Calculate on a shared unit
Divide the price by the volume, then multiply by the comparison unit you want. For example, a hypothetical 50ml moisturiser at £12 costs £24 per 100ml. A 75ml container of the same product at £15 costs £20 per 100ml. The larger size has a lower unit price, while still requiring £3 more at checkout.
Check that both listings refer to the same formula, not a day version, night version or similarly named product. If a set includes several items, do not divide the whole price by the volume of only its largest jar. Separate the contents before deciding what value the bundle offers.
Consider the amount you will finish
A lower unit price matters only if the additional product will be used appropriately. Think about how quickly you finish your current container and whether you already have unopened backups. Do not estimate value by increasing the amount you apply beyond the product's directions just to finish a large pack.
Read storage instructions and any minimum-durability or period-after-opening information. The open-jar symbol is about the period after opening, not a claim that the product will last indefinitely while sealed. Avoid opening a new container merely to compare textures if another one is already in use and you are not ready to start it.
Keep suitability ahead of size
A familiar product in a larger format can be a sensible saving. An unfamiliar formula in a large discounted jar carries a different risk: you may decide it does not suit your preferences or needs. Check the full product information and ingredients relevant to you, and follow any usage or testing instructions supplied by the manufacturer.
Delivery charges and pumps or dispensers sold separately can alter the total. Compare the delivered amount you will pay for a usable supply, then treat the millilitre figure as supporting information. It is a way to compare quantities, not evidence that a product is more effective or better for your skin.
