Hi
You have to feel around the ribcage for the BMI as to whether he is a good weight, underweight or overweight. Without knowing in which area his individual normal weight should be, the numbers don't say much. If you can just about feel the ribs, your boy is a perfect weight for his size. If you can feel single ribs or all ribs, then he is underweight and if you cannot feel any ribs at all, he is overweight.
I have had both piggies who were bordering overweight when they cracked the 800g in the peak of their lives and a large boar who was naturally borderline underweight at 1500g - yet both these piggies lived to 8 years and outlived the majority of my 'average' sized piggies. The size of a guinea pig is genetically determined. What you are aiming at is that the piggy is an ideal weight for their personal size. A small guinea pig that you overfeed in order to fit into the supposedly 'healthy' weight category has in fact a much higher risk of a shortened life span as a result because of the amount of yellow fat around their organs.
I have currently two girls who are just bout touching 600g on a good day but they are now 3 years old and - barring a genetic time bomb - have exactly the same chance of living to 5-6 years old as any larger piggies.
Health is not about being in the big huddle in the big 'average' middle; it is about being the perfect weight/size ratio. 'Average' only means that you are part of the half in the middle; it doesn't say anything else. It is the trap in our own (and even in some vets' minds) that equals 'average' with 'normal' and 'healthy' when that connection doesn't exist in reality.
This guide here explains it all:
1 Weight and Weight Loss
- Why regular weight monitoring matters
- How weight changes over a lifetime
- How to weigh on your kitchen scales (with video)
- The weight loss rules
- How critical is the weight loss for my piggy?
- Possible causes for weight loss
2 Body Mass Index (BMI) or 'Heft'
- Why is understanding your piggy's weight so important?
- 'Average' weight vs. individual weight - the big trip up
- How to check for the BMI
3 The...