© 2017 Elsevier B.V. The objective of the present study was to determine some prediction equations of gross energy content (GE), organic matter digestibility (OMd), gross energy digestibility (GEd) and the content of digestible (DE) and metabolizable energy (ME) of agro-industrial by-products for pigs, using the chemical composition and an in vitro digestibility method. Mean values of chemical composition (dry matter, DM; organic matter, OM; gross energy, GE; crude protein, CP; ether extract, EE; crude fiber, CF; neutral detergent fiber, NDF, and acid detergent fiber, ADF) were available from previous work and an in vitro OM digestibility method (IN VITRO) was performed. A total of eight by-products (liquid bakery by-product, brewer's by-product, hominy feed, high-moisture corn, mayonnaise, almond meal, cocoa meal and kiwi fruit) were used in this study, and their in vivo OMd and GEd were known from previous experiments. The best equation for GE prediction was: GE = 9.58 EE + 6.50 CP + 4.28 carbohydrates (OM-CP-EE) (R2 = 1.00; CV = 5.52). When only the chemical composition was used as independent variables, NDF and ADF were the best predictors (R2 = 0.65–0.78; CV = 12.30–10.85) for OMd and GEd, and EE plus ADF for DE and ME (R2 = 0.94–0.88; CV = 13.10–16.64). However, the best-fitted equations were: OMd = 30.90–0.302 ADF + 0.639 IN VITRO (R2 = 0.95; CV = 4.99); GEd = 34.81–0.493 ADF + 0.589 IN VITRO (R2 = 0.96; CV = 4.93); DE = 846.34 + 5.081 EE + 3.045 IN VITRO (R2 = 0.97; CV = 8.57), and ME = 687.45 + 5.057 EE + 3.150 IN VITRO (R2 = 0.97; CV = 9.78). Additionally, when the two high EE by-products (mayonnaise and almond meal; EE >50%) were excluded, the most accurate equations for DE and ME were: DE = 1415.96 + 2.764 IN VITRO (R2 = 0.97; CV = 3.10), and ME = 1262.92 + 2.868 IN VITRO (R2 = 0.97; CV = 2.68). These results indicated that GE, OMd, GEd, DE and ME may be accurately predicted from chemical composition, and especially from the in vitro OM digestibility.