Wasn't aware of this, good info. Basically allows you to take advantage of NI being calculated on each pay (weekly or monthly) while tax is calculated annually. Requires you to be able to cut your in-pocket pay drastically for part of the year I guess (and then increase it for the other part), but if that's not a problem it could save a few hundred quid over a year. If I've understood correctly you don't need to alternate actually, just have a few months where you pile into your pension so that the NI savings include a big chunk at the 8% rate, then reduce your pension contributions for the rest of the year, to make up the same overall total contribution and money in your pay over the year. You then save a bit more NI in those 'piled in' months than you would do if you kept contributions the same all year (all assuming you're in this zone generally with earnings somewhere in the 60-100K zone). So could be done with two switches of contributions in a year, if timed correctly.Let's say your Gross Pay is £60k per year (imagine that's a flat £5k per month).
You suggested contributing into your pension to bring that down to £50,270. Which works out £4189 per month and saves you 42% (40% tax & 2% NI) on the £921 you put in your pension.
Phasing of contributions makes no difference to income tax allowance, it is calculated annually. But NI is calculated based on the period in which you're paid (in this example, the month) - over £4189 per month NI is 2%, under is 8%
Which means if you alternate your contributions; month 1 £5k per month, month 2 you contribute £1842 to pension and you save 2% NI on the first £921 and 8% on the second £921.
It's more obvious when a one-off bonus is involved. Let's say your base pay is £4189 per month (£50270pa) plus a £10k bonus. Anything you salary sacrifice from your usual wage you'll save 8% in NI but any sacrifice from your bonus you'll only save 2%. So a bonus month is often the worst time to contribute to a pension, but many people do it.
Obviously, keep up any minimum contributions required to get your employer matched contributions. And don't salary sacrifice too much that takes you below minimum wage or they might remove you from the salary sacrifice scheme altogether.
Key considerations being not to let your earnings fall below minimum wage during those 'piled in' months, and also to make sure in the months you pay less into your pension that you still contribute enough to get the employer contributions at full rate too (for me I need to put in at least 6% to get their 6% contribution).
Bit of a hassle to save a few hundred but it's free money I guess.
And noting only possible if your employer runs a salary sacrifice scheme.
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