Choose the right weight before calculating canned-food nutrition
Separate net quantity from drained solids and read the nutrition heading before estimating a drained portion.
A can can display both a net quantity and a smaller drained weight. Neither number automatically tells you which material the nutrition panel describes. Read the panel's wording before deciding whether a per-100-g value belongs to the whole contents or the drained food.
Annex IX of the EU food information regulation addresses drained net weight for solid food in a liquid medium. That quantity statement alone does not replace the need to identify the nutrition basis.
Keep the two weights visible
Consider a hypothetical can marked 400 g net quantity and 240 g drained weight. Its nutrition heading explicitly says “per 100 g drained product,” with 6 g protein. If all the stated drained contents are used, the calculation is 6 × 240 ÷ 100 = 14.4 g protein.
Using the 400 g net quantity instead would produce 24 g. That result applies the drained-solids concentration to liquid that the heading excludes. It is an input mismatch rather than a multiplication mistake.
For a 90 g portion weighed after draining, the corresponding estimate is 6 × 90 ÷ 100 = 5.4 g protein. Write “90 g drained portion” in the result so its state remains clear.
Do not work backwards from an unclear panel
If a different can simply says “per 100 g,” do not infer the basis from a similar brand or the position of the drained-weight statement. Look for additional wording, preparation instructions, or manufacturer clarification. Mark the calculation pending if the basis cannot be established.
Likewise, an as-sold value cannot generally be converted to a drained value using only the weight ratio. Nutrients may be present in the liquid as well as the solids. Their distribution is not supplied by the two package weights.
Keep rinsing and retained liquid separate
Your drained portion may retain a different amount of liquid from the declared measurement basis. Rinsing introduces another change that this arithmetic does not quantify. Do not claim an exact sodium reduction merely because water was poured through the food.
A practical record includes the nutrition heading, net quantity, stated drained weight, your measured drained portion, and whether rinsing occurred. That record preserves the assumptions even if the final result is only an estimate. It also makes clear which missing information would be needed for a more precise comparison.