Two balances that do not use the same units
Brix is a mass fraction and titratable acidity is grams per litre. That single mismatch is why watering back trips people up. The sugar calculation has to go through the mass of the must — volume times density times Brix — while the acid calculation only needs the volume. Do the first one on volume alone and the water figure comes out wrong by roughly the density, nine percent on a ripe must.
2,000 litres at 1.089 kg/L and 26.5 Brix holds 577.2 kg of dissolved solids. To read 24 Brix that mass has to sit in 2,405 kg of liquid, so 226.9 kg of water goes in, which is 227.3 litres. The volume becomes 2,227 litres and every gram of acid that was in the tank is now spread across it.
The acid drops without being asked
4.8 g/L across 2,000 litres is 9.6 kg of acid. Add 227 litres of water and the same 9.6 kg is now 4.31 g/L. Nothing was removed. The acid went down by half a gram per litre purely because the denominator got bigger, and any acid target set before the water was worked out is now measured against the wrong number.
Order does not change the answer, but the arithmetic order does
Physically it makes no difference whether the water or the acid goes in first, because the final state is a mass of acid in a volume of liquid either way. What matters is which volume you sized the addition against. Work 6.5 minus 4.8 across the original 2,000 litres and you weigh out 3,400 grams. Add the water and that lands at 5.84 g/L, two thirds of a gram short of the target, because the acid you just added got diluted along with the acid that was already there.
The correct figure is 6.5 minus 4.31 across 2,227 litres, or 4,878 grams — 1,478 grams more, 43 percent above the naive number. On a 2,000 litre tank that is a bag and a half of tartaric and an adjustment that has to be made twice, which is exactly the situation the page exists to avoid.
The factor is the supplier number
The form asks how many grams of your product raise one litre by one gram per litre of titratable acidity. It is 1 when you are adding tartaric and reporting titratable acidity as tartaric, which is the common case and the reason the default is there. It is not 1 for any other acid, for a blend, or for a reporting basis other than tartaric, and the correct figure comes from the supplier of the specific product on your bench. This page will not guess it, because a wrong factor produces a perfectly plausible-looking answer that is out by a fifth.
What this page will not do
It will not lower an acidity. Bringing titratable acidity down means chemistry — carbonates, a different set of reactions and a set of consequences for pH and for what precipitates later — and that belongs with an enologist rather than with a page of mass balance.
It will not tell you a target. Not for Brix, not for acidity, not for anything. Those come from your own house style, your own block records and your own palate.
And it will not tell you what you are allowed to do. Whether water may be added to must at all, in what quantity, whether acidification is permitted in your region, what has to be recorded and how the finished wine may be described are all set by law that differs between countries and between regions inside them, and changes. If you type a limit into the optional field the page will convert your figure into a mass for your volume, and that is the entire extent of it. The regulator you answer to decides the rest.
Questions people ask
How much water do I add to bring must from 26.5 to 24 Brix?
Go through the sugar mass rather than the volume. 2,000 litres at 1.089 kg/L and 26.5 Brix holds 577.2 kg of solids; at 24 Brix that mass needs 2,405 kg of liquid, so 226.9 kg of water goes in — about 227 litres, or 11.37 percent of the starting volume. Doing it on volume alone rather than mass understates the water by roughly the density of the must.
Why is my acid addition short after watering back?
Because it was sized on the volume before the water. The acid you add gets diluted by the same water as the acid already in the tank, so a rate worked across 2,000 litres lands 0.66 g/L low once the volume is 2,227. The addition has to be worked as target acidity times the final volume, less the acid mass already present, regardless of which order you physically add things.
Does adding water lower titratable acidity?
Yes, in exact proportion to the volume. It removes no acid at all — the mass in the tank is unchanged — but the same mass over a larger volume reads lower. 4.8 g/L in 2,000 litres becomes 4.31 g/L in 2,227 litres. This is why the Brix decision and the acid decision cannot be made separately.
How many grams of tartaric raise acidity by one gram per litre?
One gram per litre, if you are adding tartaric and reporting titratable acidity as tartaric — which is why the factor field defaults to 1. For any other acid, any blended product, or a reporting basis other than tartaric, the figure belongs to the product and comes from its supplier. A wrong factor gives an answer that looks entirely sensible and is out by twenty percent or more.
Can this page tell me whether the addition is legal?
No. Water, acid and every other addition are regulated by national and regional law that differs by country and changes, labelling is regulated separately again, and none of it is arithmetic. If you enter a limit in the optional field, the page converts your own number into a mass for your own volume and says nothing else about it. Your regulator and a qualified enologist are the sources.