If one takes empty weight of various planes and divides it with end of the nosecone to end of muzzle lengths of those planes, one gets this:
F-111: 1.04
Su-27: 0.816
F-15c: 0.7
F-15e: 0.786
F-22: 1.165
F-35a: 0.946
Since F-22 and F-22 feature quite a bit of overhang of their tail surface behind the engine nozzles, here are half way values (half way between end of nozzle and end of tails) for them as well, perhaps better representtive of weight distribution:
F-22: 1.1
F-35a: 0.9
Also, sizing J-20 to its missiles, a known value, I get 19.75 meters from tip of nosecone to end of nozzles.
Which planes are most likely for comparison?
F-111: Not terribly relevant role and tech wise, but it does have a weapons bay
Su-27: No air to ground, No long airframe life, no stealth. Does carry all its fuel internally.
F-15c: Not really relevant.
F-15e: A bit more relevant in the sense that we see what sort of weight increase does strengthened airframe and longer airframe life infer.
F-22: Quite a bit relevant. Very long airframe life, full-on stealth, same role, perhaps even a bit less multirole than J20. Materials wise, weight savings may not be quite on par with J20
F-35: Quite a bit relevant. Very long airframe life, possibly similar stealth requirements to J20. Materials should be similar. A bit more oriented towards multirole.
J20 empty weight using other planes' ratios:
20.5 tons if F111
16.1 tons if Su27
15.5 tons if F15E
21.7 tons if F22
17.8 tons if F35
So what characteristics is J20 likely to have?
Long airframe life, as F35 and F22?
Airframe stressed for hauling lots of underwing ordenance, like F35?
Airframe optimized for forward sphere stealth and narrower bandwidth stealth, like F35?
Airframe optimized for broader bandwidth stealth, like F22?
If J20 is engineered to last ~5 thousand hours, not haul a lot of weight under its wings (though we did see it with four huge tanks! Something F22 never did. But maybe it was an extreme stress test? Remains to be seen how often we'll get photos of J20 with four tank configuration), if it's designed for X band stealth, primarely from frontal hemisphere...
Then it's plausible serial grade J-20 weigh 17-18 tons empty.
If J20 is engeered to last 10 or more thousand flight hours, regularly haul a few tons under its wings, if it's designed for broader band stealth (though still only frontal hemisphere)...
Then it's plausible serial J-20 weigh 20-22 tons empty.
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