The fingertip unit (FTU) was introduced by Long and Finlay in 1991 as a practical, reproducible measure of the quantity of a topical preparation without the need for weighing. One unit is the amount of cream or ointment expressed from a tube with a standard 5 mm nozzle along the distal phalanx of an adult index finger, from the tip to the first skin crease. In the original study one unit weighed 0.49 g in men and 0.43 g in women, giving the working figure of approximately 0.5 g, and covered a mean skin area of 286 cm2, roughly the area of two adult handprints with the fingers together. The rule of hand states that four hand areas require two fingertip units, that is 1 g (Long, Finlay and Averill 1992).

For adults the study established the number of units needed for a single application to each region: face and neck 2.5, one arm 3 and one hand 1 (4 combined), one leg 6 and one foot 2 (8 combined), front of trunk 7, and back of trunk including buttocks 7. A single whole-body application therefore requires about 40 units, or 20 g, and twice-daily whole-body treatment for a week uses about 280 g. These figures allow the required quantity for any prescription to be estimated from the treated area, the frequency, and the duration of the course.

Children need fewer units for the same regions because their body surface is smaller, although the unit itself is always measured on an adult finger. Long, Mills and Finlay (1998) published a paediatric table for four age bands. At 3 to 6 months the face and neck, one arm and hand, and the front of the trunk each need 1 unit and one leg and foot or the back of the trunk 1.5 units; at 1 to 2 years the figures are 1.5, 1.5, 2, 2, and 3; at 3 to 5 years 1.5, 2, 3, 3, and 3.5; and at 6 to 10 years 2, 2.5, 4.5, 3.5, and 5 for the face and neck, arm and hand, leg and foot, front of trunk, and back of trunk including buttocks respectively. Ages between the bands are treated using the nearest band, and adult figures apply from about 10 years.

Under-treatment is the most common problem with topical therapy. Fear of topical corticosteroid side effects leads many patients to apply too little, and a British expert consensus (Bewley 2008) recommended that patients be advised in fingertip units and told to apply enough to cover the affected skin, rather than being told to use the preparation sparingly. Guidelines for atopic dermatitis likewise recommend that the quantity prescribed match the treated area and the expected duration (Eichenfield et al. 2014). The unit is approximate: ointments and creams differ slightly in density, nozzle diameter and finger length vary, and the figures assume the whole region is treated. When only part of a region is affected, the quantity is scaled down accordingly.