---
title: "Federal capital-gains tax: long-term stacked across the 2026 0%/15%/20% breakpoints, short-term at ordinary marginal rates, plus the 3.8% Net Investment Income Tax."
method: POST
path: "/v1/tax/capital-gains"
tags: ["Payroll & tax"]
---

# Federal capital-gains tax: long-term stacked across the 2026 0%/15%/20% breakpoints, short-term at ordinary marginal rates, plus the 3.8% Net Investment Income Tax.

`POST /v1/tax/capital-gains`

## Request

- Base URL: `` (relative, and it does not resolve against the document’s source URL)
- URL: `/v1/tax/capital-gains`
- Auth: none declared

Federal capital-gains tax: long-term stacked across the 2026 0%/15%/20% breakpoints, short-term at ordinary marginal rates, plus the 3.8% Net Investment Income Tax.

Payment: x402 ($0.10 USDC on eip155:8453). Call unpaid to receive an HTTP 402 with requirements, then retry with the PAYMENT-SIGNATURE header. Method: LT gains stack above ordinary taxable income across 0/15/20 bands (2026 breakpoints); ST gain taxed as incremental ordinary tax; NIIT = 3.8%*min(NII, MAGI over threshold)

## Request body

- object
  - `filingStatus` 'single' | 'mfj' | 'hoh' | 'mfs'
  - `ordinaryTaxableIncome` number
  - `longTermGain` number
  - `shortTermGain` number
  - `magi` number

## Response `200`

Verified result + provenance (formula, sources, reproducible SHA-256 hash).

## Other responses

- `400`: Invalid input (no charge) — body echoes the JSON Schema.
- `402`: Payment required — PAYMENT-REQUIRED header carries the x402 accepts[] quote.

---

[API](https://skmtc.dev/x402toll/apis/tollbooth-verified-calculators-for-ai-agents.md) · [All operations](https://skmtc.dev/x402toll/apis/tollbooth-verified-calculators-for-ai-agents/llms.txt) · [OpenAPI document](https://skmtc.dev/x402toll/apis/tollbooth-verified-calculators-for-ai-agents/revisions/60cb70a3e1a5?raw)