20% off and $20 off are equal only on a $100 original price; elsewhere, they change how much the customer saves.
The arithmetic comes before the headline. A percentage grows with the product price, while a fixed amount stays the same. Check the affected products and your margin before choosing which offer to display.
| Original price | 20% off saves | Price after 20% off | $20 off saves | Price after $20 off | Equal offer at this price |
|---|---|---|---|---|---|
| $50 | $10 | $40 | $20 | $30 | $10 off equals 20% |
| $100 | $20 | $80 | $20 | $80 | $20 off equals 20% |
| $200 | $40 | $160 | $20 | $180 | $40 off equals 20% |
At $50, the copy “20% off” describes a $10 saving, while “$20 off” describes a $20 saving. Those are different offers, so a response difference would not show which headline works better on its own. At $100, the two headlines describe the same $20 reduction and can be compared as framing, provided the rest of the offer is the same.
For a $100 product, a concise comparison could be:
For a mixed-price collection, show the savings each eligible item actually receives. “20% off” and “$20 off” are not equivalent across the $50, $100, and $200 examples above. State eligibility and exclusions where customers see the offer, and avoid copy that suggests a larger saving than the checkout provides.
To test how the headline reads, choose the same product and price, discount value, eligibility, dates, placement, and audience. Change only the wording if you can. If you instead want to compare a percentage discount with a larger fixed saving, call it an offer-value test: you are changing both the deal and the copy.
Check contribution after the discount and any variable costs. For instance, a $20 reduction on a $100 product leaves $80 before other costs; compare that with your product and fulfillment costs before deciding how many discounted orders you can afford. This simple calculation does not include any change in basket size or repeat purchases, so use your own store data for the decision.
If the product itself is hard to evaluate, clarify its details before using a discount to compensate; the product-description guide can help with that work. Once you choose an offer, the guide to Instagram posts for Shopify brands covers ways to plan the message around it.
Write down the original prices, actual savings, margin floor, and offer terms before publishing. Then customers can understand the deal and your team can judge what it cost.