This could be a standalone tool as an initial component, but it would really shine as an integrated component.

The ACA tax credit has some tricky elements that make it onerous for a regular person to calculate it. The amount varies by ACA MAGI, which can be tough for people to estimate and will change based on other choices people make. It also uses family size, and the second lowest cost silver plan for your zipcode. So there’s a bit of data gathering problem to solve each year, as new regional costs are published for the “Second lowest cost silver plan”.

It also has some interesting interactions with self-employment income. e.g. you get a tax credit based on a special ACA MAGI (similar to the Roth MAGI, but not quite the same), and if you’re self-employed you get to deduct your insurance premiums that are not covered by your ACA credit – this creates an interesting iterative calculation, since the tax deduction lowers your income, which then increases your ACA credit, which lowers your deduction, etc. There’s a whole IRS publication on how to calculate this for self-employed people.

So as a standalone tool you could make it easy for people by having all the regional cost data in the backend, so that folks can just put their family size and zip code, and you could even give an estimate for the ACA deduction if they’re self-employed. But this would only be accurate for a snapshot in time, based on fixed income.

The integrated version of this would simply ask the person if they are buying an ACA plan, the cost of the plan (credit can’t be more than the cost), their zip code and number of covered family members (or would know it from other info given).

The US tax estimator wouldn’t need much more work to calculate the ACA tax credit and ACA tax deduction for self-employed. So if you decide to build the standalone estimator as a first pass, you may find it’s not that much more work to build an integrated credit calculator. I’ve actually done both versions of this with google sheets and apps scripts, it’s not that much work.