Stir constantly until the sugar completely dissolves and the mixture comes to a gentle simmer (do not boil vigorously). This should take about 3–5 minutes.

Add 1 tablespoon fresh orange zest to the saucepan and reduce heat to low. Simmer gently for 2 minutes to infuse the syrup with citrus oils from the zest.

Remove the saucepan from heat and let the syrup cool for 5 minutes, then stir in 1 cup fresh orange juice, 1 teaspoon fresh lemon juice, and 1 teaspoon food-grade citric acid until fully dissolved.

Optional: stir in 1 teaspoon orange extract if you want a brighter, more artificial Fanta-like aroma. Taste and adjust sweetness or acidity: add up to 1/4 teaspoon more citric acid for tang or up to 2 tablespoons more sugar dissolved into the warm syrup for extra sweetness.

Strain the syrup through a fine-mesh sieve into a heatproof container to remove zest and any pulp. Cover and chill in the refrigerator until cold, at least 30 minutes (can be made ahead and stored up to 5 days).

Carbonation & Serving

Assemble the Fanta-style drink

Fill a 16-ounce (about 475 ml) serving glass with 1/2 cup ice (adjust per glass). For each 16-ounce glass, combine 1/2 cup (120 ml) chilled orange syrup with 2 cups (480 ml) cold carbonated water. Pour the syrup into the glass first, then slowly add the carbonated water to preserve fizz.

Stir gently once with a long spoon to combine. Taste and adjust: if too sweet, add more carbonated water; if too weak, add up to 2 tablespoons more syrup.

Garnish with an orange slice on the rim. Serve immediately while cold and fizzy.

To make a pitcher (about 6 cups finished): combine 1 1/2 cups chilled syrup with 4 cups cold carbonated water in a pitcher, stir gently, add ice to individual glasses rather than the pitcher to avoid diluting the whole batch, then pour and garnish.