Real Interest Rate & After-Tax Return Calculator
Calculate annual real interest with Fisher adjustment, an independent after-tax return, or an inflation-adjusted total return for any matching period with optional annualization and principal.
A QUICK WALKTHROUGH
How to use this tool
- Calculate annual real interest with Fisher adjustment, an independent after-tax return, or an inflation-adjusted total return for any matching period with optional annualization and principal.
- Calculate
- Copy report / Download TXT
Formula and units
Nominal annualization: a = exp(m × log1p(r / (100m))) − 1. Effective input: a = r/100. Real mode: s = a>0 ? a(1−t/100) : a; real = (1+s)/(1+i/100)−1; shortcut = s−i/100. Independent mode: net = r(1−t/100), tax taken = r×t/100 percentage points, for the supplied period, with no annualization or inflation. Projection: balance=P(1+s)^y, basket=P(1+i/100)^y, purchasing power=balance/(1+i/100)^y, shortcut=P(1+s−i/100)^y. Same-period inflation-adjusted return: r=R/100; i=I/100; s=r>0?r(1−t/100):r; real=(s−i)/(1+i); annual(x)=expm1(log1p(x)/Y); end=P(1+s); realEnd=P(1+s)/(1+i).
Assumptions and limits
Rates and inflation cover the same year. Tax reduces only positive annualized interest in the real-interest model; negative interest assumes no refund. The independent return model always applies signed r(1−t), including losses: this is an algebraic assumption, not a tax refund. A single tax share applies to the stated return; no allowances, bands, income/gain split, deferral or personal price basket. Projection holds rates fixed; it is not a forecast or tax law. Inputs and exports stay in your browser. Every year is shown; 1000 rows is the local display budget. Return and inflation must be effective total changes over the identical period, not annual rates. Blank tax means zero; only positive returns are taxed, with no loss refund. Optional positive years annualize the total changes geometrically. Optional principal gives end values; supply a currency label with principal. Each scenario supplies its own return and inflation and shares the entered tax, years and principal. This is arithmetic, not investment or tax advice.
Local processing
Rates and inflation cover the same year. Tax reduces only positive annualized interest in the real-interest model; negative interest assumes no refund. The independent return model always applies signed r(1−t), including losses: this is an algebraic assumption, not a tax refund. A single tax share applies to the stated return; no allowances, bands, income/gain split, deferral or personal price basket. Projection holds rates fixed; it is not a forecast or tax law. Inputs and exports stay in your browser. Every year is shown; 1000 rows is the local display budget. Return and inflation must be effective total changes over the identical period, not annual rates. Blank tax means zero; only positive returns are taxed, with no loss refund. Optional positive years annualize the total changes geometrically. Optional principal gives end values; supply a currency label with principal. Each scenario supplies its own return and inflation and shares the entered tax, years and principal. This is arithmetic, not investment or tax advice.
GOOD TO KNOW
Common questions
Why can positive interest lose purchasing power?
Prices can grow faster than the after-tax balance. Enter inflation for the same year; a public index may differ from your own costs.
Why do the two modes treat a loss differently?
The Fisher model taxes only positive interest. The independent return model reproduces signed multiplication and does not determine legal loss relief.
Which rate basis should I select?
Use effective annual growth for AER/APY; annual nominal quotes require their compounding frequency. The independent mode uses the return for the supplied period without converting it.
What if the shortcut implies a negative balance?
When 1 + shortcut is negative, its purchasing-power projection has no economic meaning and is marked unavailable; the exact projection remains available.
Same-period inflation-adjusted return?
Return and inflation must be effective total changes over the identical period, not annual rates. Blank tax means zero; only positive returns are taxed, with no loss refund. Optional positive years annualize the total changes geometrically. Optional principal gives end values; supply a currency label with principal. Each scenario supplies its own return and inflation and shares the entered tax, years and principal. This is arithmetic, not investment or tax advice.