Compare cost per use as well as the checkout price. Then ask whether you will finish the bottle.
Start with comparable numbers
A $12 bottle and a $20 bottle cannot be compared fairly by price alone. You also need the amount in each bottle and your typical dose. Use the same currency and unit for both products. Millilitres and grams are not interchangeable without a density measurement.
Include delivery and tax in both prices if you want a comparison of your full spending. Or exclude both from both prices. Either approach can be useful; mixing the two creates a misleading answer.
Work through a transparent example
Imagine Product A costs $12 for 250 mL and you use 8 mL per wash. Product B costs $20 for 400 mL and you use the same 8 mL. These are fictional teaching examples, not current shop prices.
A costs $0.384 per use. B costs $0.40 per use. At three uses each week, multiplying by 52 and dividing by 12 gives average monthly costs of about $4.99 and $5.20 respectively. The more expensive bottle at checkout is not dramatically more expensive in use.
Your dose changes the result
If you use 6 mL of B instead of 8 mL, B costs $0.30 per application. That changes the comparison. Do not assume a smaller dose is sufficient because it makes the calculator look better; use a realistic estimate based on your experience and the label.
The calculator deliberately lets you enter different doses. It does not assign an effectiveness score, recommend reducing a manufacturer’s instructed dose or predict how a product will perform.
Remember the unfinished bottle
A product that sits unused still uses up your budget. Before buying a large size, consider whether you know that you like the product and can use it as directed within its labelled period after opening or expiry date.
Make a simple “finish first” list of products you already own. A good comparison may end with keeping your current routine. That is a useful outcome, even when no purchase follows.
Try the numbers yourself
Our value calculator shows price per 100 mL, estimated applications, cost per use and monthly cost. All calculations happen in your browser. The result is an estimate of spending, not a ranking of quality. This article uses original arithmetic; no retailer supplied the examples.
Sources & editorial notes
This guide uses original arithmetic and explicitly fictional examples. It does not claim to test products or offer medical advice.
Prepared with AI assistance and checked against the listed sources. This is not a hands-on product review or a dermatologist-reviewed article. Last substantive review: September 12, 2026.
