Calculation Audit: Model Checks & External Comparisons
Reproduce our Coast FIRE examples, inspect compounding differences and see which external calculator results were actually measured.
Last updated: 2026-10-27
What this audit verifies
Retire Growth model 1.0.0 uses an effective annual nominal return, exact inflation adjustment and month-end contributions in today's dollars. This audit checks arithmetic and conventions. It is maintained by Retire Growth and is not an independent professional financial review or evidence that a retirement plan will succeed.
Read the formulas, see the release and correction history, or open the Coast FIRE calculator.
Reproducible reference cases
The application calculates the contribution balance with a closed-form annuity. A separate Python decimal implementation, with 70-digit working precision, iterates the account one month at a time. It does not import the application's math engine. Our automated checks compare its published references against the application within half a cent and require the same first Coast month.
Download the reference results and download the Python script. Run python3 verify_model.py to regenerate the JSON. These are illustrative inputs, not customer financial records.
Default inputs: age 32, target retirement 65, invested savings $80,000, monthly savings $1,500, monthly retirement spending $4,000, 7% effective nominal return, 3% inflation and a 4% initial withdrawal-rate assumption. Only the listed variable changes in each row.
| Reference case | Coast number today | Balance at 65, today's USD | First Coast month |
|---|---|---|---|
| Default inputs | $341,306.84 | $1,468,003.86 | 255 (age 53 years 3 months) |
| No further deposits | $341,306.84 | $281,271.83 | Not reached |
| Return equals inflation (3%) | $1,200,000.00 | $674,000.00 | Not reached |
| Return 2%, inflation 3% | $1,655,785.19 | $566,051.02 | Not reached |
| Retirement spending $0 | $0.00 | $1,468,003.86 | 0 |
The downloadable values retain more precision than this table. A zero spending target sizes a zero portfolio; it does not establish that a person can live without expenses.
A common accumulation benchmark
A comparison is meaningful only if the question, units and growth convention match. For the external check we removed deposits, spending, taxes and benefits and used $80,000 initially invested, 33 years, 0 monthly contributions and a 7% annual rate. Retirement calculators that ask for ages received 32 and 65. These are synthetic test inputs.
The 7% label can describe two different models:
- 7% effective annual growth:
80000 × 1.07^33 = $746,027.18in future nominal dollars. - 7% nominal APR compounded monthly:
80000 × (1 + 0.07/12)^396 = $800,562.99in future nominal dollars. Its effective annual rate is approximately 7.229%.
Retire Growth uses the first convention. With 3% inflation, $746,027.18 ÷ 1.03^33 is $281,271.83 in today's purchasing power, which is the no-new-deposits line in our chart. The model's Coast number $341,306.84 answers a different question: how much is needed today to grow toward the entered $1.2 million real retirement target.
Five external tools checked
Observed October 27, 2026 in a browser. Three tools returned a measurable result; two were blocked by their security controls. Their outputs are not endorsements of this site, and agreement on one accumulation example does not validate every feature of either product.
| Tool and direct source | Settings and observed output | Comparison status |
|---|---|---|
Ramsey Retirement Calculator | Age 32→65; $80,000 savings; $0 monthly contributions; 7% return. Displayed $800,563. | Matches the rounded nominal-APR/monthly-compounding benchmark. This is an inference from the observed result, not a claim to have inspected Ramsey's internal model. No inflation field was presented in the tested form. |
Investor.gov Compound Interest Calculator | $80,000; 33 years; $0 monthly; 7%; variance 0; Annually: $746,027.18. Changing only compound frequency to Monthly: $800,562.99. | The annual setting matches our effective-annual accumulation convention before inflation. This tool does not calculate a Coast milestone. |
Calculator.net Investment Calculator | End Amount; $80,000; 33 years; 7%; annually compounded; additional contribution 0 at month-end. Displayed $746,027.18. | Annual nominal accumulation matches. Inflation and Coast target sizing are outside this test. |
Boldin Simple Retirement Calculator | Interactive page presented a security-verification screen. No result was measured. | Not numerically compared. No account was created and no security challenge was bypassed. |
NerdWallet Coast FIRE Calculator | Interactive page returned an access-block screen. No result was measured. | Not numerically compared. Public documentation was available, but a formula-based reconstruction would not be an observed product result. |
Download the external observation record. Tools can change; these results apply to the tested forms and date. We have not run a complete five-product, deposit-inclusive retirement-plan comparison. In particular, fixed nominal deposits and inflation-linked real deposits are different inputs, so we do not compare them as though they were the same.
Corrections and review status
We welcome reproducible reports: send the input values, model version, expected result, observed result and source or calculation steps to support@retiregrowth.com. Avoid including account numbers, passwords or identity documents. Confirmed numerical changes will be dated in the public changelog, with the affected convention and examples identified.
No named independent reviewer has completed a review. A professional review will be credited only after it actually occurs, with its date and scope stated.