Priming Sugar Calculator

How this works

Bottle conditioning works by giving residual yeast a small, precisely measured dose of fermentable sugar, which it ferments in the sealed bottle and produces CO2 that has nowhere to go but into solution. Get the dose right and you get a properly carbonated beer; get it meaningfully wrong on the high side and you risk over-pressurizing the bottle.

The calculation has two parts. First, this tool estimates how much CO2 is already dissolved in your beer, using a standard published fit (residual CO2 (volumes) = 3.0378 - 0.050062 x tempF + 0.00026555 x tempF²) based on the warmest temperature your beer has been at since fermentation finished — not the current fridge temperature, because CO2 the beer has already released at a warmer temperature doesn't come back just because it's now cold. Colder beer holds more dissolved CO2 at equilibrium, so a beer that's been sitting at 68°F carries noticeably less residual CO2 than one that's been fermenting at 55°F, and needs correspondingly more priming sugar to reach the same target.

Second, it computes the mass of sugar needed to produce the remaining gap between your residual CO2 and your target, using the widely-reproduced formula grams of corn sugar = 15.195 x volume (US gallons) x (target volumes - residual volumes) — note the constant is per US gallon, not per litre, so a litre input is converted to gallons before it's applied. Table sugar and dry malt extract (DME) use standard published conversion ratios relative to corn sugar — table sugar needs slightly less mass (it has no water of hydration diluting its fermentable content), and DME needs meaningfully more mass (only part of its weight is fermentable sugar).

The over-carbonation warning

Most standard homebrew bottles are rated safely to roughly 3.5-4 volumes of dissolved CO2 — beyond that, the risk of a bottle failing under internal pressure ("bottle bombs") rises sharply. This calculator flags any target above 3.5 volumes and recommends either champagne-rated bottles (built to hold sparkling-wine pressure) or kegging with force carbonation instead, which lets you dial in a high carbonation level under controlled pressure rather than relying on fermentation inside a sealed glass bottle.

Reading the result

If your beer consistently comes out under- or over-carbonated despite following a calculated dose, check two things first: whether you're measuring the temperature correctly (the warmest point since fermentation, not today's fridge temperature), and whether your bottles are sealing consistently — a slow leak at the cap will always read as under-carbonation no matter how accurate the sugar dose was.

Priming and carbonation are process-control questions, not strength questions — see the responsible-brewing note below every result on this site.

A worked example

A 19 L (5 gallon) batch that finished fermenting at 68°F, targeting a moderate 2.4 volumes of CO2 with corn sugar, needs 117.3 g of sugar — the residual CO2 already dissolved at 68°F accounts for about 0.86 volumes on its own, so the calculator is really only asking the priming sugar to make up the remaining roughly 1.54-volume gap to the 2.4-volume target, not the whole 2.4 volumes from a standing start. Switch the same batch to DME and the required mass rises to roughly 159.6 g, since only part of DME's weight is fermentable sugar.

Why temperature matters more than people expect

Two identical batches targeting the identical 2.4 volumes of CO2 — one that fermented at 65°F, one that fermented at 75°F — need meaningfully different priming sugar doses, because the warmer batch is starting from less residual dissolved CO2. Brewers who prime by a fixed rule of thumb ("always use 4-5 oz of corn sugar for 5 gallons") without accounting for temperature are effectively guessing at this correction, which is part of why the same rule of thumb can produce a nicely carbonated beer one batch and an over- or under-carbonated one the next.