A method for determining a dialysis patient's dry weight comprising the steps of: continuously measuring the extracellular resistance of a body segment during dialysis and graphing the ratio of R 0 /R t as a function of time, wherein R 0 is the extracellular resistance at the start of dialysis and R t is the extracellular resistance as a function of time; flattening the graph of the ratio of R 0 /R t as a function of time during a time interval Δt, wherein the graph of the ratio of R 0 /R t as a function of time is considered as flattened when at least one of the following equations is met: δ Δt < C 1; and k S = δ Δt / Δt < C 2; wherein λ(t) = R 0 /R t; δ(Δt) = λ(t) - λ(t+Δt); and C 1 and C 2 are constants; and wherein the flattening of the graph of the ratio of R 0 /R t as a function of time is confirmed during the time interval Δt by an algorithm of least squares; once the λ(t) function is successfully flattened, comparing the patient's normalized resistivity as a function of time (ρ N (t)) to a minimum level of normalized resistivity in healthy subjects (ρ N, H); identifying the time at which the dry weight of the patient has been achieved when ρ N t ≥ ρ N, H; and calculating the patient's dry weight; wherein the patient's normalized resistivity as a function of time (ρ N (t)) is calculated according to the following equation: ρ N t = ρ t BMI × 10 - 2 Ω ⋅ m 3 / kg wherein BMI is the patient's body mass index defined as body weight divided by body height squared; and ρ(t) is the resistivity of the body segment as a function of time, calculated according to the following equation: ρ t = χ t 2 ⋅ R t 4 ⁢ πL Ω ⋅ cm wherein L is the length of the body segment; and χ t is the circumference of the body segment as a function of time which is continuously calculated according to the following equation: χ t = χ 0 2 - 4 ⁢ π ⁢ ρ 0 ⁢ L R 0 ⁢ 1 - R 0 R t wherein χ 0 is the initial circumference of the body segment; and ρ 0 is a constant value of resistivity; wherein the patient's dry weight is calculated according to the following equation: DW = W pre - ∫ t 0 t DW ⁢ UFR t ⋅ dt where W pre is the patient's body weight prior to dialysis; UFR(t) is the ultrafiltration rate as a function of time during dialysis, with to to t DW being the time interval of a functioning UFR in which t 0 is the start time of dialysis and t DW is the time at which the dry weight of the patient is achieved. continuously measuring the extracellular resistance of a body segment during dialysis and graphing the ratio of R 0 /R t as a function of time, wherein R 0 is the extracellular resistance at the start of dialysis and R t is the extracellular resistance as a function of time; flattening the graph of the ratio of R 0 /R t as a function of time during a time interval Δt, wherein the graph of the ratio of R 0 /R t as a function of time is considered as flattened when at least one of the following equations is met: δ Δt < C 1; and k S = δ Δt / Δt < C 2; wherein λ(t) = R 0 /R t; δ(Δt) = λ(t) - λ(t+Δt); and C 1 and C 2 are constants; and wherein the flattening of the graph of the ratio of R 0 /R t as a function of time is confirmed during the time interval Δt by an algorithm of least squares; once the λ(t) function is successfully flattened, comparing the patient's normalized resistivity as a function of time (ρ N (t)) to a minimum level of normalized resistivity in healthy subjects (ρ N, H); identifying the time at which the dry weight of the patient has been achieved when ρ N t ≥ ρ N, H; and calculating the patient's dry weight; wherein the patient's normalized resistivity as a function of time (ρ N (t)) is calculated according to the following equation: ρ N t = ρ t BMI × 10 - 2 Ω ⋅ m 3 / kg wherein BMI is the patient's body mass index defined as body weight divided by body height squared; and ρ(t) is the resistivity of the body segment as a function of time, calculated according to the following equation: ρ t = χ t 2 ⋅ R t 4 ⁢ πL Ω ⋅ cm wherein L is the length of the body segment; and χ t is the circumference of the body segment as a function of time which is continuously calculated according to the following equation: χ t = χ 0 2 - 4 ⁢ π ⁢ ρ 0 ⁢ L R 0 ⁢ 1 - R 0 R t wherein χ 0 is the initial circumference of the body segment; and ρ 0 is a constant value of resistivity; wherein the patient's dry weight is calculated according to the following equation: DW = W pre - ∫ t 0 t DW ⁢ UFR t ⋅ dt where W pre is the patient's body weight prior to dialysis; UFR(t) is the ultrafiltration rate as a function of time during dialysis, with to to t DW being the time interval of a functioning UFR in which t 0 is the start time of dialysis and t DW is the time at which the dry weight of the patient is achieved.