Exact discount distribution for multiple products in the order

Shopify Discussion

Topic summary

A user is questioning Shopify’s method for distributing fixed-amount discounts across multiple products in a cart. According to Shopify’s help documentation, discounts are applied proportionally across all items. However, the user disputes this claim with a specific example:

Example scenario:

  • Cart contains: $50 item + $100 item
  • $50 discount applied
  • Actual distribution: First item discounted by $16.50, second by $33.50
  • True proportional distribution would be: $16.67 and $33.33 (1/3 and 2/3 split)

Core issue:
The user seeks clarification on Shopify’s exact rounding methodology, as the actual discount allocation ($16.50 and $33.50) doesn’t match true proportional distribution. The discrepancy suggests Shopify uses a specific rounding algorithm that isn’t clearly documented.

Summarized with AI on November 13. AI used: claude-sonnet-4-5-20250929.
1

I need to know how exactly Shopify distributes a discount among multiple products in the order.

Your help page https://help.shopify.com/en/manual/discounts/discount-types/percentage-fixed-amount states:

“If your discount is set to discount all products, then the discount is applied proportionally across all items in the cart. For example, if you have a $50 discount applied to a cart with a $50 and a $100 item, then the first item is discounted by $16.50 and the second one by $33.50”

This is NOT proportionally. Proportionally would be $16.67 and $33.33, for example (1/3 and 2/3).

How is Shopify “rounding” proportions to $16.50 and $33.50?

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