The purpose of this prospective single center study is to investigate if the accuracy of length based body weight estimation by the already investigated algorithm (CLAWAR) can be improved by adding another parameter. For this study 500 patients are required to collect anonymized data (length, weight, age, mid upperarm circumference and patient habitus by visual estimation) for achieving a power of 80% during statistical analysis. The main hypothesis ist that CLAWAR's accuracy can be improved.

This prospective single center study is performed at the university children's Hospital zurich. Patients planned for surgery in general anaesthesia with intubation, aged 0 to 16 years and with a body length suitable for CLAWAR. Patient and parental information is performed during the pre-anaesthetic visit.

After written consent the patient will be included. Patient ́s length and weight will be measures earliest one day before data collection. Data collection during anaesthesia has no influence on the daily anaesthesia routine and has no impact on patient safety and anaesthesia. Patient's data is made anonymous for further inspection. Data is documented in Microsoft Excel and statistical analysis calculated with SPSS.

500 patients are needed for a power of 80%. Primary outcome parameter is the accuracy of length based weight estimation using either patient habitus by visual estimation or an improved version of the algorithm CLAWAR using the mid upperarm circumference.

Inclusion Criteria:

• body length suitable with the investigated algorithm
• all patients aged 0 -16 years

Exclusion Criteria:

• already included in this study once
• missing patient or parental consent

Pediatric patients planned for elecitve surgery in general anaesthesia