Pay tax before or after commission?

therivierakid

I recently started tutoring students in my spare time. I have registered with HMRC as self-employed and I informed them of the fact that I have a full time job through which I pay my standard income tax and NICs. They said I could apply for a low income NI Exemption certificate (and I'm still waiting on that).

But my question is this: I currently get all my work through an agency and I have to pay the agency a flat commission of £3 per session that I give. So when I come to do my self-assessment do I calculate the tax I owe before or after deducting the commission fee?

chappers

would have thought after , would have thought the commission is an allowable business expense