To calculate diammonium phosphate (DAP) dosage: grams of DAP = (target YAN increase in mg/L × batch volume in liters) ÷ 210. For example, a 20L batch that needs its YAN raised by 100 mg/L requires (100 × 20) ÷ 210 ≈ 9.5g of DAP.
What YAN is, and why must often falls short
Yeast Assimilable Nitrogen (YAN) covers the forms of nitrogen — ammonia and free amino acids — that yeast can actually use to build new cells and complete fermentation. Grape must's natural YAN varies a lot by variety, ripeness, and growing conditions, and higher-sugar musts need proportionally more nitrogen to ferment cleanly to dryness.
Common target YAN ranges for wine fermentation run from roughly 150 to 250 mg/L, with higher-sugar or higher-alcohol-target musts sitting at the top of that range. If a lab test or estimate shows your must falling short, DAP is the standard way to close the gap.
What's the DAP dosing formula?
Diammonium phosphate is roughly 21% nitrogen by weight. Dissolved into a must, 1 gram of DAP per liter raises YAN by approximately 210 mg/L. That gives a straightforward conversion:
grams of DAP = (target YAN increase in mg/L × batch volume in liters) ÷ 210
210 comes from DAP's nitrogen content by weight (roughly 21%), expressed as milligrams per gram. If your specific DAP product's label states a different purity, substitute that number — most commercial DAP sits close to this standard figure, but it's worth checking.
Worked example
Say a YAN test (or a conservative estimate for the grape variety) shows your must needs an extra 100 mg/L of nitrogen, across a 20 L batch:
(100 × 20) ÷ 210 = 9.5 g of DAP
Dissolve the weighed DAP in a small amount of water or must first, then stir thoroughly into the full batch — undissolved powder settling to the bottom doesn't distribute evenly to the yeast population.
Why splitting the dose matters
Adding the full calculated amount in one shot at pitch is a common mistake. Early in fermentation, the yeast population is still small and can't efficiently take up a large nutrient dose all at once — part of it goes unused, and an overwhelmed, stressed colony is more likely to produce hydrogen sulfide (a rotten-egg off-aroma) as a byproduct.
Splitting the total DAP dose across two or three additions during the first third of active fermentation — rather than dosing everything at pitch — keeps yeast in consistently favorable conditions and measurably reduces sulfur off-aroma risk in the finished wine.
Skip the manual math
The wine calculator applies this formula directly to your batch size and target style — no separate conversion needed.
For the full picture on nitrogen's role in clean fermentation and avoiding off-aromas, see the fermentation chapter of Home Winemaking Science.
Handle DAP and other fermentation salts as directed on the product label. Over-fertilizing a must beyond recommended YAN targets doesn't speed fermentation further and can encourage excess yeast biomass at the expense of wine quality.