Compound interest calculator: fees & inflation

By Xavier Carraté · Editor & researcher · Prepared: 2026-09-07

Compare three editable compound-growth scenarios with monthly contributions, fees, inflation and negative returns. Free USD tool with a monthly table and CSV.

Compare scenarios

Starting values are examples. Replace them with your own assumptions. All amounts are in US dollars.

Use a decimal point or comma, up to two decimal places, and no thousands separators. Enter whole months from 1 to 600.

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1. Amount and time

Starting amount ($)

Money available at the start. Range: $0–$1,000,000,000.

Monthly contribution ($)

A constant contribution at the end of each month. Range: $0–$1,000,000,000.

Time in months

For example, 12 is one year. Whole months: 1–600.

2. Try three return assumptions

These labels are for comparison, not probabilities, guarantees or lower bounds. Replace any rate, including with a negative number. Each rate stays constant for every month; market returns do not.

Conservative annual return (%)

Illustration, not a guaranteed minimum. Range: −99% to 100%.

Base annual return (%)

Your working assumption, not an expected return. Range: −99% to 100%.

Alternative annual return (%)

Try a different assumption. Range: −99% to 100%.

3. Fees and purchasing power

Annual proportional fee (%)

Effective reduction, 0%–10%. Use 0 if already included in the return; fixed-dollar fees are excluded.

Annual inflation (%)

Purchasing-power assumption, −50% to 100%. Enter 0 for no adjustment.

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Start with the question you can control

What happens if I contribute $25 more each month? Compare the same amounts and horizon with three return assumptions, then change the contribution and recalculate. The new-contributions line separates money you add from modeled growth. A larger projected balance can come mostly from saving more, not from earning more.

Worked example you can check without JavaScript

With $1,000 at the start, $100 added at the end of each month, 12 months, 0% return, 0% fees and 0% inflation: new contributions are $1,200 and the final balance is $2,200. Month 1 closes at $1,100; month 12 closes at $2,200. Growth is $0. This is an arithmetic example, not a suggested investment.

Use the result for one next step

Choose an amount that your cash-flow budget can support, then decide when to review it. If you need the money soon, consider the need for access and the possibility of losses before choosing where to hold it. Use the savings-goal tool if your main question is how much to put aside for a specific deadline.

How the calculation works

The annual return is an effective rate. We convert it using (1 + return / 100)^(1/12) − 1, not by dividing it by 12. Each month: apply gross growth or loss to the opening balance, deduct the modeled proportional fee, then add your contribution. A contribution made in month 1 first earns a return in month 2. Growth and fees round to cents each month, so results can differ from an unrounded formula or a real account statement.

Fees and today’s dollars

The annual fee is a separately entered effective proportional reduction: its monthly fraction is 1 − (1 − fee / 100)^(1/12). It applies after gross return and before the monthly contribution. This is a planning convention, not a specific fund’s fee schedule. If your return assumption is already net of that fee, enter 0 here. Today’s-dollar values divide each nominal balance by (1 + inflation / 100)^(month / 12). They estimate purchasing power under your assumption; they are not extra money.

What this cannot tell you

A constant-rate path does not represent volatility, the order of good and bad months, investment suitability or a probability of success. Negative example returns show a possible loss mechanism, not the worst possible outcome. The model excludes taxes, withdrawals, variable contributions, transaction charges and fixed account fees. It supports 1–600 months and rejects scenarios with nominal or real amounts beyond the supported range instead of showing an incomplete projection.

Sources and model distinction

Sources checked on September 7, 2026. They explain the concepts; the monthly rounding and effective-fee convention above are our own explicit model, not a claim that these agencies use the same calculation.