# Investment growth calculator with fees and inflation documentation

Inputs, formulas, assumptions and API/MCP examples for the investment growth calculator with fees and inflation. Reproduce a calculation and cite its sources.

Canonical calculator: https://klarfort.com/tools/investment-growth-calculator/
Content updated: 2026-09-07
Assumptions reviewed: 2026-09-07

## Overview
See how contributions, withdrawals, asset fees, tax assumptions and inflation change a projected investment balance. Choose whether the headline shows gross, after-fee, after-tax or real value.

Use volatility and simulation settings to explore a range of possible model outcomes alongside the constant-return projection. These scenarios describe the assumptions entered, not a forecast for a specific investment.

## How to use
1. Enter your starting balance, projection months, contributions and any regular withdrawals.
2. Set the assumed annual return, contribution growth, asset fee, tax on positive gains and inflation.
3. Choose the headline result and set volatility, path count and an optional simulation seed.
4. Calculate, then compare each value basis, withdrawal shortfall, fees, taxes and simulated percentiles.

## Understanding the result
Gross value excludes fees and tax. After-fee value deducts the asset fee; after-tax value also applies the entered tax to positive monthly gains. Real value adjusts that after-tax balance for inflation.

The percentile results always describe simulated after-tax nominal ending balances, whichever headline mode is selected. Depletion counts paths with a zero closing balance at any point, even if later contributions rebuild it.

## Questions
### What does the tax input represent?
It deducts the entered percentage from positive monthly gains after the fee. Losses produce no tax credit. This is a tax-drag illustration and does not model tax allowances, realization rules or a particular account.

### Why does the simulated median differ from the constant-return result?
Simulations apply changing monthly returns drawn from the entered return and volatility assumptions. Their median is the middle modeled ending balance, which need not equal the constant-return path.

### What happens if a withdrawal exceeds the balance?
The model pays only the available assets and reports the unmet amount as withdrawal shortfall. Contributions and withdrawals occur at month start, before that month's return, fee and tax.

## Default worked example
Projected value: 10,000 USD. Illustrative investment projection with the fee, tax and inflation assumptions you entered. Assumptions as of 2026-09-07.

## Formula
Each month: add contribution, pay withdrawal up to available assets, apply return, deduct an asset fee, then tax positive gains. Simulated monthly factors = exp(log(1 + annual return)/12 − volatility²/24 + volatility/√12 × Normal(0,1)).

- Contributions are made at the beginning of each month. Annual returns are effective rates converted to monthly compound factors. All figures use one entered currency.
- Tax applies to positive monthly gains only; there is no tax credit for losses. This is a tax-drag illustration using your rate, not jurisdiction-specific tax accounting.
- The annual fee is an effective asset deduction: monthly fee = 1 − (1 − annual fee)^(1/12). Downside percentiles always describe after-tax nominal terminal balances. A path is depleted if any closing balance is zero.

## Inputs
- currency: Currency; default "USD"
- principal: Starting balance; default 10000
- months: Projection months; default 12
- monthlyContribution: Monthly contribution; default 0
- annualReturn: Annual return (%); default 0
- mode: Displayed result; default "after-tax"
- monthlyWithdrawal: Monthly withdrawal; default 0
- annualVolatility: Annual volatility (%); default 0
- annualFee: Annual asset fee (%); default 0
- taxRate: Tax on positive gains (%); default 0
- inflationRate: Annual inflation (%); default 0
- contributionGrowth: Annual contribution growth (%); default 0
- paths: Simulation paths; default 1000
- seed: Leave empty to derive a reproducible seed from your inputs, or enter 16 hexadecimal characters.; default ""

## Interfaces
Input schema: https://klarfort.com/tools/investment-growth-calculator/schema.json
Worked request and response: https://klarfort.com/tools/investment-growth-calculator/examples.json
Public table: https://klarfort.com/tools/investment-growth-calculator/table.json
JSON API: POST https://klarfort.com/tools/api/v1/investment-growth-calculator
MCP endpoint: POST https://klarfort.com/tools/mcp
MCP tool name: investment-growth-calculator

## JSON request body
```json
{
  "currency": "USD",
  "principal": 10000,
  "months": 12,
  "monthlyContribution": 0,
  "annualReturn": 0,
  "mode": "after-tax",
  "monthlyWithdrawal": 0,
  "annualVolatility": 0,
  "annualFee": 0,
  "taxRate": 0,
  "inflationRate": 0,
  "contributionGrowth": 0,
  "paths": 1000,
  "seed": "44af4bef64b738ab"
}
```

## Usage and credit
Free for personal, commercial and any other lawful use with credit to KlarFort at https://klarfort.com/. Third-party data terms still apply.

Computed with KlarFort Calculators, https://klarfort.com/

Place a visible credit near the reused material or in the credits for your work or integration. One clear credit per work or integration is sufficient; no separate credit per API request is required. The credit_url field contains the homepage. The citation_url field is an optional calculator reference. Full permission: https://klarfort.com/tools/docs/license/

## Sources
KlarFort calculation methodology: https://klarfort.com/tools/methodology/ (1)