Well it's probably less than you'd think - let's have a (very rough and wholly unscientific) stab at it:
£20000 income means (under current tax bands, and assuming a tax code of 503L which is about normal) that you will pay income tax of £3034.30. Your national insurance contribution will be something like £1645.
That's just the tax on earnings though, there are lot of taxes on spending too, but these depend on your lifestyle and where you live.
The average council tax in this country was just over £1000 last year, but we'll stick about £1500 down for that (assuming he's the only person in his household contributing to it).
Petrol tax is confusing, but if we assume our subject drives his car about 15000 miles a year (a canny amount). it's fair to say that he'll be coughing up about £1000.
So our subject has been taxed about £7180 so far, leaving him with about £12820.
You pay a VAT of 17.5% on most purchases, but not all as some things (e.g. some foodstuffs) are exempt and some stuff has a lower rate. For arguments sake we'll say he spends ALL of his remaining income on normal VAT goods (highly unlikely as he'll probably spend a load of it on his mortgage or whatever!), that means he's paying tax of 17.5% * 12820 = about £2250.
Bringing his total tax bill for the year to about £9450. Which is probably a fair bit higher than the actual figure for most people of that income in reality!
Sorry if my figures aren't that accurate but I'm very tired and it's time for bed... I know, I'm a sad git, and should be down the pub on a friday night.
Tax and NI would be about 22%, so about £4400, CBC mentioned about £4600.
Again agree with CBC on council tax although he may be living at home.
Talking about tax on purchases is confusing and I do not think he asked for this. Someone on £20k with a family would not be a big spender.
Wages after tax and NI = about £15,500
The next part is complex and every family varies; hence, I do not want to take about figures.
There may be rent/mortgage to pay, other bills, car, etc. However, tax credits, child benefit, etc. do help families with costs.
If you are a family man on £20k and with a child, you would probably be living in a council flat, or renting a low demand area with proper a high crime rate.
In a report a few years ago, they said to purchase a house in Hull, one needs to on a wage of at least £13k.
Steven, you thought about moving to Hull?