Where our formulas come from
Every calculator starts with a primary source, not a blog summary. We name the source on each tool page and link out so you can check our work.
- Mortgage: standard amortization (M = P x [r(1+r)^n] / [(1+r)^n - 1]). We follow CFPB homeowner guides and Freddie Mac rate publications for context, not for live rates.
- Federal income tax: IRS inflation-adjusted brackets and standard deductions for the labeled tax year (currently 2026, marked TODO_VERIFY until we confirm against irs.gov each fall).
- Max heart rate: published equations by Tanaka, Gellish, Gulati, and Inbar, plus the classic 220 minus age rule. Zone math uses percent of max or Karvonen (heart rate reserve) when you enter resting HR.
- Cumulative GPA: credit-weighted grade points per standard US 4.0 and 5.0 mappings, with 10-point and percentage modes documented on the page.
- ERA: MLB's official definition: earned runs times innings per game, divided by innings pitched, with baseball outs notation (.1 and .2).
- Tip and financial freedom: arithmetic formulas documented on each page. Compound growth uses monthly compounding when you opt in.
How we test
Before a calculator ships, we run known inputs by hand and compare against at least one independent calculator or spreadsheet. Example: a $400,000 home, 20 percent down, 30 years at 6.5 percent should produce about $2,021 monthly principal and interest. A single filer at $75,000 with the 2026 standard deduction should owe about $8,341 in federal tax in our model. ERA of 18 runs over 45.1 innings at nine innings per game should read 3.57.
We keep those test values in our content and code comments so a future edit does not silently break the math. If you find a mismatch, email us with your inputs and we will reproduce it.
How often we update
Tax brackets and standard deductions get reviewed every filing year when the IRS publishes inflation adjustments (usually late fall). Mortgage, GPA, heart rate, ERA, and tipping content update when formulas, norms, or official definitions change. Every tool page shows a last updated date at the bottom. Constants that still need owner verification carry TODO_VERIFY in the repo until someone confirms them against the official source.
Corrections
If you spot an error, email [email protected]. We fix confirmed mistakes within a week when we can and note the change on the page date. Disagreements about financial planning philosophy are not bugs; wrong arithmetic is.
What these tools are not
eCalculator calculators are educational estimates. They are not financial, medical, tax, or legal advice. They do not know your full tax return, your lender's fees, your doctor's orders, or your league's official scorer. Use the numbers to plan and ask questions. Use a qualified professional when the decision is big.
Browse our disclaimer for full limitations, or jump straight to any tool: