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Spending Opportunity Cost After Tax and Inflation

Compare an immediate purchase with monthly-compounded investment growth, a flat gains tax, and inflation-adjusted purchasing power.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Results are estimates for planning and education. Confirm rates, taxes, fees, and legal requirements with the relevant institution or a qualified professional before making a financial decision.

Formula and method

With no investment growth, 1,100 exposed to 10% price inflation for one year has purchasing power of 1,000, a real loss of 100.

Nominal savings = P(1+r/1200)^(12t); tax applies to the gain; real value = after-tax savings/(1+i/100)^t.

Worked example

Enter these known values and leave the other values blank.

Money to spend
1100 USD
Annual return on savings
0 %
Investment period
1:0 yrs / mos
Income tax
0 %
Annual inflation rate
10 %
Total savings (after tax)
1100 USD
Opportunity cost — inflation adjusted
1000 USD

Assumptions and limitations

  • Rates are scenario assumptions, not forecasts. The model taxes the final nominal gain once at the entered flat rate and compounds the investment monthly.
  • Real purchasing power divides by the price-level growth factor. This corrects the source use of (1−inflation)^years, which understates purchasing power differently.
  • Use one currency throughout. Retained currency selector factors are a historical snapshot, not live exchange rates.
  • Clear an editable unknown before solving an inverse. Selection controls and forced outputs cannot be inferred.

Common questions

What does this model assume?

Rates are scenario assumptions, not forecasts. The model taxes the final nominal gain once at the entered flat rate and compounds the investment monthly.

Can I use the currency menu as a current quote?

No. Keep amounts in one currency; conversion factors are a captured historical snapshot.

References

The calculation follows this reviewed mathematical model: Nominal savings = P(1+r/1200)^(12t); tax applies to the gain; real value = after-tax savings/(1+i/100)^t. Required model inputs: Use a positive investment horizon. Inflation must exceed minus 100 percent. The monthly growth factor must be positive. Allow negative real investment returns: inflation need not be below the nominal investment return.

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