The formula
ideal weight = current weight ÷ (1 + (score − 5) × 0.1)
Where it comes from
There is no breed weight table here, and that is deliberate: they are published by individual breed clubs, they disagree with each other and they are useless for the most common animal of all, the crossbreed. What vets use is the nine-point body condition score, where five is ideal and every point above is roughly ten per cent of excess weight. That relationship is published and works for any animal, pedigree or not, because it measures fat rather than breed.
How to work it out by hand
- Score the body condition: ribs, waist seen from above and abdominal profile
- Every point above five is 10 % of excess
- Divide the current weight by one plus that excess
- Plan the loss at 1.5 % of the ideal weight per week
What is worth knowing
Condition is scored with the hands, not the scales: on an ideal animal the ribs can be felt under light pressure but not seen, there is a waist looking down from above, and the abdomen tucks up in profile. A long coat hides two whole points of the scale. Safe loss is one to two per cent of bodyweight a week, so taking five kilos off a dog takes months rather than weeks: crash plans fail and in cats they are dangerous.