Background: The estimation of body surface area involvement is an important tool. Hand surface area (HSA) or palm surface area (PSA) is commonly used for the estimate, with an assumption that HSA represents 1% of the total body surface area (TBSA). Objectives: To establish (i) the most accurate values for mean HSA% and PSA% of TBSA, and (ii) the variability of these with patient variables. Methods: The PubMed, Embase and Cochrane databases were searched and 14 eligible studies were identified. Weighted means of HSA% and PSA% were produced. The meta-analysis examined systematic variation associated with sex, age (for children), body mass index (BMI) and ethnic group using random-effects models. Results: HSA% is 13% lower than the accepted 1% value for all adults (P = 0·004). PSA% is not significantly different from the accepted 0·5% value (P = 0·82). Men have a significantly higher HSA% than women (P < 0·0001). Children have a significantly higher HSA% than adults (P < 0·0001). HSA% falls with increasing BMI in adults (P < 0·0001). A comparison of European, Chinese and Indian subcontinent ethnic groups showed that each group was different from the others (P < 0·05). Conclusions: The use of HSA equating to 1% TBSA results in an overestimate for adults (particularly women) and an underestimate for children. PSA equating to 0·5% TBSA appears to be suitable for adults. Patient variables including sex and BMI result in variation of HSA as a percentage of TBSA. The heterogeneity of the included studies and the lack of data for children are the major limitations of this study. What's already known about this topic? Hand surface area (HSA) and palm surface area (PSA) as percentages of total body surface area (TBSA) are commonly used to estimate surface area involvement. Commonly used estimates are HSA equalling 1% TBSA and PSA equalling 0·5% TBSA. What does this study add? For adults, HSA is 0·87% TBSA, 13% lower than the accepted value of 1% TBSA. Body mass index, sex, age and ethnicity affect this value. The accepted value of PSA equalling 0·5% TBSA is accurate. It may be more accurate to use PSA to estimate surface area involvement in adults. Patient variables may affect this estimate. © 2013 The Authors BJD © 2013 British Association of Dermatologists.