סימן מט סעיף ג
יַיִן שֶׁנִּתְעָרֵב בְּמַיִם, אִם אֵין בַּיַּיִן אֶלָּא אֶחָד מִשִּׁשָּׁה חֲלָקִים שֶׁבַּמַּיִם, וַדַּאי בָּטֵל הַיַּיִן וְאֵינוֹ רַק כַּמָּיִם. וְאִם יֵשׁ בּוֹ יַיִן יוֹתֵר, אִם דֶּרֶךְ אַנְשֵׁי הַמָּקוֹם לִמְזֹג אוֹתוֹ כָּל כָּךְ וְלִשְׁתֹּתוֹ בִּמְקוֹם יַיִן, מְבָרֵךְ עָלָיו בּוֹרֵא פְּרִי הַגָּפֶן וּלְאַחֲרָיו עַל הַגֶּפֶן. וְאִם לֹא, בְּטֵלָה דַּעְתּוֹ (ר"ד).
If wine was diluted with water, and the wine is only one sixth of the mixture [i.e., five parts water to one part wine,]9Though the Gemara states that a mixture less than three parts water to one part wine is not considered wine, the poskim explain that this applied only to wine made at the time of the Mishnah when the wine was so strong that it could not be drunk “raw”, i.e. undiluted. However, that sort of wine making is a lost art and we now no longer produce such strong wines and we judge each mixture according to its time and place. But, in all cases, a mixture exceeding five parts water to one part wine is no longer wine. the wine is definitely nullified, and it is considered as water. But, if it has more wine [than one sixth], and it is the custom of the people of that area to dilute it to that degree and to drink it as wine, the berachah over it is Borei peri hagafen and afterwards Al hagefen. However if the people of that area do not drink it as wine, the individual's opinion does not count [and the berachah is Shehakol].
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)