Topical Medication Calculator
Calculate topical medication quantities using the Fingertip Unit (FTU) method for accurate prescriptions.
About this calculator
This calculator applies the fingertip unit (FTU) method, the standard dermatology convention for estimating how much topical cream or ointment a treatment actually needs. Long and Finlay first described it in 1991 as a reproducible alternative to vague "apply a thin layer" instructions. One FTU is the ribbon of product squeezed from a 5 mm tube nozzle from the crease of the distal finger joint to the fingertip, and it weighs about 0.5 g -- enough to cover roughly 2% of an adult's body surface. Each anatomical region has a published FTU count for full coverage: one hand takes 1 FTU, one foot 2, one arm 3, one leg 6, the face and neck 2.5, and the trunk front or back 7 each.
Affected Area is a percentage of the SELECTED REGION itself, not of the whole body -- entering 50% for the leg means half of the leg is affected, regardless of how much of total body-surface-area the leg represents -- and this calculator scales the region's full-coverage FTU count directly by that percentage. Grams per application then multiply the scaled FTU count by a formulation-specific weight -- 0.5 g for creams and ointments, a higher 0.7 g estimate for lotions, which spread thinner and need more product to achieve the same FTU-equivalent coverage. Multiplying by applications per day and treatment days gives the total grams needed, which the calculator rounds up to a standard 15 g, 30 g, or 60 g tube size. This is a dispensing-quantity estimate, not a dosing instruction for the medication itself -- follow the prescriber's concentration and frequency orders.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
Total Needed
7g
Tube Size
0.02oz
Figures current as of 1991. Source: Long CC, Finlay AY. The finger-tip unit -- a new practical measure. Clin Exp Dermatol. 1991;16(6):444-447.
How to Use This Calculator
- Select the Body Area being treated from the anatomical list.
- Enter Affected Area as a percentage of that selected region (not of total body surface area) and the number of Applications per Day.
- Set the Treatment Duration (days) and select the Formulation (cream, ointment, gel, lotion).
- Review FTU per Application and Grams per Application to determine dispensing quantity.
- Use Total Grams Needed and Recommended Tube Size to prescribe the correct supply.
How the result changes with Affected Area
| Affected Area | Total Needed | Tube Size |
|---|---|---|
| 10 | 4g | 15g |
| 15 | 5g | 15g |
| 30 | 11g | 15g |
| 50 | 18g | 30g |
What each input means
- Body Area
- 1=face/neck, 2=arm, 3=hand, 4=trunk front, 5=trunk back, 6=leg, 7=foot, 8=scalp.
- Affected Area
- Percentage of the SELECTED body region that's affected (not a percentage of total body surface area).
- Applications/Day
- Daily application frequency.
- Treatment Duration
- Total treatment days.
- Formulation
- 1=cream, 2=ointment, 3=lotion.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersBody Area = 1, Affected Area = 20, Applications/Day = 2, Treatment Duration = 14 = 5 input(s) provided
- Calculate Total NeededTotal Needed = gramsPerDay * treatmentDays7 = 7
- Calculate Tube Size15 = 15
- Calculate FTU Per ApplicationFTU Per Application = ftuForArea * affectedFraction0.5 = 0.5
- Calculate Grams/ApplicationGrams/Application = ftuPerApplication * gpf0.3 = 0.3
Figures and sources
- Fingertip unit (FTU) method for topical medication dosing (1991) — Long CC, Finlay AY. The finger-tip unit -- a new practical measure. Clin Exp Dermatol. 1991;16(6):444-447.
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What exactly is a fingertip unit, and why use it instead of just 'a thin layer'?
A fingertip unit is a standardized ribbon of cream or ointment -- from a 5 mm nozzle, laid from the crease of the last finger joint to the fingertip -- that weighs about 0.5 g. Dermatology adopted it because "apply a thin layer" varies wildly between patients, while an FTU gives a reproducible, teachable amount that maps to published per-body-region coverage tables.
Why doesn't changing the Formulation change the FTU Per Application number?
FTU Per Application depends only on Body Area and Affected Area -- it is a measure of how much product ribbon covers the treated surface, independent of what that ribbon is made of. Formulation only changes the Grams/Application figure, because a cream, ointment, or lotion FTU has a different average weight even though it represents the same coverage.
Why does treating a smaller percentage of a body area lower the FTU count so much?
The calculator scales each region's full-coverage FTU count directly by Affected Area as a percentage of that region -- treating 25% of a trunk uses a quarter of the trunk's full 7 FTU, and treating 100% of it uses the full 7. Spot treatment of a small patch should never cost as much product as covering the whole region.
How is the recommended tube size chosen?
The calculator compares Total Needed in grams against three standard dispensing sizes -- 15 g, 30 g, and 60 g -- and recommends the smallest one whose actual capacity covers the full course on its own (up to 15 g, up to 30 g, otherwise 60 g), so a short course on a small area gets a single 15 g tube while a longer or larger-area course steps up to 30 g or 60 g. Tubes to Prescribe then shows how many of that size are needed if the course exceeds even the 60 g size.
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