20% off → $3.20 per bag
Expected volume during promo (brand manager’s estimate)	16,000 bags/week (+60%)
Share of the extra bags that are regulars stocking up	about 25%

6. The Concepts

Price elasticity of demand

Price elasticity of demand measures how much volume responds to a price change: the percentage change in quantity divided by the percentage change in price. Nordwell’s promo cuts price from $4.00 to $3.20 — a 20% cut — and the brand manager expects volume to rise from 10,000 to 16,000 bags, a 60% lift. So the implied elasticity is:

elasticity = 60% ÷ 20% = 3.0

An elasticity of 3.0 sounds strong — three points of volume for every point of price. Hold that number; the whole question is whether 3.0 is enough to pay for a 20% discount. It is not obvious that it is, and the answer is often no.

A discount does not just add volume; it lowers the margin on every bag sold at the promo price. Contribution margin is price minus the variable cost the unit causes:

At list: $4.00 − $2.50 = $1.50 a bag (a 37.5% margin).
On promo: $3.20 − $2.50 = $0.70 a bag.

The 20% price cut nearly halves the contribution margin — from $1.50 to $0.70. That is the hidden cost of the promo: not only the discount on the incremental bags, but a smaller margin on the 10,000 bags Nordwell was already selling. Weekly contribution is what actually matters:

Baseline (no promo): 10,000 × $1.50 = $15,000.
Promo week at the +60% estimate: 16,000 × $0.70 = $11,200.

Volume is up 60% and contribution is down $3,800. That is the trap in one line: a promo that “lifts volume” can still destroy margin.

So how much lift would the 20% promo need? Enough that the shrunken $0.70 margin, spread over the new volume, at least matches the old $15,000:

required volume = $15,000 ÷ $0.70 = 21,429 bags — a lift of +114%.
break-even elasticity = 114% ÷ 20% = 5.7.

The 20% discount needs an elasticity near 5.7 just to break even. The brand manager’s optimistic 3.0 is nowhere close, so the promo destroys contribution. There is a clean shortcut for the floor: the smallest elasticity any discount could break even at is 1 ÷ the contribution-margin ratio — here 1 ÷ 0.375 = 2.67 — and it only climbs from there as the discount deepens. At Nordwell’s numbers an elasticity of 3.0 supports a discount of only about 4%. Check it: a 4% cut prices the bag at $3.84 (contribution $3.84 − $2.50 = $1.34), and a 3.0 elasticity lifts volume 3 × 4% = 12%, to 11,200 bags; 11,200 × $1.34 = $15,008 ≈ the $15,000 baseline. So about 4% is the most a 3.0 elasticity can carry — a 20% cut needs far more response than the brand ever gets.

(An interactive calculator sits here — enter your own price, cost, baseline, discount, and expected lift, and watch the decision flip as the observed elasticity crosses the break-even.)

Incremental versus pull-forward volume

The −$3,800 above compares the promo week with a normal week, and it still flatters the promo — because not all of the extra 6,000 bags are new demand. Roughly 25%, about 1,500 bags, are regulars stocking up: bags they would have bought next week at full price. That is pull-forward (or cannibalized) volume, and it is not incremental at all.

Follow one pulled-forward bag across two weeks. With no promo it sells next week at $4.00 and earns its full $1.50. With the promo it sells this week at $3.20 for $0.70, and then does not sell next week — that customer is already stocked. Pulling the bag forward turns a future $1.50 into a present $0.70: a $0.80 loss on that bag (its full margin minus the promo margin). Over 1,500 bags that is 1,500 × $0.80 = $1,200 of contribution that pull-forward quietly costs.

Now fold that into the headline correctly — this is the easy place to slip. The −$3,800 week-one figure already credits these 1,500 bags their $0.70 of promo contribution, as if the volume were new. It is not; it is next week’s full-price sale, moved forward and marked down. So the promo does not merely under-earn $0.80 a bag — measured against the −$3,800 view it gives up the entire forgone future sale of $1.50 a bag: 1,500 × $1.50 = $2,250. (That $2,250 is the $1,200 real loss plus the $1,050 of promo contribution the headline should never have counted as incremental.) The promo’s true two-week effect is −$3,800 − $2,250 = −$6,050 — worse than the headline, and the calculator’s net change after pull-forward line shows exactly this.

The lesson is to count only the volume that would not have happened without the promo — the truly incremental bags — and never let stock-up buying by existing customers dress a discount up as growth. Incremental volume earns its keep; pull-forward volume just moves a full-price sale into a discounted week.

7. Your Call

You have seen how elasticity, the break-even lift, and pull-forward decide Nordwell’s call. Now a different one lands on your desk.

Vireo Sparkling is a composite maker of canned sparkling drinks — a different company in a different corner of consumer goods from Nordwell’s snacks. It sells a bottle at $2.00 with a variable cost of $1.00, so its contribution margin is $1.00 (a 50% margin), and it sells 8,000 bottles a week. A competitor has just cut its own price about 10%, and Vireo’s marketing team wants to defend share with a 20%-off promo ($1.60 a bottle). Their best elasticity estimate is 2.5. Rather than a plain yes/no, your job is to say whether the 20% promo pays — and if not, name the deepest discount that still would.

How this differs from the taught case (the transfer): this is a different company and sector (a beverage brand, not a snack maker), the figures are different so the arithmetic must be redone, there is an added constraint — a competitor is discounting at the same time and the elasticity is only an estimate — and it is a different kind of decision: you set the deepest discount worth running, not a simple accept/reject of one fixed promo. The core concept is the same: elasticity and promo lift weighed against the margin the discount gives up.

8. Self-check

Before you write the memo, make sure you can say each of these in one line:

Why is “volume is up 60%, so run the promo” the wrong test?
What single number decides whether a given discount adds or destroys contribution?
How do you turn an elasticity estimate into the deepest discount worth running?
What is pull-forward volume, and why does counting it flatter a promo?

If any is fuzzy, reread section 6 — elasticity, the margin the discount moves, the break-even lift, and pull-forward are the whole course.

9. Stretch

Push the decision further on your own:

Back at Nordwell: at what discount depth does the 3.0 elasticity stop breaking even? (Solve for the discount where the shrunken margin times the lifted volume equals $15,000 — it is smaller than you would guess.)
The genuinely harder one: suppose the retailer will co-fund half the discount, so Nordwell’s effective price is $3.60 while shoppers pay $3.20. Does the 20% promo now pay at a 3.0 elasticity, and what is the new break-even lift?
Write the one sentence you would say to a brand manager who insists “but look how many bags we moved.”

10. Ship it — your decision memo

Write a one-page memo to Vireo’s head of marketing. State the call (do not run the 20%-off promo; the break-even lift is +67% and the 2.5 estimate gives only +50%, a $800 weekly loss — the deepest discount that holds contribution is about 10%). Show the two-line arithmetic (promo contribution 12,000 × $0.60 = $7,200 vs baseline 8,000 × $1.00 = $8,000). Name what you rejected (reading volume or revenue as success; matching the competitor’s cut for its own sake) and why. Name the one thing that would change your mind (a genuinely higher elasticity, or a co-funded discount that protects the margin). Keep it to a single page a marketing lead grasps in two minutes. This memo is your own argued claim — not a credential.

11. Sources

Nordwell Snacks and Vireo Sparkling, and every figure attached to them, are composite — constructed for clean teaching arithmetic, not drawn from or claimed about any real company. The concept definitions used to reason about them are standard; references below.

Concept / claim	Source (publisher)	URL	Accessed
Price elasticity of demand = % change in quantity ÷ % change in price	Wikipedia — Price elasticity of demand	https://en.wikipedia.org/wiki/Price_elasticity_of_demand	2026-07-19
Contribution margin = price − variable cost	Wikipedia — Contribution margin	https://en.wikipedia.org/wiki/Contribution_margin	2026-07-19
Gross vs contribution margin on a discounted price	Wikipedia — Gross margin	https://en.wikipedia.org/wiki/Gross_margin	2026-07-19
Definition of a variable cost	Wikipedia — Variable cost	https://en.wikipedia.org/wiki/Variable_cost	2026-07-19
Sales promotions and temporary price discounts	Wikipedia — Sales promotion	https://en.wikipedia.org/wiki/Sales_promotion	2026-07-19

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