Methodology
Every live calculator ships with fixtures. The build computes each fixture and stops if the result misses the stated tolerance.
By David Miller Updated Sources
How calculators are built
Pure TypeScript in src/domain/calculators/ owns the math. The page shell only
renders a view model. Money uses decimal.js with half-up rounding to the cent. There are 3 live calculators in the registry today.
Update schedule
- Annual IRS and SSA figures when each agency publishes them.
- Weekly mortgage rates once the live rate file replaces the sample file.
- Corrections are logged under Updates when a published number changes.
Limits of the models
These tools estimate a household figure from the inputs you type. They do not model every statute, escrow item, or agency rounding rule. Your lender, plan administrator, or official notice is the final word.
Data sources
- Revenue Procedure 2025-32
Internal Revenue Service. Retrieved 2026-10-11.
- TODO verify: Social Security wage base
Social Security Administration. Retrieved 2026-10-11.
- TODO verify: 401(k) and IRA contribution limits
Internal Revenue Service. Retrieved 2026-10-11.
- TODO verify: 2027 federal inflation adjustments
Internal Revenue Service. Retrieved 2026-10-11.
- Cost-of-Living Adjustment (COLA) information
Social Security Administration. Retrieved 2026-10-11.
- Sample mortgage rates (not a live quote)
fintrivex sample. Retrieved 2026-10-11.
- Loan estimate: how a fixed mortgage payment is built
Consumer Financial Protection Bureau. Retrieved 2026-10-11.
- Compound interest calculator
U.S. Securities and Exchange Commission, Investor.gov. Retrieved 2026-10-11.
Money is rounded half up to the cent. The table below is the same check the test suite runs: expected value, computed value, and the source that states the method.
| Calculator | Fixture | Field | Expected | Computed | Source |
|---|---|---|---|---|---|
| Mortgage payment calculator | 100k at 6 percent for 30 years | payment | 599.55 | 599.55 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 100k at 6 percent for 30 years | totalInterest | 115838.45 | 115838.45 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 100k at 6 percent for 30 years | totalPrincipal | 100000 | 100000 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 100k at 6 percent for 30 years | endingBalance | 0 | 0 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 100k at 6 percent for 30 years | months | 360 | 360 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 400k at 6.5 percent for 30 years | payment | 2528.27 | 2528.27 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 400k at 6.5 percent for 30 years | totalInterest | 510179.81 | 510179.81 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 400k at 6.5 percent for 30 years | totalPrincipal | 400000 | 400000 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 400k at 6.5 percent for 30 years | endingBalance | 0 | 0 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 400k at 6.5 percent for 30 years | months | 360 | 360 | Standard fixed-rate amortization, monthly interest rounded half up to the cent |
| Mortgage payment calculator | 250k at 6.5 percent for 15 years with 100 extra | payment | 2177.77 | 2177.77 | Standard fixed-rate amortization with a level extra principal payment |
| Mortgage payment calculator | 250k at 6.5 percent for 15 years with 100 extra | totalInterest | 130610.48 | 130610.48 | Standard fixed-rate amortization with a level extra principal payment |
| Mortgage payment calculator | 250k at 6.5 percent for 15 years with 100 extra | totalPrincipal | 250000 | 250000 | Standard fixed-rate amortization with a level extra principal payment |
| Mortgage payment calculator | 250k at 6.5 percent for 15 years with 100 extra | endingBalance | 0 | 0 | Standard fixed-rate amortization with a level extra principal payment |
| Mortgage payment calculator | 250k at 6.5 percent for 15 years with 100 extra | months | 168 | 168 | Standard fixed-rate amortization with a level extra principal payment |
| Mortgage payment calculator | 250k at 6.5 percent for 15 years with 100 extra | interestSaved | 11387.67 | 11387.67 | Standard fixed-rate amortization with a level extra principal payment |
| Compound interest calculator | 10000 at 6 percent compounded monthly for 10 years | futureValue | 18193.97 | 18193.97 | Compound interest, A = P(1 + r/n)^(nt), monthly |
| Compound interest calculator | 10000 at 6 percent compounded monthly for 10 years | principal | 10000 | 10000 | Compound interest, A = P(1 + r/n)^(nt), monthly |
| Compound interest calculator | 10000 at 6 percent compounded monthly for 10 years | contributions | 0 | 0 | Compound interest, A = P(1 + r/n)^(nt), monthly |
| Compound interest calculator | 10000 at 6 percent compounded monthly for 10 years | interest | 8193.97 | 8193.97 | Compound interest, A = P(1 + r/n)^(nt), monthly |
| Compound interest calculator | zero rate is principal plus contributions | futureValue | 16000 | 16000 | Identity: at a zero rate the balance is the sum of deposits |
| Compound interest calculator | zero rate is principal plus contributions | principal | 10000 | 10000 | Identity: at a zero rate the balance is the sum of deposits |
| Compound interest calculator | zero rate is principal plus contributions | contributions | 6000 | 6000 | Identity: at a zero rate the balance is the sum of deposits |
| Compound interest calculator | zero rate is principal plus contributions | interest | 0 | 0 | Identity: at a zero rate the balance is the sum of deposits |
| Compound interest calculator | 5000 at 7 percent with 200 monthly for 20 years | futureValue | 124379.03 | 124379.03 | Future value of a present amount plus an end-of-month contribution |
| Compound interest calculator | 5000 at 7 percent with 200 monthly for 20 years | principal | 5000 | 5000 | Future value of a present amount plus an end-of-month contribution |
| Compound interest calculator | 5000 at 7 percent with 200 monthly for 20 years | contributions | 48000 | 48000 | Future value of a present amount plus an end-of-month contribution |
| Compound interest calculator | 5000 at 7 percent with 200 monthly for 20 years | interest | 71379.03 | 71379.03 | Future value of a present amount plus an end-of-month contribution |
| Social Security COLA calculator | 2000 at 2.8 percent | newMonthly | 2056 | 2056 | Hand-checked 2000 × 1.028 |
| Social Security COLA calculator | 2000 at 2.8 percent | increaseMonthly | 56 | 56 | Hand-checked 2000 × 1.028 |
| Social Security COLA calculator | 2000 at 2.8 percent | increaseAnnual | 672 | 672 | Hand-checked 2000 × 1.028 |
| Social Security COLA calculator | 1500 at 3.0 percent | newMonthly | 1545 | 1545 | Hand-checked 1500 × 1.03 |
| Social Security COLA calculator | 1500 at 3.0 percent | increaseMonthly | 45 | 45 | Hand-checked 1500 × 1.03 |
| Social Security COLA calculator | 1500 at 3.0 percent | increaseAnnual | 540 | 540 | Hand-checked 1500 × 1.03 |
| Social Security COLA calculator | zero benefit | newMonthly | 0 | 0 | Hand-checked zero benefit |
| Social Security COLA calculator | zero benefit | increaseMonthly | 0 | 0 | Hand-checked zero benefit |
| Social Security COLA calculator | zero benefit | increaseAnnual | 0 | 0 | Hand-checked zero benefit |
Official figures
A cell that is not ready says being updated. It is never filled with a guess.
- 2026 standard deduction, single
- $16,100
- Social Security wage base
- being updated
- 401(k) employee deferral limit
- being updated
- 2027 COLA
- being updated
- Mortgage rate file
- sample data
Recent updates
-
Social Security COLA calculator published
2026-10-11 · The Social Security COLA calculator is live ahead of the scheduled October 14, 2026 announcement.
-
2026 federal tax figures added
2026-10-11 · Standard deductions and ordinary-income brackets for 2026 were copied from IRS Revenue Procedure 2025-32.