I have kept regular records of the weights of my three male rats, Widget, Cricket and Snip, starting from their arrival at age 6 weeks. I have used a Healthometer dial kitchen scale, which has an error margin of about 25 grams and goes up to 5 lbs ( 2275 g).

The blue dots represent the average weight of my three rats at a given time. The error bars represent one standard deviation. The rats gained weight rapidly up to their third month. After the third month their weight gain continued but at a slower rate.

Here are the actual weight measurements of the three rats. All three rats follow a similar pattern of weight gain, with very rapid gain up to three and a half months of age followed by a slower increase. Snip is consistently the lightest rat. Widget was heaviest early on, but was surpassed by Cricket during the fifth month. Cricket continues to be the heaviest rat by a large margin.