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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.
Content updated . Assumptions reviewed .
Formula and assumptions
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.
Assumptions reviewed:
Sources
- KlarFort calculation methodology. Free to use with the credit line; vintage 1.
Parameters
| Name | Description | Type | Default | Minimum | Maximum |
|---|---|---|---|---|---|
| currency | Currency | string | "USD" | ||
| principal | Starting balance | number | 10000 | 0 | 1,000,000,000,000 |
| months | Projection months | integer | 12 | 1 | 600 |
| monthlyContribution | Monthly contribution | number | 0 | 0 | 1,000,000,000,000 |
| annualReturn | Annual return (%) | number | 0 | -99 | 100 |
| mode | Displayed result | string | "after-tax" | ||
| monthlyWithdrawal | Monthly withdrawal | number | 0 | 0 | 1,000,000,000,000 |
| annualVolatility | Annual volatility (%) | number | 0 | 0 | 200 |
| annualFee | Annual asset fee (%) | number | 0 | 0 | 100 |
| taxRate | Tax on positive gains (%) | number | 0 | 0 | 100 |
| inflationRate | Annual inflation (%) | number | 0 | -99 | 100 |
| contributionGrowth | Annual contribution growth (%) | number | 0 | -99 | 100 |
| paths | Simulation paths | integer | 1000 | 1 | 1,000 |
| seed | Leave empty to derive a reproducible seed from your inputs, or enter 16 hexadecimal characters. | string | "" |
JSON API
GET accepts one input parameter containing URL-encoded JSON. POST sends the JSON input object. Prefer POST for personal figures. Inputs are limited to 16 KiB. POST results are not stored; GET results may be cached for up to 24 hours.
curl -X POST https://klarfort.com/tools/api/v1/investment-growth-calculator -H "Content-Type: application/json" --data '{"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"}'fetch("https://klarfort.com/tools/api/v1/investment-growth-calculator", {method:"POST", headers:{"Content-Type":"application/json"}, body:JSON.stringify({"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"})})import json, urllib.request
request = urllib.request.Request("https://klarfort.com/tools/api/v1/investment-growth-calculator", data=json.dumps({"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"}).encode(), headers={"Content-Type":"application/json"})
print(urllib.request.urlopen(request).read().decode())Connected assistants
Connect using https://klarfort.com/tools/mcp. Discover this calculator with tools/list, then call investment-growth-calculator. Protocol versions 2026-07-28 and 2025-11-25 are supported. Modern requests carry protocol metadata and matching HTTP headers. Set Mcp-Name to params.name for tools/call and to params.uri for resources/read. Older clients initialize first.
Limits: 120 requests per minute per address and approximately 3,000 per minute per edge location. Simulations are capped at 1,000 paths, 600 months and 150,000 path-months (paths multiplied by months). Shorten the duration or reduce paths when that work limit is exceeded. A limit response returns 429 and Retry-After. No exact remaining-request count is available.
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.
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. Links to individual calculators are optional.
Computed with KlarFort Calculators, https://klarfort.com/
The API returns this text in credit and the homepage in credit_url. The optional citation_url identifies the calculator for reference. See usage and credit for the full permission.
KlarFort is not financial, investment, tax, or legal advice. Verify outputs before making material decisions.