Is this the same as an IRS deduction calculator?
No. It is a planning calculator. Use IRS rules and tax guidance to decide whether a mileage deduction or reimbursement applies.
Should cost per mile include fuel only?
It can, but label the basis. Fuel-only rates are narrower than all-in rates that include wear, insurance, depreciation, or reimbursement assumptions.
How do I calculate annual driving cost?
Use annual miles as the miles input, or use trip miles with the number of trips you expect across the year.
Why might the real-world result differ?
Match the result to the task type: shopping tools depend on the same unit and usable quantity, home-project tools depend on field measurements and waste, date/time tools depend on counting rules, and conversion tools depend on the unit system.
Should I round the result?
Round for readability after checking the formula and units. Keep more precision when the result feeds another calculation, and add a task-specific buffer only when shortage, waste, or timing risk matters.
Why might another calculator show a different output?
Different tools may use different rounding, assumptions, default rates, methods, formulas, or input timing. Compare the visible method and inputs before relying on the output.