The Zero-Guess Principle
Calculator results depend on formulas, entered values, display rounding, and assumptions. Each tool should explain material assumptions near the result or in its method notes. We link relevant primary or product sources when available; a source for one product or location does not make its value universal.
At FivoTools, we operate under a permanent source-based calculation policy. For each calculator, we aim to:
- We identify the relevant mathematical, physical, or statutory method and consult primary sources when available.
- We identify the kind of value used and state material assumptions.
- We add known-value test fixtures where a tool's test suite supports them.
- We implement pure calculation engines isolated from UI presentation.
- We check representative edge cases when we have tests for the tool, including relevant zero values and input boundaries.
How We Treat Values and Sources
Values used by a tool fall into different groups. Mathematical identities (for example, 12 inches per foot) can be applied directly. Schedule-based defaults (such as 40 hours × 52 weeks = 2,080 hours) are planning inputs, not rules for every worker. Product coverage, yield, dimensions, and bulk density vary by product, material, and conditions; use the label or supplier data when available. Tax, mileage, payroll, and building rules depend on jurisdiction, eligibility, and effective date. User-entered percentages such as waste, overhead, and target margin are editable scenarios, not recommendations that fit every project or business.
Source Selection
We prioritize official rules and primary product documentation for jurisdiction-dependent or product-dependent values. When no universal source exists, we identify a value as an example or planning assumption.
Official, Primary, and Product Sources
Primary regulatory publications, statutory codes, official tax authority notices, national standards institutes, and primary manufacturer technical specifications:
- IRS Bulletins & Notices: Used for standard business, medical, and charitable mileage rates.
- Department of Labor & FLSA rules: Used to explain applicable U.S. federal overtime rules, alongside a reminder that coverage, exemptions, and state or local rules can affect a worker's pay.
- Construction references: Product guidance and dimension references inform relevant construction estimates; geometry is based on the formulas shown on each tool page.
Calculation Checks
Mathematical checks, test fixtures where available, and user reports reviewed against the assumptions and sources shown for the relevant tool.
Planning Examples
Where no universal value exists, a tool may provide an illustrative default or range (for example, paint coverage, soil or aggregate density, or carpet roll width). These examples are not specifications or endorsements. Check the product label, supplier, installer, or applicable local authority.
Precision, Rounding & Unit Rules
Calculation accuracy requires rigorous floating-point discipline:
- Calculation precision: Tools retain internal precision where the method allows. A documented method may round intermediate values; that can affect the displayed final quantity.
- Display rounding: Output formatting follows the method of each tool. Currency is commonly displayed to two decimal places, while a tool may round purchasable units up to whole items.
- Ceiling Behavior for Purchasable Units: Some materials are sold only in whole packages. When appropriate, tools round estimates up to the next whole package based on the selected coverage and allowance. Check current packaging and project conditions before ordering.
Review Cadence & False Freshness Policy
Rates and rules can change. Rate- and rule-based calculators (such as IRS mileage rates) should identify the jurisdiction, effective period, and governing source. Check that source before using a value for filing, reimbursement, payroll, or compliance decisions.
No Manufactured Freshness: We do not automatically update page "last reviewed" timestamps to give search engines an illusion of artificial freshness. A review date changes only when our team has materially audited or updated the underlying formula, content, or reference data.
Report a Discrepancy
We welcome corrections. If a formula, source, default, or example appears inaccurate or unclear, please describe the issue through our contact page.