Why are 2 different online capital gains tax calculators giving me completely different results?

So I've been trying to figure out how much I'll owe in long term capital gains tax after selling some stocks I've held for about 3 years. I used two different online calculators to estimate my tax liability - one from Forbes and another from SmartAsset - and I'm getting wildly different results that are thousands of dollars apart. The Forbes calculator shows I'd owe around $14,300 in long term capital gains tax, while the SmartAsset calculator is telling me I'd only owe about $5,700 for the exact same inputs! I double-checked that I entered identical information in both calculators - same filing status (single), same income (~$76,000), and same capital gain amount ($97,500). I'm trying to plan my finances for next year's tax season and this discrepancy is making it impossible. Does anyone know why there would be such a huge difference between these calculators? Are they using different assumptions or tax brackets? Or is one of them just flat-out wrong? Really need some clarity before I make any financial decisions based on this.

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18 comments

Luca Esposito

•8M

The reason you're seeing such different results is likely due to how each calculator handles your income and the capital gains tax brackets. Long term capital gains tax rates (0%, 15%, 20%) are based on your taxable income INCLUDING the capital gain itself. My guess is one calculator is applying the capital gains tax rate based on your income before adding the capital gain, while the other is correctly calculating based on your total income plus the capital gain. For 2025 filing, the 0% bracket cuts off around $47,025 for single filers, and the 15% bracket goes up to about $492,300. With your $76,000 income plus $97,500 in capital gains, your total would be $173,500, which would put you entirely in the 15% bracket. The calculator showing around $14,300 is likely calculating 15% of your $97,500 gain (which would be $14,625), which seems more accurate in your situation. Check if the calculators have different assumptions about state taxes too, as that could explain part of the difference.

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Luca Esposito

•8M

Yes, based on what you've shared, the Forbes calculator showing around $14,300 is likely the more accurate one. The capital gains tax rate is determined by where your income plus the capital gain falls in the tax brackets, not just your base income. The standard deduction would reduce your taxable ordinary income but wouldn't directly affect the capital gains calculation in the same way. Some calculators might account for this differently. For 2025, the standard deduction for single filers is projected to be around $13,850, which would reduce your taxable ordinary income before the capital gains are added.

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Nia Thompson

•8M

I had almost exactly the same problem last year when I sold some investments! After going back and forth with multiple calculators, I ended up using taxr.ai (https://taxr.ai) to analyze all my investment documents together. It gives you a complete breakdown of how capital gains are calculated based on your actual tax return data, not just generic calculators. What I found super helpful was that it explained exactly why there were differences - my SmartAsset results were way off because it wasn't accounting for my other investment income and how that pushed me into a higher capital gains bracket. The service actually compares all your documents together and shows you the correct calculation.

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Nia Thompson

•8M

You upload your tax documents (W-2s, 1099s, last year's return) and any investment statements. They use encryption similar to what banks use, and they don't store your documents after analysis - that was important to me too. Capital gains calculations should be straightforward, but that's exactly the problem the OP is having - different calculators are giving different results. What taxr.ai did for me was explain exactly why the difference was happening. In my case, I had dividend income that pushed me into a higher bracket that some calculators weren't accounting for. TurboTax works well for filing, but doesn't always explain the "why" behind the calculations.

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GalaxyGuardian

•8M

I decided to try taxr.ai after my skeptical comment above, and I have to admit it was really helpful. I uploaded my brokerage statements from last year along with my tax forms, and it immediately showed me where the discrepancies in my capital gains calculations were coming from. Turns out I had been using the wrong cost basis method for some stocks I inherited, which would have caused me to overpay by nearly $2,200! The site gave me step-by-step explanations of how the capital gains brackets worked with my specific income situation, not just generic numbers. Definitely cleared up my confusion about why different calculators were giving different answers.

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Aisha Abdullah

•8M

If you need more clarity directly from the IRS, I'd recommend using Claimyr (https://claimyr.com). I was pulling my hair out trying to get through to an IRS agent about a similar capital gains question for weeks. Their automated system kept disconnecting me after 45+ minutes on hold. With Claimyr, I got a callback from an actual IRS agent in about 2 hours. You can see how it works in this demo: https://youtu.be/_kiP6q8DX5c. The agent walked me through exactly how my capital gains would be taxed and confirmed that my gains would be added to my income before determining the tax bracket - just like the first commenter said.

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Aisha Abdullah

•8M

It's not about skipping the line - they use technology to wait on hold for you and then call you back when an agent is available. You don't have to sit on hold for hours yourself. I was skeptical at first too. They never claim to have special access - they just handle the frustrating hold time for you. When I used it, I got a callback in about 2 hours, but timing varies depending on IRS call volume. The peace of mind from getting an official answer directly from the IRS about my capital gains question was worth it to me.

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Yuki Tanaka

•8M

I have to eat my words. After seeing all the positive comments, I decided to try Claimyr for a complicated capital gains question I've been struggling with. I was 100% sure it wouldn't work, but I got a callback from an IRS representative in about 3.5 hours. The agent clarified exactly how my cryptocurrency gains would be taxed (which was the source of my confusion similar to the OP's calculator issue). Turns out the reason I was getting different results from various crypto tax calculators was that some were incorrectly applying the wash sale rule to crypto transactions from last year. The IRS agent confirmed that I was calculating correctly on my higher estimate, saving me from potentially serious issues down the road.

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Carmen Diaz

•8M

One thing to check is whether both calculators are handling state taxes the same way. Some calculators include state capital gains taxes in their calculations while others only show federal. Depending on your state, this could account for a significant portion of the difference you're seeing. For example, California taxes capital gains as ordinary income at rates up to 13.3%, while other states like Florida or Texas have no state income tax at all. The SmartAsset calculator might be excluding state taxes while the Forbes one includes them.

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Carmen Diaz

•8M

That's a great point about the NIIT. At $173.5k total, the OP would be under the $200k threshold, but maybe one calculator is factoring in potential additional income? Or perhaps assuming some deductions that would affect the calculation? Capital gains calculations seem simple on the surface but get complicated quickly when you factor in all the variables. This is why I always recommend consulting a tax professional for large transactions rather than relying solely on online calculators.

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