How much does a light-weight bike weigh? Interested in 18''-20'' bikes. Brand maybe? i gotta be able to carry it up to 3rd floor every day!?

Are you talking abour FRAME size or WHEEL size?

And what do you mean by "light-weight"?

And what do you want to use the bike for?

It's perfectly possible to get bikes below 10 kg /22 lbs, but I wouldn't expect to find one under $1000. Probably closer to $2000.

Below 8 kg/18 lbs I guess would be above $3000.

And this is assuming you're talking about frame sizes 18-20".

These would be fairly exotic and delicate road bikes, not particularly well suited for everyday use.

I can carry my ~37lb/17kg fully equipped 19" 700C tourer to the 4th floor easily enough. The trick is to get the top tube over your right shoulder with your left hand on the bars so the frame is slightly low in the front end, but clear of the step ahead of you and controlled.

The actual bike is somewhat lighter, but having a decent D-lock, racks on front and back, mudguards, dynamo lights, toolkit and at least one bag (plus whatever water is being taken out (shouldn't really be much coming back though)... it adds around 5kg to the bare weight.

Trying to carry the bike any other way is awkward and difficult though... even when it is a bit lighter.

I can manage the bike up and down short flights even with an extra 40-50 lbs in 4 panniers, but it is generally easier to unload them when inside my building and to take the bike and bags separately if I need to.

I'm not large nor especially strong (~110lbs, 5'5") so there shouldn't be significant issues with bikes and stairs for most people. (Note that despite the 'hefty' weight this is a high quality bike, but one built for loaded strength and reasonable speed and comfort - it isn't a light weight racing bike, but has a riding position that isn't too far off, but is comfortable to ride for up to 12 hours, and which I can cruise at 20mph on the flat).

Go to the local bike shop and talk with them a hybrid would be the lightest unless you want a road bike.

Around 4-5kg. Not cheap, tho.

It depends on material used to build the bicycle.