What's your top rate of tax? You may think it's 20 per cent or 40 per cent but the convoluted mess created by successive Chancellors means you may be taking home as little as 12p in every pound on some of your income.
That effective 88 per cent tax rate can even hit basic-rate taxpayers, putting them in a higher bracket than the richest people in the land.
Those on modest incomes of less than £20,000 a year may end up with as little as 27p of every £1 they earn.
In a bizarre quirk, parents of large families could pay tax of more than 90 per cent on parts of their income while a £1,000 pay rise could leave some people worse off.
The convoluted mess created by successive Chancellors means you may be taking home as little as 12p in every pound on some of your income
These anomalies, calculated for Money Mail by accountant Deloitte, are caused by government policies that give with one hand and take away with the other.
While these are extreme examples, official figures confirm that millions are paying extraordinarily high tax rates on portions of their income.
In the worst circumstances, this can render pay rises, overtime payments and bonuses practically worthless, with far more money going to the Government than to those who earn it.
Patricia Mock, tax director of accountant Deloitte, says: 'The UK income tax system is complicated with no fewer than seven rates of income tax, including a zero rate that can apply to savings and dividends.
'When National Insurance, tax credits and child benefit clawback are taken into account, this can lead to some very unexpected results.'
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Our taxing tax system
On the surface, our income tax system seems to be simple. There's a tax-free personal allowance of £11,500.
In England, Wales and Northern Ireland the next £33,500 — the portion between £11,501 and £45,000 — is taxed at 20 per cent. In Scotland the 20 per cent rate applies from £11,501 to £43,000.
Income above this is taxed at 40 per cent and any income over £150,000 a year is taxed at 45 per cent.
But then you get the wrinkles. Those earning more than £100,000 a year have their tax-free allowance gradually taken away.
There is withdrawal of benefits and, on top of that, National Insurance (Nics). Nics looks like a tax and works like a tax, being docked from the wages of anyone below state pension age.
It is supposed to give us certain benefits, primarily the state pension. Nics is charged on earnings over £8,164 per year at 12 per cent for the employed and 9 per cent for the self-employed (because they don't qualify for certain contribution-based benefits).
There's a tax-free personal allowance of £11,500. In England, Wales and Northern Ireland the next £33,500 is taxed at 20%, then 40% and any income over £150,000 a year is taxed at 45%
It drops to 2 per cent on earnings over £45,000. Once you hit state pension age, you no longer pay Nics.
So our effective tax rates actually end up looking more like this: up to £8,164 per year — no tax; £8,165 to £11,500 — 12 per cent; 11,501 to £45,000 — 32 per cent; £45,000 to £150,000 — 42 per cent; and £150,000 and above — 47 per cent.
For the Scots, things are slightly worse, as we will see later.
When you dig deeper, nasty twists start to emerge in the 'marginal rates'. This is the tax you pay on a specific portion of your income.
Due to the complex way our tax and benefits systems interact, many people have extraordinarily high marginal tax rates on certain earnings. These may come in the form of child tax credits being withdrawn, child allowance being taken or the whittling-away of the personal allowance.
With the help of tax experts we delved into the mire.
The traps to watch out for
Families on low incomes can benefit from working and child tax credits. But once income is over £6,420, the entitlement is reduced by 41p for every extra £1 earned. The actual level when withdrawal starts will depend on your circumstances.
As a result, the marginal tax rate can be as high as 73 per cent — 41 per cent tax credit withdrawal, 20 per cent income tax and 12 per cent Nics.
Deloitte offers the example of someone with three children, but no childcare costs, who earns £20,000 a year and works more than 30 hours a week.
If they receive a £1,000 pay rise, they will pay £200 in tax and £120 Nics, and lose £410 from withdrawal of tax credits, leaving them with just £270 extra a year.
In other words, they'd take home 27 per cent of the pay rise, with 73 per cent going to the Government.
Families on low incomes can benefit from working and child tax credits
Estimates from the Institute for Fiscal Studies suggest more than a million families lose some or all of their child benefit because one parent earns more than £50,000 a year.
The benefit is snatched at a rate of 1 per cent for every £100 earned over this threshold, so by the time earnings reach £60,000 the child allowance is gone.
The withdrawal is based only on the salary of the higher-earning parent. If one parent earns £52,000 and the other £15,000 (total £67,000) they will lose 20 per cent of their benefit. But if both earn £49,000 (total £98,000) they lose nothing.
The effective tax rate depends on how many children you have. For one child you would lose just under 21p for every £100 of extra annual earnings, making a marginal tax rate of 52.6 per cent on up to £10,000 of income.
This is based on 40 per cent tax, 2 per cent Nics and 10.56 per cent for the withdrawal of Child Benefit. So for every £100 earned between £50,000 and £60,000 you would take home £47.40.
A family with two children would face a marginal tax rate of 59.2 per cent. With four children the marginal rate would rise to 73.46 per cent.
In the most extreme situation, Deloitte says a family with eight children could see a marginal tax rate of 101.33 per cent because the rate for Child Benefit withdrawal would be 59.33 per cent added to the 42 per cent for tax and Nics. And it could be even worse for someone entitled to childcare help and tax credits.
A woman working more than 30 hours a week with three children and childcare costs of £200 a week can continue to claim tax credits when her income is in the £50,000 to £60,000 a year range.
In this case a salary rise of £1,000 a year would actually leave her worse off. While earning £55,000 a year her total income would be £41,958. This is because she would pay £10,700 income tax and £4,620 Nics but then gain £1,251 Child Benefit and £1,027 tax credits.
But out of a £1,000 pay rise to £56,000 she would pay £400 tax and £20 Nics, and lose £251 Child Benefit and £410 tax credits. That's a total deduction of £1,081.
There's an unlikely trap waiting for families with one earner who earns just under the higher-rate tax band.
In England, Wales and Northern Ireland this is £45,000 a year. In Scotland it is £43,000.
The marriage allowance lets basic-rate taxpayers use £1,150 of a non-taxpaying partner's personal allowance — effectively raising their tax-free pay to £12,650.
The non-taxpayer must apply to transfer this portion and it can cut the taxpayer's bill by £230 a year.
But if, for example, someone in England earns £45,000 and receives a bonus of £500 they will become a higher-rate taxpayer and lose the right to use the allowance. So on that bonus their marginal tax rate would be 88 pc, leaving them just £60 better off.
The marriage allowance lets basic-rate taxpayers use £1,150 of a non-taxpaying partner's personal allowance — effectively raising their tax-free pay to £12,650
For those in Scotland, the 40 per cent higher-rate tax cuts in on earnings over £43,000. But their Nics thresholds are the same as in the rest of the UK, so they are charged 12 per cent Nics on earnings up to £45,000.
This gives the Scots a marginal tax rate of 52 per cent on income between £43,001 and £45,000, before it drops back to 42 per cent.
Higher earners with incomes of more than £100,000-a-year gradually have their personal allowance withdrawn.
The allowance is reduced by £1 for every £2 of extra income. This creates a marginal tax rate of 62 per cent on income between £100,000 and £123,000 a year (at which point there is no allowance left) based on tax and Nics.
The £100,000 starting point for withdrawing the allowance has not changed since it was introduced by Labour Chancellor Alistair Darling in April 2010.
HMRC estimates that 893,000 taxpayers will pay this 62 per cent rate this tax year — that's 3 per cent of all taxpayers.
Universal credit — the new payment that replaces a range of benefits including housing benefit, child tax credit and jobseeker's allowance — is withdrawn at a rate of 63p for every £1 of extra income, but based on after-tax earnings.
Paul Falvey, tax partner at accountant BDO, explains: 'If your income takes you from basic to higher-rate tax you will lose out a lot.'
Someone whose income rises from £44,000 to £46,000 will pay £740 tax from their £2,000 increase.
But then their benefit will also be reduced by £794, so on their £2,000 pay rise they will be only £466 better off. The loss of tax and benefits would take 77 per cent of the money.
'We create these boundaries where people pay very high rates of tax because the system is far too complicated,' Mr Falvey says.
Most taxpayers can put up to £40,000 a year into their pension tax-free, if they have the spare cash. Anything above this limit is taxed at your top tax rate.
But the Government has made sure that those with the most money to spare have a much smaller allowance.
Once someone earns more than £150,000 a year, their annual pension allowance is reduced by £1 for every £2 above the threshold, until it reaches a lower limit of £10,000.
By preventing high earners using tax relief to cut their tax bill the Government has effectively imposed a 69.5 per cent tax rate on earnings between £150,000 and £210,000.
This is made up of 45 per cent income tax, 2 per cent Nics and 22.5 per cent for the loss of the allowance.
How you can fight back
The simplest way to cut your tax bill is to pay more into your pension
The simplest way to cut your tax bill and dodge the highest marginal tax rates is to pay more into your pension if you can afford to.
Workplace pension contributions are deducted from your wages before tax is calculated.
If you contribute to a personal pension, the effect is much the same, though make sure you declare the pension payment to HMRC to get the benefit.
For someone who benefited from the marriage allowance and received a £500 pay rise that put them just into the higher-rate tax band, the results could be astonishing.
If they paid the full £500 into their pension, instead of losing the marriage allowance and paying 88 per cent tax on their pay rise, they could keep the allowance and the £500 contribution would effectively cost just £60.
The perks are terrific for those who face withdrawal of Child Benefit. They could pay money into their pension and gain tax relief of at least 52.6 per cent.
In other words, every £100 pension contribution would come at an effective cost of less than £48.
But the biggest perk goes to those most likely to be able to afford it: those with earnings between £100,000 and £123,000 a year who have their personal allowance withdrawn.
Someone on £123,000 could have £23,000 paid into their pension by their employer using salary sacrificeand it would cost them £8,740 — an effective tax relief rate of 62 per cent.
Even if they paid it in without salary sacrifice, they would get 60 per cent tax relief making their contribution £9,200.
Salary sacrifice schemes offered by employers can also reduce your tax and National Insurance bill if you join a Childcare Vouchers scheme before the end of the tax year on April 5, 2018, or make use of the cycle-to- work scheme.
So you could buy a smart new bike and preserve your Child Benefit at the same time.
Or you could give a bonus to charity using Gift Aid. In the worst cases of marginal taxes you could make a substantial gift that would cost you practically nothing.