A refill can cost less per millilitre but more each time you use it. Concentration and recommended dose can change the comparison. First establish that the products serve a comparable purpose, then compare total cost and the number of uses.
- Bottle: ₩8,000, 500 mL, 5 mL per use
- Bottle: 100 uses → ₩80 per use
- Refill: ₩12,000, 1,000 mL, 8 mL per use → ₩96
- Refill costs ₩16 more per use here
Do not reduce the labeled dose just to save money. No equal-performance test is implied.
Hypothetical KRW example with no new-container cost. Match the task and product dose conditions.
Calculate with your own numbers
The boundary dose is a calculated result, not a recommended dose. Follow the manufacturer's instructions.
This tool does not transmit your entries or store them on a server. The CSV is created in your browser. All sample numbers are hypothetical; use one currency and consistent units.
Divide purchase cost by the expected number of uses
Number of uses = quantity ÷ dose. Cost per use = total purchase cost × dose ÷ quantity. Include relevant delivery and taxes. Use the same measurement unit, and match the manufacturer's dose conditions for the same task where possible.
Enter a required new bottle or pump as an extra container cost for the refill. This tool allocates that expense entirely to the first refill pack, keeping the first cash outlay visible. Enter zero only when an existing compatible container is available.
The refill dose boundary is where its calculated cost per use matches the bottle's. A higher dose costs more under the entered conditions. This is a cost boundary, not a recommended dose: never reduce the manufacturer's dose just to reach it.
Worked example: cheaper per mL, dearer per use
Hypothetical KRW amounts: bottle 8,000 for 500 mL at 5 mL per use; refill 12,000 for 1,000 mL at 8 mL per use; no new container cost. The bottle costs 80 per use and the refill 96. Its lower price per mL does not win this comparison.
The refill boundary is about 6.67 mL per use. If an equivalent task genuinely calls for 5 mL of the refill, its cost is 60 per use. No performance test is implied: changing a dose in a calculator cannot establish equivalent effectiveness.
Check these before purchasing a refill
- Confirm the exact product and container compatibility.
- Match the task and the dose conditions.
- Include any required pump, container, shipping and taxes.
- If some contents may remain unused, also compare used-quantity cost.
- When the saving is small, consider storage and convenience.
A cost comparison is not an environmental or performance test
The tool does not measure packaging impact, effectiveness, hygiene or durability. It does not instruct you to mix products or dilute them. Follow manufacturer use and storage instructions. If your dose varies, compare explicit low, typical and high assumptions.
Common questions
Can I spread the container cost over future refills?
This tool charges it to the first pack. A long-term average needs a separate assumption about repeat purchases and container life.
Can I use weight instead of volume?
Yes, if both quantity and dose use the same weight unit. Do not mix grams and millilitres.
Sources and how this guide was written
Background on unit pricing: the UK Competition and Markets Authority (CMA). Durudam developed the calculations and hypothetical examples in this guide.
Durudam prepared this calculation and record-keeping guide with AI assistance. It does not claim product testing or on-location reporting. Examples are hypothetical; formulas and assumptions are disclosed. Sources checked: 4 October 2026.
Related reading
- Is Free Shipping Worth Buying More? A Total-Cost Calculator
- Is Buying in Bulk Cheaper? Calculate the Cost of What You Use
What to check next in this situation
- What Will an Online Return Really Cost? A Refund Worksheet
Before buying a new container or larger pack, check the actual refund and return-postage terms if it does not suit your needs.