An experiment was conducted to determine the optimal dietary concentration of standardized total tract digestible (STTD) Ca for growth and feed efficiency of nursery pigs. A total of 2,185 pigs (initial BW =11.2 ± 1.4 kg) were blocked according to BW and randomly allotted to 1 of 6 dietary treatments, with STTD Ca formulated to 0.38, 0.46, 0.54, 0.62, 0.70, or 0.78% by increasing dietary limestone at the expense of corn. Each treatment had 16 replicate pens with 23 pigs per pen. Corn and soybean-based diets were formulated to contain 0.36% STTD P, which exceeds NRC 2012 (+9%). This resulted in STTD Ca: STTD P ranging from 1.1 to 2.2. To minimize variation, 2 basal diets were produced (0.38 and 0.78% Ca diets), and then blended on site to produce the intermediary levels. Data were analyzed using the MIXED procedure of SAS (Cary, NC) with fixed effect of diet and random effect of block. Prediction equations were generated using the FITTED GROUP of JMP with goodness of fit techniques to optimize the R2 value and RMSE. Feed conversion ratio (FCR) declined linearly (P = 0.011) with increasing levels of STTD Ca in the diet, and is described by: FCR = [1.456 + (0.115 x STTD Ca)] (R2 = 0.698, RMSE = 0.013). Final pig weight tended to increase (quadratic, P = 0.110) as dietary STTD Ca increased, and is described by: BW = [22.77 – (0.55 x STTD Ca) – (7.72 x (STTD Ca – 0.58)2] (R2 = 0.696, RMSE = 0.135). Exceeding 0.70% STTD Ca (2.0:1.0 STTD Ca: STTD P) reduced pig growth, FCR, and profitability. Adequate STTD Ca levels ranged from 0.46 to 0.62% (STTD Ca:STTD P, 1.3 to 1.8), but growth and profit were optimized at 0.54% (1.5:1.0 STTD Ca: STTD P).