Run the calculator’s defaults, $3,000 in expected medical costs against a $1,500 deductible and 20 percent coinsurance, and here’s what actually happens: the first $1,500 comes straight out of your pocket since that’s the deductible. The remaining $1,500 of expenses is then split with your insurer at the coinsurance rate, so you pay 20 percent of that, $300, for a total of $1,800 out of pocket before any HSA is factored in. Add a $3,900 HSA contribution taxed at a 30 percent rate, saving $1,170, and the calculator’s net cost figure drops to $630. Notice that the $1,800 out-of-pocket figure is nowhere near the full $3,000 in medical costs; the deductible and coinsurance structure caps what you actually pay well below the sticker price of your care.
The calculator above works through your costs in the same order your insurance actually does: it applies your deductible first, dollar for dollar up to whatever you enter, then applies your coinsurance percentage to whatever medical spending is left above that deductible. Separately, it estimates the tax value of an HSA contribution by applying a flat 30 percent savings rate, since HSA contributions reduce your taxable income, and nets that estimated tax savings against your out-of-pocket total. It doesn’t model an out-of-pocket maximum, so if your plan has one and your expenses are high, your real-world costs may cap out lower than what this tool shows.
One tax fact is stable enough to state plainly: unreimbursed medical expenses above 7.5 percent of your adjusted gross income are deductible if you itemize on your federal return. Beyond that, HSA contribution limits, eligibility rules, and exactly which expenses qualify change often enough, and depend enough on your specific plan and filing situation, that this is genuinely a case for checking the current IRS publications or talking to a tax professional rather than relying on a number pulled from a calculator built for rough planning.
How the deductible and coinsurance actually stack
Your deductible is the amount you pay first, dollar for dollar, before insurance starts sharing costs at all. Once you’ve paid that amount, coinsurance kicks in, meaning you and the insurer split whatever comes next at the percentage your plan specifies, commonly somewhere between 10 and 40 percent on your side. Your total out-of-pocket cost, before any out-of-pocket maximum caps it, is the deductible plus your coinsurance share of everything above it.
What an HSA actually saves you
An HSA contribution reduces your taxable income in the year you make it, so the calculator estimates the value of that at a flat 30 percent, meaning a $3,900 contribution is treated as worth $1,170 in tax savings. That’s a simplification of your actual marginal tax rate, which could be higher or lower depending on your income and filing status, but it gives a reasonable ballpark for how much an HSA effectively lowers your net healthcare cost.
Why this tool doesn’t model an out-of-pocket maximum
Real health plans cap your total annual spending at an out-of-pocket maximum, after which insurance covers 100 percent of costs for the rest of the year. The calculation above stops at deductible plus coinsurance and doesn’t apply that cap, so if you have unusually high medical expenses relative to your plan’s maximum, your actual costs may end up lower than the figure shown here. Check your plan documents for the specific out-of-pocket maximum that applies to you.
High-deductible plans are not automatically the wrong choice
A high-deductible plan usually comes with a lower monthly premium and HSA eligibility, and for someone who is generally healthy with low expected medical spending, that combination can genuinely cost less over a year than a low-deductible plan with a higher premium. The calculator lets you test this by running your expected costs through different deductible and coinsurance combinations to see where the total actually lands for your situation.
The one tax rule worth remembering
If you itemize deductions on your federal return, unreimbursed medical expenses above 7.5 percent of your adjusted gross income are deductible. Everything else about medical tax treatment, HSA limits, what specifically counts as a qualified expense, and how state taxes handle any of this, changes often enough and depends enough on your specific situation that it’s worth confirming against current IRS publications or a tax professional rather than a general rule.