# tax-penalty-calc

Works out the IRS underpayment penalty on missed quarterly taxes: shortfall per quarter, the current rate, and days late.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 364 installs · ★ 98
- Author: openaccountant
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/openaccountant/skills/tree/HEAD/personal/tax-penalty-calc/SKILL.md
- Updated: 2026-04-09
- HTML: https://affpapa.org/en/nevibecode/skills/tax-penalty-calc

## What it does

This skill estimates the IRS underpayment penalty for taxpayers who came up short on quarterly estimated payments. The math is short: underpayment times the annual rate, divided by 365, times the days from the quarterly due date to the payment date or April 15. The rate is set each quarter, so check irs.gov for the period you are filing.

It walks through the whole sequence. Figure the year's total tax liability, get the required annual payment (the lesser of 90% of this year's tax or 100%/110% of last year's), divide by four, then pull every payment with `transaction_search` and compare each quarter against its minimum. A table of due dates and a worked example ending at $133.15 are included. The no-Wilson path does the same by hand from Lines 24 and 25 of Form 1040.

Watch the threshold: if you owe under $1,000 after withholding and credits, the penalty usually does not apply. Safe harbor is another exit — paying 100% of last year's liability, or 110% when AGI tops $150K. The late balance due on April 15 is a separate 0.5% per month charge. Filing decisions still belong to a CPA.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add openaccountant/skills --skill tax-penalty-calc -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/tax-penalty-calc/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/tax-penalty-calc/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add openaccountant/skills --skill tax-penalty-calc -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/tax-penalty-calc/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add openaccountant/skills --skill tax-penalty-calc -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/tax-penalty-calc/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install openaccountant/skills tax-penalty-calc
```

2. **Manually: put the files in place**
- File: `.github/skills/tax-penalty-calc/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add openaccountant/skills --skill tax-penalty-calc -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/openaccountant/skills --path personal/tax-penalty-calc
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/tax-penalty-calc/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add openaccountant/skills --skill tax-penalty-calc -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/openaccountant/skills/tree/HEAD/personal/tax-penalty-calc/SKILL.md

## When to use it
- estimate the penalty on a missed quarterly payment
- pull IRS and EFTPS payments with dates and amounts
- compare each quarter's paid amount against its minimum
- check whether the $1,000 threshold or safe harbor applies
- gather payment history for Form 2210

## Example prompts
- Estimate my IRS underpayment penalty for this year
- Find my estimated tax payments and show which quarters fell short
- Export my payment history for Form 2210

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 6060f8971d6b3e2a.

BEGIN THIRD-PARTY FILE 6060f8971d6b3e2a
````text
---
name: tax-penalty-calc
description: >
  Estimate IRS underpayment penalties for missed quarterly tax payments.
---

# Tax Underpayment Penalty Calculator

## Overview

Estimates the IRS underpayment penalty (Form 2210) for taxpayers who did not pay enough estimated tax during the year. The penalty is essentially interest charged on the underpaid amount for each quarter, calculated from the quarterly due date to the payment date or April 15 filing deadline.

## Wilson Tools Used

- `transaction_search` — Find estimated tax payments made to the IRS, EFTPS, or state tax authorities with dates and amounts
- `spending_summary` — Summarize total tax payments for the year
- `export_transactions` — Export payment history for Form 2210 preparation

## Workflow

1. Determine the total tax liability for the year (from your completed return or estimate).
2. Calculate the required annual payment (lesser of 90% of current year tax or 100%/110% of prior year tax).
3. Divide the required annual payment by 4 to get the per-quarter minimum.
4. Use `transaction_search` to find all estimated tax payments, noting the date and amount of each.
5. For each quarter, compare what was paid by the due date against the required amount.
6. For any quarter with a shortfall, calculate the penalty:

### Penalty Calculation

```
penalty_per_quarter = underpayment_amount x (annual_rate / 365) x days_late
```

- The IRS penalty rate is set quarterly based on the federal short-term rate + 3 percentage points.
- As of early 2025, the rate is approximately **8% annually** (verify at irs.gov/newsroom for current quarter).
- Days late = number of days from the quarterly due date to the earlier of: the payment date or April 15.

### Quarterly Due Dates and Penalty Periods

| Quarter | Due Date | Penalty Runs Until |
|---------|----------|-------------------|
| Q1 | April 15 | Payment date or April 15 of following year |
| Q2 | June 15 | Payment date or April 15 of following year |
| Q3 | September 15 | Payment date or April 15 of following year |
| Q4 | January 15 | Payment date or April 15 |

### Example

- Required quarterly payment: $5,000
- Q2 payment made: $3,000 (shortfall of $2,000)
- Days from June 15 to April 15 = 304 days
- Penalty: $2,000 x (0.08 / 365) x 304 = **$133.15**

### Exceptions (Penalty May Be Waived)

- Total tax owed is less than $1,000 after withholding and credits.
- You paid at least 100% of prior year tax liability (110% if AGI > $150K).
- The underpayment was due to a casualty, disaster, or other unusual circumstance.
- You retired (after age 62) or became disabled during the tax year.

## Without Wilson
````
END THIRD-PARTY FILE 6060f8971d6b3e2a

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First 2649 of 3895 bytes. © openaccountant · MIT · https://github.com/openaccountant/skills/tree/HEAD/personal/tax-penalty-calc/SKILL.md

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