Process for producing chlorosilanes in a fluidized bed reactor by reaction of a hydrogen chloride-containing reaction gas with a particulate contact mass containing silicon and optionally 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 = 1-3 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 60 m 3 and d hyd is 0.7 to 1.8 m; - the constitution of the contact mass without catalyst is described by an index K 2 uncat = R Si ⋅ B AK d 32, where B AK = breadth of the particle size distribution of the contact mass [pm], d 32 = particle Sauter diameter [pm], R Si = purity of the silicon, wherein B AK is 10 to 1500 μm and d 32 is 10 to 1000 μm; - the constitution of the contact mass with catalyst is described by an index K 2 cat = δ rel R Si, where δ rel = relative catalyst distribution in the contact mass, wherein δ rel is 0.005 to 3 and R Si is 0.75 to 0.9999; - 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] and p diff = pressure drop over fluidized bed [kg/m*s 2], wherein u L is 0.05 to 4 m/s, ν F is 3*10 -6 to 2.5*10 -5 m 2 /s, ρ F is 1.5 to 5 kg/m 3 and p diff is 10 000 to 400 000 kg/m*s 2; and wherein K1 has a value of 1 to 10, K2 uncat has a value of 0.1 to 10, K2 cat has a value of 0.005 to 3 and K3 has a value of 1 to 700. - 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 60 m 3 and d hyd is 0.7 to 1.8 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 60 m 3 and d hyd is 0.7 to 1.8 m; - the constitution of the contact mass without catalyst is described by an index K 2 uncat = R Si ⋅ B AK d 32, where B AK = breadth of the particle size distribution of the contact mass [pm], d 32 = particle Sauter diameter [pm], R Si = purity of the silicon, wherein B AK is 10 to 1500 μm and d 32 is 10 to 1000 μm; B AK = breadth of the particle size distribution of the contact mass [pm], d 32 = particle Sauter diameter [pm], R Si = purity of the silicon, wherein B AK is 10 to 1500 μm and d 32 is 10 to 1000 μm; - the constitution of the contact mass with catalyst is described by an index K 2 cat = δ rel R Si, where δ rel = relative catalyst distribution in the contact mass, wherein δ rel is 0.005 to 3 and R Si is 0.75 to 0.9999; δ rel = relative catalyst distribution in the contact mass, wherein δ rel is 0.005 to 3 and R Si is 0.75 to 0.9999; - 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] and p diff = pressure drop over fluidized bed [kg/m*s 2], wherein u L is 0.05 to 4 m/s, ν F is 3*10 -6 to 2.5*10 -5 m 2 /s, ρ F is 1.5 to 5 kg/m 3 and p diff is 10 000 to 400 000 kg/m*s 2; and wherein K1 has a value of 1 to 10, K2 uncat has a value of 0.1 to 10, K2 cat has a value of 0.005 to 3 and K3 has a value of 1 to 700. u L = superficial gas velocity [m/s], ν F = kinematic viscosity of the fluid [m 2 /s], ρ F = fluid density [kg/m 3] and p diff = pressure drop over fluidized bed [kg/m*s 2], wherein u L is 0.05 to 4 m/s, ν F is 3*10 -6 to 2.5*10 -5 m 2 /s, ρ F is 1.5 to 5 kg/m 3 and p diff is 10 000 to 400 000 kg/m*s 2;