Watermelon syrup is the rare syrup you should never put on the stove. Watermelon's flavour lives in a set of volatile compounds that cook off fast; simmer the juice and you get something that tastes like a melon-flavoured candy rather than a melon. So this one is built cold — juice, sugar, stir, done — and it is measured by weight in a 3:1 ratio of juice to sugar, which is the reverse of the 2:1 rich syrups most bars keep on the rail.
Three Parts Juice, One Part Sugar
The 3:1 here is juice to sugar, by weight — not the sugar-forward ratio the name might suggest if you're used to reading simple syrup as sugar-first. It is a deliberately lean syrup. Watermelon juice is already sweet and extremely watery, so a conventional 1:1 or 2:1 sugar load would bury the fruit under sugar and give you a syrup that only reads as sweet. At 3:1 the sugar does its structural job — body, a little preservation, something for the acid to push against — while the watermelon stays the loudest thing in the glass.
Weigh it, don't measure it by volume. Watermelon varies enormously in water content between melons, and a cup of juice from a dead-ripe melon in August is a different ingredient from a cup pressed out of a pale one in March. A scale makes the ratio mean something.
The Build
Blitz seedless watermelon flesh and strain it through a fine sieve — you want juice, not pulp, because pulp drops out of suspension in a day and turns the bottle cloudy and sour. Weigh the strained juice, add a third of that weight in granulated sugar, and stir at room temperature until the sugar has fully dissolved. That is the whole method. The small measure of vodka at the end is a preservative, not a flavour: roughly two percent by weight is enough to slow spoilage without making the syrup taste boozy.
Storage and Shelf Life
Up to a week refrigerated, and it is genuinely at its best in the first few days. Raw fruit juice with no heat treatment is a hospitable place for yeast, and a syrup that has started to ferment will smell faintly of cider and fizz when you pour it. That's your cue to bin it and press another melon. Make it in small batches and use it.
Bottom Line
A five-minute syrup with a one-week clock on it, and worth restarting that clock every time. Nothing you can buy in a bottle tastes like fresh watermelon, because the part that tastes like watermelon is the part that does not survive being cooked, shipped, or shelved.