Discount Math for Ecommerce: When a Sale Makes You Money and When It Burns Cash
Discounts are one of the fastest ways to increase ecommerce revenue. They are also one of the fastest ways to hide a profit problem.
A 20 percent sale can look great in Shopify. Orders go up, conversion improves, and the marketing team feels momentum. But if the discount pushes contribution margin too low, the business may be buying revenue with cash it cannot afford to lose.
The question is not "should we discount?" The question is "what has to happen for this discount to be profitable?"
Start With Gross Margin, Then Go Deeper
Most discount decisions start with gross margin:
Gross margin = Sale price - product cost
That is useful, but incomplete.
A better discount decision uses contribution margin:
Contribution margin = Sale price - product cost - payment fees - packaging - fulfillment - shipping subsidy - expected return cost - ad cost
If a discount reduces sale price but the other costs stay mostly fixed, margin compresses quickly.
For example:
- Normal price: $80
- Product cost: $28
- Payment and fulfillment: $8
- Shipping subsidy: $7
- Expected return cost: $3
- Ad cost per order: $14
Normal contribution margin:
$80 - $28 - $8 - $7 - $3 - $14 = $20
At 20 percent off, the sale price becomes $64.
Discounted contribution margin:
$64 - $28 - $8 - $7 - $3 - $14 = $4
Revenue is still coming in. Profit per order has dropped 80 percent.
Calculate the Volume Needed to Break Even
A discount can still make sense if it produces enough extra order volume, repeat customers, or inventory relief. But you need to know the break-even point.
Use this formula:
Required volume lift = Original contribution margin / Discounted contribution margin
Using the example above:
$20 / $4 = 5x
The sale would need to produce five times as many orders just to match the same contribution profit. That is a high bar.
If your normal campaign sells 100 orders and produces $2,000 in contribution profit, the discount needs 500 orders at $4 contribution profit to tie.
Most sales do not create that kind of lift.
Discounts Work Better on Some Products Than Others
Do not apply the same discount across the whole store without checking product economics.
Products that can handle discounts usually have:
- High contribution margin
- Low shipping cost
- Low return rate
- Strong repeat purchase potential
- Inventory you actually want to move
- Good attach rate with other products
Products that should be protected from discounts usually have:
- Thin margin
- Heavy or expensive shipping
- High return rate
- Limited inventory
- High paid acquisition cost
- Strong full-price demand
If a product already sells well at full price, discounting it may train customers to wait without creating enough incremental profit.
The Free Shipping Plus Discount Trap
Discounts become more dangerous when they stack with free shipping.
For example:
- 20 percent discount
- Free shipping
- Paid social traffic
- High return rate
Each piece may be acceptable alone. Together, they can erase margin.
Before a sale goes live, model the combined effect:
| Input | Question |
|---|---|
| Discount | How much revenue is removed from each order? |
| Shipping subsidy | How much cost does the business absorb? |
| Ad spend | How much does it cost to create the order? |
| Return rate | How much profit is lost after refunds and handling? |
| Payment fees | How much is paid on gross transaction value? |
If the discounted order cannot carry those costs, the campaign is not a growth campaign. It is a cash drain.
When Discounts Do Make Sense
Discounts are not bad. Bad discount math is bad.
A sale can be strategic when it:
- Clears slow-moving inventory
- Increases repeat purchase from existing customers
- Pushes customers into a profitable bundle
- Improves cash position before a supplier payment
- Converts trial customers into subscription buyers
- Helps test demand for a new product
The key is to define the goal before launching. A clearance sale should be judged by cash recovered and inventory freed. A new customer sale should be judged by contribution profit and repeat purchase. A bundle sale should be judged by combined order margin.
One promotion cannot optimize for everything.
Build Discount Guardrails
Set simple rules before campaigns are created:
- Minimum contribution margin per order
- Products excluded from blanket discounts
- Maximum discount by product category
- Free shipping threshold based on margin
- Required payback period for paid campaigns
- Return-rate review after each sale
These guardrails stop discount decisions from being made only by conversion rate.
If a marketer wants a bigger discount, the finance question is simple: what volume lift, AOV lift, or retention improvement makes this worth it?
How to Review a Sale After It Ends
Do not judge a sale by revenue alone.
Review:
- Gross sales
- Discounts given
- Net revenue
- Ad spend
- Contribution profit
- Average order value
- Return rate
- New versus returning customers
- Inventory cleared
- Cash generated
Then compare it to a normal period. If the sale lifted revenue but reduced total contribution profit, be careful about repeating it.
How Nummbas Helps
Nummbas connects the data you need to judge discounts properly: revenue, product cost, ad spend, shipping, fees, expenses, and margin.
That makes it easier to answer:
- Which products can safely be discounted?
- Which campaigns grow profit, not just orders?
- Which discounts create return problems?
- Which bundles improve margin?
- Which sale helped cash and which one only inflated revenue?
Discounts should be a lever, not a habit. Use the math first.