How "% left" is computed

The idea

A bottle on the scale weighs glass plus liquid. Take away the glass and you know how much liquid is left.

Everything rests on one identity:

full weight = empty weight + size × density

• Empty weight is the bottle with nothing in it, in grams.
• Size is the bottle’s volume in millilitres.
• Density is how much one millilitre of the liquid weighs.

Any two of these give the third. So when the product registry knows a bottle’s empty weight and its size, the app can turn a weight into “% left”. Knowing the density makes the result more accurate.

A worked example

Take a 700 ml bottle of spirit at 40% ABV.

• The liquid in a full bottle weighs 663.6 g.
• A typical 700 ml spirit bottle weighs about 480 g empty.
• So a full bottle weighs about 480 + 663.6 = 1,143.6 g.

Now put an open bottle on the scale. It reads 812 g.

• Liquid left: 812 − 480 = 332 g.
• 332 g out of 663.6 g is half.

The app shows about 50% left and about 350 ml, and adds 0.50 of a bottle to the count.

Where the numbers come from

Not every product has been weighed yet, so the app uses the best figure available. From best to worst:

Empty weight

1. Measured. People have weighed empty bottles of this product. Odd readings, such as a pourer left on or the wrong bottle, are rejected so they do not spoil the average.
2. Worked out from a full bottle. The sealed-full weight minus size × density. For spirits this is accurate to about ±3 to 5 g.
3. Category estimate. A typical glass weight for that category and size, for example about 480 g ± 60 g for 700 ml spirit bottles. This is low confidence and is flagged as an estimate.

Density

1. Measured. Worked out for the product once both empty and full bottles have been weighed at least 3 times each.
2. From the ABV. Very good for unsugared spirits. Wrong for liqueurs and syrups.
3. Category average. The average of measured products in the same category. This is how liqueurs become accurate: the first few measured cream liqueurs calibrate the whole shelf.

Why there is a ± figure

The app shows millilitres with a ± margin instead of false precision. For a 700 ml spirit, the scale itself is good to about 1 to 2 g, and glass varies between production runs by about 10 to 20 g. Liqueurs carry a wider margin until their category has enough measurements.

Help improve it

Every accepted pair of empty and full weights improves that product, its category, and future estimates. See File sealed-full and empty weights.