OpenCalculator pages explain the method beside the tool. This guide describes the common assumptions and points you to primary sources for rules that can change. Calculation tests check worked examples and edge cases; they cannot establish that every real-world situation is covered.
Math, percentages, and rounding
The percentage calculator uses part = whole × percent / 100. This tool calculates percent change by dividing the difference by the absolute value of the original value; that convention also supports negative starting values. Percent change is different from a change in percentage points. The fraction calculator uses rational arithmetic. Other tools generally use floating-point arithmetic and round the displayed result, so adding rounded rows can differ slightly from a total computed before rounding.
Savings and interest
The compound interest calculator applies the chosen compounding frequency and contribution timing. The savings calculator uses the stated APY and deposits. These are scenarios with a fixed rate, not forecasts of investment performance. Fees, inflation, changing returns, and taxes are not included unless the tool explicitly provides an input for them.
U.S. tax and paycheck estimates
The federal tax, tax refund, and paycheck calculators use the tax year stated on their pages. Federal 2026 brackets and deductions are based on IRS Revenue Procedure 2025-32. Payroll calculations include Social Security and Medicare; see the SSA contribution and benefit base for the Social Security wage limit.
A state's page identifies estimated tables where applicable. Local taxes, individual payroll elections, special credits, and some deductions may not be modeled. Annual income-tax estimates are not an employer's exact withholding calculation or a filed tax return. Confirm your own circumstances with current official guidance.
Grades and test scores
The GPA calculator uses a 4.0 letter-grade scale with optional honors and AP/IB weighting. Your school's scale or weighting rules may differ. Course grades and final-exam targets use the weights you provide.
AP, SAT, and ACT raw-score conversions here are estimates, not official score reports. Forms are equated differently, and the digital SAT is adaptive. College Board explains how SAT scores are calculated. Use the relevant exam's official practice-test scoring instructions when available.
College Board's AP score guide explains how section scores contribute to a final score. For ACT tests administered since September 2025, the composite averages English, Math, and Reading; Science is reported separately. See ACT's scoring explanation. The historical ACT option on this site retains the older four-section composite.
VA disability ratings and compensation
The VA disability calculator combines ratings using the remaining-efficiency method, rather than adding percentages, and estimates compensation for the selected household. The payment data cites the VA's 2026 disability compensation tables. Eligibility, special monthly compensation, and an individual award require a VA determination.
Health estimates and unit conversions
The body fat calculator uses circumference-based and BMI-based formulas. Measurements and population assumptions affect the estimate; it is not a clinical measurement or diagnosis. The peptide calculator converts supplied quantities between mass, concentration, and volume. Example inputs are not dose recommendations, preparation instructions, or evidence that a substance is safe to use.
Graphing, algebra, and calculus
Graphs sample a function across a viewport and can miss features outside that range. Symbolic derivatives and integrals support the forms described on their pages. A numeric definite integral is an approximation, while unsupported symbolic forms may have no result. Check domain restrictions, discontinuities, and the displayed method.
Shared links and Excel files
The same calculation engines power the homepage and full pages. A shared link restores inputs and computes the answer again, so a later data update can change a tax or benefit estimate. PDF and image exports preserve the displayed result. Supported Excel templates use formulas; generic exports contain values and steps. Spreadsheets can use different rounding or date conventions, so review their assumptions before extending them.