About This Calculator
In hot vintages, grape must often arrives at crush above 26–28 °Brix. Fermenting it straight can stress yeast, produce stuck fermentations, and result in wines that are too alcoholic and unbalanced. The standard remedy — used in California wineries and by experienced home winemakers — is to water back the must before pitching yeast.
The catch: plain water dilutes sugar and acidity together. If your must is already borderline low in TA (common in hot-vintage fruit), diluting with plain water pushes it even lower. The solution is to acidulate the water — dissolve tartaric acid into the dilution water at the same concentration as your must's TA — so only the sugar is diluted while acidity holds steady.
How to Use This Calculator
- Step 1 — Measure your must: Take a Brix reading (refractometer or hydrometer), measure TA with a titration kit, and optionally read pH with a calibrated meter. Enter total must volume including pomace for red must.
- Step 2 — Set targets: Choose a wine style for auto-filled typical targets, then adjust. Set the acidulated water TA to your must's current TA (default) to hold acidity constant, or to your final target TA if you want to build acidity during dilution.
- Step 3 — Read results: The calculator shows how many litres (or gallons) of water to add, how many grams of tartaric acid to dissolve per litre of that water, post-dilution Brix and TA, and whether any additional direct tartaric acid top-up is needed.
- Step 4 — Add in two passes: Mix in 70–80% of the calculated water, wait 30 minutes, stir and re-measure Brix. Heterogeneous berries can shift the reading. Add the remaining water as needed. Similarly, add tartaric in stages and retest TA.
Typical Must Chemistry Targets
| Style | Brix | Potential ABV | TA (g/L) | pH |
|---|---|---|---|---|
| Red dry | 23–25° | 12.7–13.8% | 6.0–9.0 | 3.4–3.65 |
| White dry | 20–23° | 11.0–12.7% | 6.5–8.0 | 3.1–3.4 |
| Rosé dry | 20–22° | 11.0–12.1% | 6.5–7.5 | 3.2–3.5 |
Sources: MoreWine complete must adjustment guide; WineMaker Magazine; Vinmetrica pH/TA management guide.
Formulas Used
Vwater = Vinitial × (Binitial ÷ Btarget − 1)
Tartaric acid in dilution water:
g tartaric = TAwater (g/L) × Vwater (L)
Post-dilution TA (mixed volume):
TApost = (TAjuice × Vjuice + TAwater × Vwater) ÷ (Vjuice + Vwater)
Additional tartaric top-up:
g extra tartaric = (TAtarget − TApost) × Vfinal (L)
Potassium bicarbonate (deacidification):
g K-bicarb = (TApost − TAtarget) × 0.67 × Vfinal (L)
Potential ABV:
ABV% ≈ Brix × 0.55
The Brix dilution formula uses mass-conservation of dissolved sugars (C₁×V₁ = C₂×V₂). The volume-change factor from the added water is accounted for directly. The 0.55 Brix-to-ABV factor is the standard home-winemaking estimate confirmed by MoreBeer / Brehm Vineyards and WineMaker Magazine. Sources: Brehm Vineyards acidulated water guide; MoreBeer complete must adjustment; WineMaker Magazine — Adjust Your Must.
Why the Juice Fraction Matters
Red grape must contains grape skins, seeds, and pulp (pomace). Your Brix refractometer reads the juice, but the total volume in your fermenter includes the pomace. Typically, red must is 60–70% juice by volume (Bordeaux varieties ~60%, Rhône-style and Zinfandel ~70%). This matters because you're acidulating the water to the juice TA, and the TA-change calculation should be applied to the juice volume, not the total pomace-including volume. White and rosé juice that has already been pressed is 100% liquid.
Common Mistakes
- Adding plain water to high-acid must when TA is already at target: dilution will undershoot your acidity. Acidulate to maintain.
- Adding all the water at once before stirring and re-reading Brix. Raisined or heterogeneous berries will keep releasing sugar, shifting the reading.
- Adding tartaric acid too aggressively. Buffering capacity means the first 75% of a calculated addition often does most of the work. Always add in stages, mix thoroughly, wait 15–30 minutes, and retest.
- Ignoring the juice fraction for red must. Calculating acid against total must volume (including pomace) over-estimates how much tartaric acid you need.
- Using citric acid instead of tartaric for must acidulation. Citric acid can be metabolised by lactic acid bacteria during MLF into acetic acid, increasing volatile acidity. Tartaric acid is stable.
Frequently Asked Questions
Plain water dilutes both sugar and acidity proportionally. If you acidulate the dilution water to the same TA concentration as your must, the acid level stays constant after dilution while only the Brix drops. This is standard California winery practice for hot-vintage water-back corrections, described in the Brehm Vineyards acidulated water guide and the MoreBeer complete must adjustment article.
Red must contains grape skins, seeds, and pulp — not just juice. Only the liquid fraction (typically 60–70% of total must volume) is relevant for acid calculations. Using total must volume over-estimates how much tartaric you need. White and rosé juice that has been fully pressed is 100% liquid, so no fraction adjustment is needed.
Adding 1 gram of tartaric acid per litre raises Titratable Acidity (expressed as tartaric acid equivalents) by approximately 1 g/L. However, buffering capacity means the actual result can vary by 10–15% in either direction. Best practice: add 75% of the calculated amount, mix thoroughly, wait 30 minutes, retest, and add the remainder only if needed.
For red dry wine: Brix 23–25° (potential ABV ~12.7–13.8%), TA 6.0–9.0 g/L, pH 3.4–3.65. For white dry wine: Brix 20–23° (potential ABV ~11–12.7%), TA 6.5–8.0 g/L, pH 3.1–3.4. Rosé sits between the two. Hot vintages regularly produce red grapes above 27°Brix, making water-back necessary to avoid stuck fermentations and over-alcoholic wines.
Use potassium bicarbonate: approximately 0.67 g/L of potassium bicarbonate reduces TA by about 1 g/L. Dissolve it in a small volume of water, stir into the must, and retest after 30 minutes. Don't try to remove more than 2 g/L in one correction, as higher doses can impart a salty or bitter taste. The calculator shows potassium bicarbonate quantity when post-dilution TA exceeds your target.
Pre-fermentation dilution is strongly preferred — sugar readings are accurate, acidity readings are stable, and additions mix homogeneously. Once fermentation begins, dissolved CO₂ interferes with hydrometer and TA readings. If you must add water during fermentation, do it during the first 24–48 hours before alcohol rises significantly.
These results are estimates for guidance only. Actual must chemistry varies with grape variety, vintage, harvest heterogeneity, and instrument calibration. Always verify with properly calibrated instruments and taste. This tool is not professional winemaking advice.