Process for producing chlorosilanes in a fluidized bed reactor by reaction of a hydrogen- and silicon tetrachloride-containing reaction gas with a particulate contact mass containing silicon and a catalyst, wherein the chlorosilanes have the general formula H n SiCl 4-n and/or H m Cl 6-m Si 2 where n = 1-4 and m = 0-4, characterized in that - the reactor design is described by an index K 1 = φ ⋅ V reactor , eff A tot , cooled ⋅ d hyd , where φ = fill level of the reactor, V reactor , eff = effective volume of the reactor [m 3 ], A tot , cooled = sum of cooled surface areas in the reactor [m 2 ], d hyd = hydraulic reactor diameter [m], wherein V reactor, eff is 1 to 300 m 3 and d hyd is 0.5 to 2.5 m; - the constitution of the contact mass is described by an index K 2 = R Si ⋅ B AK . δ rel d 32 , where B AK = breadth of the particle size distribution of the contact mass [pm], d 32 = particle Sauter diameter [μm], R Si = purity of the silicon, δ rel = relative catalyst distribution in the contact mass, wherein δ rel is 0.001 to 7, d 32 is 10 to 2000 μm, B AK is 10 to 1500 μm and R Si is 0.75 to 0.99999; - the reaction conditions are described by an index K 3 = u L ν F ⋅ 10 6 ⋅ p diff g ⋅ 1 ρ F , where u L = superficial gas velocity [m/s], ν F = kinematic viscosity of the fluid [m 2 /s], ρ F = fluid density [kg/m 3 ], p diff = pressure drop over the fluidized bed [kg/m*s 2 ], g = acceleration due to gravity [m/s 2 ], wherein p diff is 10 000 to 200 000 kg/m*s 2 , u L is 0.05 to 2 m/s, ρ F is 2 to 20 kg/m 3 and ν F is 3*10 -7 to 5.4*10 -6 m 2 /s; and wherein K1 has a value of 2 to 20, K2 has a value of 0.001 to 200 and K3 has a value of 0.5 to 10 000. - the reactor design is described by an index K 1 = φ ⋅ V reactor , eff A tot , cooled ⋅ d hyd , where φ = fill level of the reactor, V reactor , eff = effective volume of the reactor [m 3 ], A tot , cooled = sum of cooled surface areas in the reactor [m 2 ], d hyd = hydraulic reactor diameter [m], wherein V reactor, eff is 1 to 300 m 3 and d hyd is 0.5 to 2.5 m; φ = fill level of the reactor, V reactor , eff = effective volume of the reactor [m 3 ], A tot , cooled = sum of cooled surface areas in the reactor [m 2 ], d hyd = hydraulic reactor diameter [m], wherein V reactor, eff is 1 to 300 m 3 and d hyd is 0.5 to 2.5 m; - the constitution of the contact mass is described by an index K 2 = R Si ⋅ B AK . δ rel d 32 , where B AK = breadth of the particle size distribution of the contact mass [pm], d 32 = particle Sauter diameter [μm], R Si = purity of the silicon, δ rel = relative catalyst distribution in the contact mass, wherein δ rel is 0.001 to 7, d 32 is 10 to 2000 μm, B AK is 10 to 1500 μm and R Si is 0.75 to 0.99999; B AK = breadth of the particle size distribution of the contact mass [pm], d 32 = particle Sauter diameter [μm], R Si = purity of the silicon, δ rel = relative catalyst distribution in the contact mass, wherein δ rel is 0.001 to 7, d 32 is 10 to 2000 μm, B AK is 10 to 1500 μm and R Si is 0.75 to 0.99999; - the reaction conditions are described by an index K 3 = u L ν F ⋅ 10 6 ⋅ p diff g ⋅ 1 ρ F , where u L = superficial gas velocity [m/s], ν F = kinematic viscosity of the fluid [m 2 /s], ρ F = fluid density [kg/m 3 ], p diff = pressure drop over the fluidized bed [kg/m*s 2 ], g = acceleration due to gravity [m/s 2 ], wherein p diff is 10 000 to 200 000 kg/m*s 2 , u L is 0.05 to 2 m/s, ρ F is 2 to 20 kg/m 3 and ν F is 3*10 -7 to 5.4*10 -6 m 2 /s; u L = superficial gas velocity [m/s], ν F = kinematic viscosity of the fluid [m 2 /s], ρ F = fluid density [kg/m 3 ], p diff = pressure drop over the fluidized bed [kg/m*s 2 ], g = acceleration due to gravity [m/s 2 ], wherein p diff is 10 000 to 200 000 kg/m*s 2 , u L is 0.05 to 2 m/s, ρ F is 2 to 20 kg/m 3 and ν F is 3*10 -7 to 5.4*10 -6 m 2 /s; <EOS>