The objective of this study was to evaluate the use of a digital Brix refractometer to assess the levels of IgG in maternal colostrum and to measure passive transfer in newborn calves. Colostrum (n = 1,590) and serum (n = 1,756) samples from 1- to 5-d-old calves were collected from 130 dairy farms across the US from April through December 2014. Colostrum and serum IgG were determined by radial immunodiffusion (RID), and a digital refractometer was used to indirectly estimate IgG concentrations by %Brix. The mean IgG colostrum level was 76.7 g/L (SEM ± 0.84) with a range of 3.0 to 227.4 g/L. The mean %Brix in colostrum samples was 23.3 (SEM ± 0.12) with a range of 2.3% to 40%. The mean IgG serum level was 21.8 g/L (SEM ± 0.26) with a range of 0.1 to 79.3 g/L. The mean %Brix in serum samples was 9.1 (SEM ± 0.02) with a range of 6.4% to 18.7%. The %Brix in colostrum and serum samples was highly correlated with IgG levels in colostrum and serum (r = 0.76 and r = 0.85, respectively). A receiver-operating characteristic curve was created to plot the true positive rate against the false positive rate for consecutive %Brix values for colostrum and serum samples. Compared with a cut-point of 50 g/L IgG (consistent with good quality) as determined by RID in colostrum samples, the lowest false-positive rate occurred at 22% Brix, with a combined sensitivity and specificity of 78% and 84%, respectively. The positive predictive value (PPV) and negative predictive value (NPV) of 22% Brix were 94% and 53.5%, respectively. For serum samples, the optimal combination of sensitivity (88.8%) and specificity (90.6%) compared with a cut-point of 10 g/L (consistent with adequate passive transfer) occurred at 8.4% Brix. The PPV and NPV of 8.4% Brix were 98.3% and 56.9%, respectively. The digital brix refractometer is a convenient method to assess colostrum quality and adequacy of passive transfer in calves at the herd level. However, individual %Brix values below these threshold values underestimate the actual IgG level in about 45% of the samples.