Method & sources

Anyone reporting a figure to the tax office should know where it came from. This page sets out which legal provisions capi.tax calculates from, which official data feeds in, and how the calculations are checked.

Legal basis

The calculations reproduce the relevant provisions — not a private interpretation of them:

• § 20 EStG — capital income: dividends, interest, disposal gains, writer premiums (Abs. 1 Nr. 11), forward transactions (Abs. 2 S. 1 Nr. 3), cost basis and FIFO (Abs. 4), loss offsetting (Abs. 6), the Sparer-Pauschbetrag (Abs. 9)
• § 20 Abs. 4a EStG — corporate actions: carrying forward cost basis and acquisition date
• § 23 EStG — private disposals, in particular foreign-currency balances within the one-year period
• § 32d EStG — the separate tax rate (Abs. 1), the duty to declare income not taxed at source (Abs. 3), crediting foreign tax (Abs. 5), the Günstigerprüfung (Abs. 6)
• §§ 16, 18, 20 InvStG — investment income, the Vorabpauschale and the Teilfreistellung
• Alongside these, the relevant BMF circulars on the Abgeltungsteuer and the applicable double-taxation treaties for creditable withholding tax

Official data

Two inputs do not come from your broker export but from official publications:

• Exchange rates: the reference rates of the Deutsche Bundesbank / ECB, taken for the trade day of each individual transaction. Not broker rates, not yearly averages.
• The base rate (Basiszins) for the Vorabpauschale: the annual BMF announcement under § 18 Abs. 4 InvStG. For a year with no published value the calculation stops rather than quietly using zero.

Rules apply per tax year

Tax law changes. capi.tax therefore keeps a separate rule set per year: a 2022 return is calculated under the law of 2022, a 2025 return under today's. Past years are never rewritten retroactively — otherwise a return you already filed would change underneath you.

One example: the separate loss bucket for forward transactions, with its €20,000 cap, was removed retroactively by the 2024 Annual Tax Act. capi.tax reflects the current position.

How it is checked

Three layers that run independently of one another:

1. A readable specification. The tax rules are written as scenarios in plain German — each with its legal basis, the facts, and the expected result in euro. Those scenarios run on every change, and they are written so a tax advisor can follow them without reading code.
2. Regression comparison. A frozen set of worked examples makes sure a change in one place does not silently move a figure somewhere else.
3. Exact arithmetic. Every amount is computed as a decimal, never as a floating-point number. Rounding happens only for display, so the method itself introduces no cent-level drift.

Where it deliberately does not calculate

Just as important as what is computed: when something is missing — the fund type, an opening trade, a redemption price — nothing is estimated. Those cases surface as a warning and the affected trades stay out of the totals until you supply the detail. See what we need from you.

Limits

capi.tax is a preparation tool, not tax advice. It does not know your personal situation — the Sparer-Pauschbetrag, church tax, the Günstigerprüfung or other income do not enter into it. What it outputs is the taxable base, not your finished tax bill. Rare edge cases are not covered, and where a result would be uncertain the tool says so rather than inventing a number.

capi.tax prepares your data and is not tax advice. Please review the figures and ask a tax advisor (Steuerberater) about your personal situation.