Calculating child support

I am currently awaiting to find out how much I am likely to pay for child support. I earn approx £2,000 net per month and will be supporting my sons who are 3 & 6 years old. I am due to be moving in with my girlfriend and her children (aged 11, 12 & 16) soon, will this affect how much I pay as although these children are not mine, I will still be living with them. Depending on how I read it on the CSA website, I will be paying either approx £300 or £400. Does anyone know?

It's 20% of your wage. full stop.. I Don't believe this is per child.. But what else if anything are you paying for in regards to your ex? it all reduces the payment

If the non resident parent has children living with them there is an allowance deducted from the NRPs income before the usual child support calculation. In this case you will have three children living with you so your income will be reduced by 25% to £1,500.

However, if you earn more than your new partner any Working Families Tax Credits are then added to your income before the usual calculation of 20% (minus any deductions for overnight stays your children have with you) for your two children is made.