Objectives
The accuracy of gestational age (GA) wheels has been shown to be poor, yet they remain commonly used. We surveyed their use within our institution and determined the inter‐observer and inter‐device variability of a range of devices. We have devised a procedure for validating device accuracy.

Methods
All clinicians within our maternity unit were asked their most recent method of calculating GA and whether they felt this was accurate to within one day. Ten clinicians assessed 16 devices: 14 manual, 2 electronic. Five dates represented the last menstrual period (LMP) and were used to calculate the estimated date of delivery (EDD) of the 5 dates for each device compared to a 280 day control. ANOVA was used for statistical analysis.

Results
73% last used a manual device to calculate GA. Seventy‐two per cent believed their method was accurate. There was a significant bias (difference in the mean) between device‐calculated and control EDD for all manual devices except one, with individual differences of up to 4 days. Variability altered throughout the year. Electronic devices consistently had no error. Inter‐observer variability was insignificant.

Conclusions
Clinicians should be aware of the inaccuracy of manual devices. Electronic devices are recommended. Manual devices should be validated before use by comparing the device‐calculated EDD with a 280 day control at five points throughout the year.