There's a straightforward sum that is used by the UK government, and the rest of the EU, to define "poor" for the purposes of benefit (welfare) targeting, etc.
Anyone (more properly, any household) who earns less than 60% of the median income, after deducting housing costs, is "poor". It is a rule of thumb, rather than a hard-and-fast rule, but it works reasonably well for most purposes.
(If you don't already know, the "median" is the income figure that has 50% of the population above and 50% below it. It is not necessarily the same as the "mean" - what we usually talk about as the "average", and certainly isn't for income distribution, where the bottom end is fixed at zero and the top end is not fixed but has the likes of Bill Gates to pull it upwards.)
The median income (from memory) is currently $63,278 for a four-person family (for calendar 2001) (
source), compared to £14,800 per taxpayer in the UK (
source - for UK fiscal 2001: April 2001 to April 2002. In the same place, the
mean is given as £20,300, but in a Poisson-like distribution, the mean is skewed upwards by the few very high incomes at the top end, of which more later.)
If we assume that the four-person household contains two taxpayers (a reasonable assumption?), an assuming that the median British household also contains two taxpayers (another reasonable assumption?) the direct comparison is $31,639 to £29,600.
Not knowing what housing costs are in the USA, and since many of the things we pay for through tax here are paid by individuals or companies there (most notably healthcare), I don't think there
is a direct comparison, but for the sake of argument (and 'cos it's fun!), let's assume the costs of housing are about the same in both nations, and just compare pre-tax incomes levels.
Which gives us "poverty lines" at
$18,983 and
£17,760 per household, respectively for the US and UK.
When comparing ourselves to the Third World, we should use global median incomes and housing costs. I couldn't find these anywhere, but I'm guessing than the global median would be less than $1,000, so yes, we'd all be "rich".
Also, by implication, if a two-person household earns more than 140% of the median, that household is "rich". So, if you live in a household that earns more than $44,295 in the US or £41,440 in the UK, you're technically "rich" and should stop complaining!
I think this is the core of the problem - no matter how wealthy we are in absolute terms such as these, there is almost always someone richer than we are. We tend to measure our wealth by our disposable incomes, forgetting that the non-disposable part is earmarked in our minds on those expensive cars, plush houses, foreign holidays, etc.
And, we tend to think about income distribution as if it is a "normal" or "Bell" distribution, with ourselves at about the "average" level, when in fact it is tails of a very long way upwards, with a bunch at the downward end (a bit like the Poisson probability distribution for those with a statistical bent).
The truth is, the state of being "rich" starts comfortably in the middle classes. As indeed does the state of being "poor".