It is important that you keep a regular record of your piggies weight. I would recommend that you create a pattern for weighing your guinea pig. Why not weigh once a week – or two at the most?
Create a folder or place where you can list the date and the weight of your piggie. At any point if you feel your piggie is unwell, then you need to increase how often you weigh your piggie. If your piggie is really unwell then you will definitely be needing to weigh daily at those times.
It is usual for a weight to vary by 25 grams (adult piggie) due to urine loss. However if you are recording consistently lower and lower weights then you know that your guinea pig is loosing weight daily and there is a problem.
Guinea pigs do loose some weight when they age but this really does not start happening until they are 5 years or so of age and it is a slow process.
To weigh your piggie you will need scales that can acurately measure to 2kg in weight. Usually your kitchen scales will do this. However it may not be appropriate to use the kitchen scales to have a piggie weighed. This is easily rectified. Make sure you have a container for the piggie to sit inside. Ice-cream containers are perfect!
Place the ice-cream container onto the scales – then turn them on. This way the ready at zero will already take into account the container. Now add your piggie into the container to get an accurate measurement.