9 CFR 319.106(b)(7)

Dry-curing weight loss

Percentage lost against the green weight, and the weight a piece has to reach for a stated target.

There is exactly one weight-loss figure in US law: a country ham or dry cured ham must finish at least 18 percent lighter than its fresh uncured weight. Every other target in circulation, whether 30, 35 or 40 percent, is trade practice rather than a published parameter, and this site does not print those as though they were.

The arithmetic is trivial. What is not trivial is what the number means.

The one figure in US law

Minimum loss: 18 %
Measured against: fresh uncured weight
Applies to: dry cured ham and shoulder

9 CFR 319.106(b)(7). Applies to Country Ham, Country Style Ham, Dry Cured Ham, Country Pork Shoulder, Country Style Pork Shoulder and Dry Cured Pork Shoulder.

Calculate

Green weight (g)

Weight now (g)

Target loss (%)

18 percent is the figure in 9 CFR 319.106(b)(7) for dry cured ham. It is a legal minimum for that named product, not a general target.

Lost so far

20.00 %

Working

loss % = (green - now) / green x 100
       = (5,000.0 - 4,000.0) / 5,000.0 x 100
       = 20.000 %

weight at 18.0 % loss
       = green x (1 - 18.0 / 100)
       = 4,100.0 g

still to lose = 0.0 g

Past the 18.0 percent target.

Weight loss is a proxy, not a measurement of safety. What controls the growth of Staphylococcus aureus is water activity, and the relationship between the two depends on the fat content of the piece: fat carries almost no water, so a fatty piece reaches a given water activity at a lower percentage loss than a lean one.

Worked example

A leg at 5,000 g green, now 4,000 g.

loss = (5,000 - 4,000) / 5,000 x 100 = 20.0 %

to reach the 18 percent figure in 9 CFR 319.106(b)(7):
  5,000 x 0.82 = 4,100 g
  already past it

What this does not tell you

Weight loss is a proxy for water activity, and a poor one. Fat carries almost no water, so a fatty piece reaches a given water activity at a lower percentage loss than a lean one.
A piece can hit a weight-loss target with a wet centre. Case hardening is exactly that failure.