For a 1-gallon (3.785 L) batch at 68°F targeting a standard 2.4 volumes CO2, the priming sugar dose is 23.4 g of corn sugar — proportionally almost exactly a fifth of the 116.7 g a full 5-gallon batch needs at the same temperature and target, since the calculation scales linearly with volume. Check this exact dose.
The arithmetic isn't the hard part here — the scale is
Scaling down a batch scales down the sugar dose proportionally, and that's the easy half of this. The real challenge at 1-gallon scale is weighing 23.4 g accurately, which is a very different problem than weighing 116.7 g. A typical kitchen scale reads in 1-2 g increments — a small but tolerable error on a 5-gallon dose, where a 2 g miss is under 2% of the total, but a meaningfully larger relative error on a 1-gallon dose, where the same 2 g miss is closer to 9% of the total dose. If you're bottling small batches regularly, a scale that reads to 0.1 g removes this problem almost entirely and is a worthwhile investment for the price.
Why this isn't "just divide the 5-gallon recipe by 5"
It genuinely is that simple mathematically — priming dose scales linearly with volume at a fixed temperature and target, so dividing a known 5-gallon dose by 5 gets you very close to the right 1-gallon number. Where this goes wrong in practice is when the 1-gallon batch's actual residual CO2 differs from the 5-gallon assumption — a small batch that fermented in a different location, or at a different peak temperature than assumed, needs its own temperature entered rather than reusing the larger batch's figure. Always enter your batch's own actual peak temperature, not an assumption carried over from a different batch.
A practical tip for small-batch dosing
Dissolving the calculated sugar in a small amount of boiled and cooled water, then stirring it gently into the batch before bottling (rather than trying to distribute a tiny quantity of dry sugar evenly across a handful of individual bottles) gives far more even carbonation across bottles than dosing bottle-by-bottle — and it's the same technique standard batch priming uses at any scale, just working with a proportionally smaller amount of both sugar and water.
If you're scaling the other direction
For the full 5-gallon version of this same calculation, including the sugar-type comparison and how the dose shifts with a different CO2 target, see how much priming sugar for 5 gallons.