Two useful comparisons
Price per millilitre is the bottle price divided by volume. Monthly cost is different: it depends on how much you use and how often. Our calculator lets you enter the actual purchase price and your known use rate; it does not recommend an application amount.
Use a single currency for each calculation. Add unavoidable shipping and taxes to the amount you expect to pay. If a charge covers several products, decide how you are allocating it rather than quietly counting it as zero.
Keep assumptions visible
Some labels say a few drops; others provide a measured amount or approximate bottle duration. Those are not equally precise. If you do not know the daily use rate, compare price per millilitre and mark monthly cost as uncertain. Do not turn a guess into a promise about how long a bottle lasts.
Do not compare a subscription discount with a competitor’s one-time price unless the recurring commitment is genuinely part of your choice. A promotional bundle also needs to be compared against how much you expect to use before its storage or expiry guidance becomes relevant.
Value includes a routine you will use
The cheapest theoretical millilitre is not useful if the product’s scent or application makes you avoid it. Equally, a pleasant routine does not justify a clinical claim. Compare cost, preference and evidence as three separate considerations.
Use the calculator below to explore your own numbers. Its output is arithmetic based on what you enter, not a forecast of results or a suggested dose.
Your routine, your budget.
Enter the delivered price and bottle volume. Add a known use rate only if available from your actual routine or product directions.
Arithmetic estimate only. This calculator does not recommend how much product to use. Entries stay in this page and are not sent to a server.
Sources & further reading
Product pages establish brand claims and label facts. They do not independently validate efficacy. Sources checked 14 September 2026.
- The Ordinary: Multi-Peptide Serum for Hair Density — official product and ingredients
- The INKEY List: Caffeine Stimulating Scalp Treatment — official product and ingredients
- RevivSerums: RevivHair Max Hair Stimulating Serum — official product and ingredients
