What's the exact formula for calculating employers NI?

I have a spreadsheet to calculate Er's NI contributions per employee. I can't quite get the roundings right though, we're only talking a penny here and there but they all add up and my grand totals are often wrong.

 

For example, I have one employee who gets £145.92 per week gross. I calculate NI to be 145.92 - 144 = 1.92 x 13.8% = 0.26496. The HMRC calculator says it should be 0.26 so I assume they round down.

 

But, another employee gets £152.00. NI should be 152 - 144 = 8.00 x 13.8% = 1.104. If I round this down it is 1.10. But HMRC says it is 1.11

 

So can anyone help me with the exact way in which I should be calculating these?

Comments
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It's complicated

dnicholson |

Just what I wanted! Thanks.

ajnmx |