Posted on Academic board and didnt get any replies, thought the the B&I geniuses would know...

Do only HSA contributions made from payroll deduction help lower SAI or do contributions I also make on my own to my HSA also help lower SAI? TIA

HSA contributions you make lower your AGI which is used for SAI, regardless of where they come from.

You are better off most of the time to stick with payroll deductions instead of doing it yourself because payroll deductions also don't have to pay payroll tax. If you make a contribution on your own after the fact you don't get refunded the payroll tax you might have already paid on those dollars.

Lowering your payroll taxes this way might have a small effect on your social security income which is used if/when you start collecting social security, but most people would rather have the savings today rather than later.

Edit: you have until you file your taxes (or April 15,2025) to make an HSA contribution for 2024. I have seen a case where someone made a HSA contribution to keep their AGI under the threshold of having to pay long term capital gains tax.