A method for the manufacture of a polyether diol product having a polyether hydroxyl number PolOH of from 4 to 120 mg KOH/g by reaction of an initial starter substance S i having two hydroxyl groups (initial starter S i) with one or more alkylene oxides in the presence of water and a double metal cyanide catalyst, comprising the steps of a. providing a starter mixture, comprising the initial starter S i and the DMC catalyst, b. continuously adding an alkylene oxide to the starter mixture, c. continuously adding water to the starter mixture, wherein i. steps b and c either start simultaneously or step b starts before step c, preferably steps b and c start simultaneously, ii. the initial starter S i has a starter hydroxyl number StartOH of from 6 to 250 mg KOH/g, iii. the water is added in such an amount that the concentration of the water, based on the total masses of the water and the alkylene oxide, is from 150 ppm to 3500 ppm, iv. the build ratio BR, defined as the mass of polyether diol product divided by the mass of initial starter S i in the starter mixture, is at least 4, v. the polydispersity index PDI of the polyether diol product is 1.5 or less, and PDI is calculated as Mw/Mn, wherein Mw and Mn are determined according to the method outlined in the description vi. StartOH is 1.25*PolOH or larger, vii. StartOH and PolOH are determined by ASTM D4274-16, met. D, viii. the amount of the continuously added water is adjusted such that cH2O, calculated as {100*(total mass of continuously added water in g)/[(total mass of continuously added water in g) + (total mass of continuously added alkylene oxide in g)]} fulfills the equation cH 2 O = PolOH − A − B * StartOH / C wherein A is in mg KOH/g and is calculated as A = 0.034*BR + 0.16, B is calculated as B = 0.0068* 2 - 0.1153*BR + 0.6098, C is in mg KOH/g and is calculated as C = -0.3836* 2 + 6.6482*BR + 24.48, and ix. the ratio of PolOH/StartOH is in the range between 1 and 1/BR. a. providing a starter mixture, comprising the initial starter S i and the DMC catalyst, b. continuously adding an alkylene oxide to the starter mixture, c. continuously adding water to the starter mixture, i. steps b and c either start simultaneously or step b starts before step c, preferably steps b and c start simultaneously, ii. the initial starter S i has a starter hydroxyl number StartOH of from 6 to 250 mg KOH/g, iii. the water is added in such an amount that the concentration of the water, based on the total masses of the water and the alkylene oxide, is from 150 ppm to 3500 ppm, iv. the build ratio BR, defined as the mass of polyether diol product divided by the mass of initial starter S i in the starter mixture, is at least 4, v. the polydispersity index PDI of the polyether diol product is 1.5 or less, and PDI is calculated as Mw/Mn, wherein Mw and Mn are determined according to the method outlined in the description vi. StartOH is 1.25*PolOH or larger, vii. StartOH and PolOH are determined by ASTM D4274-16, met. D, viii. the amount of the continuously added water is adjusted such that cH2O, calculated as {100*(total mass of continuously added water in g)/[(total mass of continuously added water in g) + (total mass of continuously added alkylene oxide in g)]} fulfills the equation cH 2 O = PolOH − A − B * StartOH / C wherein A is in mg KOH/g and is calculated as A = 0.034*BR + 0.16, B is calculated as B = 0.0068* 2 - 0.1153*BR + 0.6098, C is in mg KOH/g and is calculated as C = -0.3836* 2 + 6.6482*BR + 24.48, and A is in mg KOH/g and is calculated as A = 0.034*BR + 0.16, B is calculated as B = 0.0068* 2 - 0.1153*BR + 0.6098, C is in mg KOH/g and is calculated as C = -0.3836* 2 + 6.6482*BR + 24.48, and ix. the ratio of PolOH/StartOH is in the range between 1 and 1/BR.