How this calculator works
A container holds a fixed volume, and each use draws a small amount from it. Dividing the volume by the amount used per application gives the number of applications in the container, and dividing the price by that gives the cost per application. Multiplying by how often you apply it gives a weekly and yearly cost. The product presets are typical usage estimates: a mascara tube is used a fraction of a millilitre at a time, while nail polish lays down a much thicker coat. They are starting points, not measurements, so if you know how long a container really lasts, use the custom option and work back to the amount per layer.
Formula
Amount per application = typical use per layer x layers. Applications per container = volume / amount per application. Cost per application = price / applications per container. Cost per week = cost per application x applications per week. Cost per year = cost per week x 52. Weeks per container = applications per container / applications per week.
Worked example
A 30 mL foundation at 40, applied in one layer using 0.5 mL each time, gives 60 applications. That is 0.67 per application, 4.67 a week at seven applications, and 242.67 a year. One container therefore lasts about 8.6 weeks.
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Frequently asked questions
How accurate are the product presets?
They are rough industry-typical figures chosen to show the right order of magnitude, not measurements of a particular brand. Application size varies with the brush, the formula and your own technique. The custom option lets you enter a figure based on how long your last container actually lasted.
Why is cost per application more useful than price per millilitre?
Price per millilitre ignores how much you use at a time. A dense foundation may need less per application than a thin one, so the cheaper bottle by volume can be the more expensive product to use. Cost per application captures both the price and the usage.
Should I include products I use only occasionally?
Yes, if you want a yearly figure that reflects your real routine. Set the applications per week to a fraction only if the field allows it; otherwise work in a longer period, such as four applications a month, and divide accordingly.
Does the calculator account for the product expiring before it is used?
No. Cream products carry a period-after-opening mark, often 6 or 12 months, and a product used rarely can expire before the container is finished. If that is your situation, the yearly cost is better estimated from the replacement interval than from the number of applications.