The IRS doesn't just want your tax paid by April — it wants it paid roughly on schedule throughout the year, through withholding or quarterly estimated payments. Fall short of that pace and the shortfall accrues interest, charged as an underpayment penalty, even if you write a check in full when you file. This calculator gives you a fast, honest estimate of that penalty from three numbers you already have: this year's expected tax, what you've paid so far, and last year's total tax bill.
How it works
The IRS lets you avoid the penalty by meeting a "required annual payment" — the smaller of two thresholds. The first is 90% of this year's tax, a number you can only estimate mid-year. The second is 100% of last year's tax, a number that's already locked in (110% if your prior-year adjusted gross income was over $150,000, a stricter version Congress built in for higher earners). Because the rule always takes the smaller of the two, this calculator computes both and keeps the lower figure as your target.
From there the math is simple: subtract what you've actually paid — withholding plus any estimated payments made — from that required annual payment. Whatever's left over is your underpayment. Multiply it by the current IRS underpayment interest rate, and you get an estimated penalty. If you've already paid at least the required amount, the underpayment floors at zero and so does the penalty.
Worked example
Say your estimated tax for this year is $12,000, you've paid $7,000 so far through withholding and estimated payments, and last year's total tax was $9,000.
- 90% of current-year tax: $12,000 × 90% = $10,800
- Required annual payment: the smaller of $10,800 and $9,000 = $9,000
- Underpayment: $9,000 − $7,000 = $2,000
- Penalty estimate: $2,000 × 7% = $140
Because last year's tax ($9,000) was lower than 90% of this year's projection ($10,800), the prior-year figure wins and becomes the target. This taxpayer only needed to pay $9,000 across the year to stay penalty-protected — they paid $7,000, so $2,000 went unpaid on schedule and now carries an estimated $140 in interest.
How to interpret your result
The penalty estimate is directional, not a bill. It tells you roughly what one flat rate applied to your annual shortfall would cost — useful for deciding whether to send in a catch-up payment now, but not a number you can copy onto a form. If it comes back at $0, you've either met the safe harbor or overpaid relative to it; either way, you're not accruing underpayment interest as things stand.
If the underpayment amount is large, the fix is usually a same-quarter estimated payment through IRS Direct Pay, not waiting until filing season — interest keeps accruing on the unpaid portion for every day it stays outstanding.
Be honest about what this tool is not. The real IRS calculation, done on Form 2210, works quarter by quarter: it tracks what you owed and paid at each of the four due dates and charges interest on each quarter's specific shortfall for the exact number of days it was outstanding, using whatever rate applied that quarter. This calculator collapses all of that into one annual figure and one flat rate, which is a reasonable planning approximation but will not match the IRS's own number to the cent — especially if your income or payments were uneven across the year. It also covers US federal tax only; state estimated-tax penalties use their own rates and rules. Treat this as a planning estimate, not tax advice or a substitute for Form 2210 or a tax professional.
Methodology & sources
requiredAnnualPayment = min(currentYearTax × 0.9, priorYearTax). underpayment = max(0, requiredAnnualPayment − totalPaid). penaltyEstimate = underpayment × UNDERPAYMENT_RATE / 100, rounded to the cent from unrounded intermediates. The safe-harbor logic mirrors IRC Section 6654: pay the smaller of 90% of current-year tax or 100% of prior-year tax (110% if prior-year AGI exceeded $150,000 — not modeled as a separate input here, see this site's dedicated safe harbor calculator for that AGI-aware version).
UNDERPAYMENT_RATE is the IRS's individual underpayment interest rate — the federal short-term rate plus 3 percentage points, reset every calendar quarter under IRC Section 6621. It is currently 7%, confirmed for the quarter beginning July 1, 2026 on the IRS's own Quarterly interest rates page; see also the IRS's underpayment of estimated tax by individuals penalty page for the underlying rule. Because this rate changes quarterly, a penalty accrued over several quarters would technically blend multiple rates — this calculator applies the current rate to the full underpayment as a simplification.