Assuming you’re not in Scotland, your tax code works like this:
£12,500 personal allowance - no tax due. But, this is reduced by £1 for every £2 you earn over £100,000.
That’s £20,000 for you, so £20,000 / £2 is £10,000. So you lose £10,000 of you personal allowance.
So you pay:
0% tax on £2,500
20% tax on £2,500-£37,500
40% tax on £37,500-£120,000
(you would start to pay 45% over £150,000)
This is simplified, as it doesn’t take into account any pension payments, and you may have other benefits with tax and/or NI relief - such as share save schemes, childcare vouchers or cycle to work scheme.
You can easily use an online calculator to simulate your take home pay if you vary your pension payment. This example isn’t salary sacrifice but if you take the £20,000 and pay it into your own personal pension at £1667 per month. It’s just to give you an idea, as I can’t account for your exact circumstances:
£120,000 salary = £74,139 take home
£120,000 salary with £1667 per month (£20,000) put in a pension = £66,137 take home plus £20,000 in pension plus £13,336 added to pension by HMRC.
So you lose £8,002 in take home pay and gain £33,336 in pension.
Honestly, I cannot understand why you have not spoken to an IFA. If you earn £120K but don’t understand tax codes, it’s the obvious thing! They’ll save you more money in one meeting than you’ll pay them.
Agree with PP that you need to watch out for the annual allowance - but you can carry over some unused allowance. I won’t post all the rules here - go see an IFA!