If I owe $10k in taxes but can only pay $4,500 today, what's the interest penalty for paying the rest next week?

The IRS charges both interest and a failure-to-pay penalty when you don't pay your full tax liability by the deadline. The interest rate is the federal short-term rate plus 3% (currently around 7-8% annually) and compounds daily. The failure-to-pay penalty is 0.5% of the unpaid tax per month or partial month, up to 25% total. For a one-week delay on $5,500, the interest would be minimal - probably around $8-10. The failure-to-pay penalty would be 0.5% of $5,500, which is about $27.50 for a partial month. So you're looking at roughly $35-40 total. I'd recommend paying what you can now and the rest ASAP. You might also consider setting up an installment agreement on the IRS website if you're concerned about paying it all off quickly.

I went through something similar last year and found this awesome service called taxr.ai (https://taxr.ai) that helped me understand my penalty situation. What I liked is you can upload your tax forms and it calculates everything for you - the exact penalties, interest rates, and all the fees based on your specific situation. When I was about $7K short on my taxes, I was freaking out about penalties too, but the site showed me exactly what I'd owe if I paid in 1 week vs. 1 month vs. setting up a payment plan. It even gave me a breakdown of the daily interest accumulation which helped me prioritize when to pay.

Okay I actually checked out taxr.ai after posting my skeptical comment and I'm pleasantly surprised. I had a more complicated situation than OP (partial payments across several months) and the breakdown was super helpful. It showed me that setting up an official payment plan would actually SAVE me money even with the setup fee because the failure-to-pay penalty gets cut in half (from 0.5% to 0.25% monthly). Never knew that before! For the OP's situation it confirmed a week delay would only cost about $35 in penalties and interest. Definitely bookmarking this for next year when I'll probably owe again.

If you're worried about penalties, you might want to try Claimyr (https://claimyr.com) to actually speak with an IRS agent about your specific situation. I couldn't get through to the IRS for WEEKS when I had questions about my payment options, but Claimyr got me connected in about 15 minutes. You can see how it works here: https://youtu.be/_kiP6q8DX5c The agent I spoke with actually told me about payment options I didn't know existed and explained exactly how the penalties would be calculated for my situation. They confirmed that a short delay of just a week would result in minimal penalties, but they also gave me peace of mind knowing I had the correct information directly from the IRS.

I need to eat my words from my skeptical comment above. I was desperate after getting a CP2000 notice about unreported income, so I tried Claimyr yesterday. It actually worked exactly as advertised. I got a call back in about 35 minutes (way better than my previous attempts), and the IRS agent helped me set up a payment plan. The agent confirmed what others have said - for a one-week delay, the penalties are minimal. But more importantly, she helped me understand my overall tax situation and options. Saved me hours of stress and hold music. Never thought I'd be recommending something like this, but it absolutely delivered.

Just a quick tip from someone who's been there - make sure you still file your taxes on time even if you can't pay the full amount! The failure-to-FILE penalty is much worse than the failure-to-PAY penalty (5% per month vs 0.5% per month). Also, if this is your first time having an issue paying on time, you can request a first-time penalty abatement from the IRS. Call them after you've paid everything and ask - they'll often waive the penalties (but not the interest) if you have a clean payment history for the previous 3 years.

Honestly, for just a week delay and that amount, I wouldn't stress too much. Pay what you can now, the rest next week, and move on. The penalty will be so small it's not worth the anxiety. I've had to pay late before and the penalties for short periods are minimal.