Impossible to say with any degree of accuracy I’d say. I had several carp yesterday that I thought could be doubles which were around 6lb, length or even overall appearance are, in my humble opinion, not a good indicator of weight.
Going by your carp comment , I have done the same ,thinking it was a double and finding out the bloody thing was hollow and weighed in at around 5lb and 6lb.
Yet on another venue had carp of the same length and similar girth or broader shoulders and they have been low doubles .
The hollow ones were on a commercial , the others on gravel pit and a lagoon .
My theory as daft as it sounds is , the commercial ones basically are feeding and crapping, or competing for bait and not putting the pounds due to possibly being caught a few times a day and end up rushing round the water either on the end of a pole and elastic or the end of a rod n line .
The gravel pit and lagoon ones don't have to compete for food, they can graze without fear of competition therefore put weight on easier.