An energy system is described in which, in both single-stomached and ruminant animals, the heat increment of feeding is considered to be linearly related to five measurable quantities. For both kinds of animals three of the quantities, with their heat increments in parentheses, are urinary N (wu; kJ/g), faecal organic matter (wd; kJ/g) and positive protein retention (wp; kJ/g). In ruminants the other two, with their heat increments in parentheses, are CH4 energy (wm; kJ/kJ) and positive lipid retention (w1; kJ/g); in single-stomached animals they are positive lipid retention from feed lipid (wII; kJ/g), and positive lipid retention not from feed lipid (wI; kJ/g). Data from suitable experiments on steers, pigs and chickens were used to test the system and to estimate wu 29.2, wd 3.80, wp 36.5, wm 0.616, wI 16.4 and wII 4.4. The values for wu, wd, wm and (wI - wII) allow an energy scale, called effective energy, to be defined for both single-stomached animals and ruminants. On this energy scale the values of wp and wI, together with the heats of combustion of protein and lipid of 23.8 and 39.6 kJ/g respectively, allow the energy requirement to be expressed as (MH + 50PR + 56LR) for both kinds of animal, where PR and LR are the rates of positive protein and lipid retention (g/d), and MH is the maintenance heat production (kJ/d) which can be estimated as 0.96 of the fasting heat production. The effective energy (EE) yielded to a ruminant animal by a feed ingredient can be estimated as EE (MJ/kg organic matter) = 1.15ME - 3.84 - 4.67DCP, where ME is the metabolizable energy value (MJ/kg organic matter) and DCP is the digested crude protein content (kg/kg organic matter) with both measured at maintenance. Alternatively, EE can be estimated as EE (MJ/kg) = GE (d - 0.228) - 4.67DCP, where GE is the gross energy (MJ/kg) and d is the energy digestibility (MJ/MJ) also measured at maintenance. The EE yielded to a single-stomached animal can be estimated as EE (kJ/g) = 1.17ME - 4.2CP - 2.44, where ME (kJ/g) is measured at, or corrected to, zero N-retention and CP (g/g) is the crude protein (N x 6.25) content of the feed ingredient. The system is simpler for ruminants, and more accurate for both kinds of animal, than those now in use. As effective energy values can be tabulated for ingredients, and are additive to the extent that ME values are additive, they can be used to formulate diets using linear programming.