Sales Tax Calculator
This calculator supports three workflows: calculate tax from a subtotal and rate, reverse-calculate tax from a tax-inclusive total, or apply a verified custom jurisdiction rate. It is intended for quick estimates and bookkeeping convenience.
The tool emphasizes transparency: each method returns explicit intermediate values (subtotal, tax amount, pre-tax amount) so you can record how the figure was derived. Always cross-check rates with your jurisdictional tax authority.
Governance
Record 88deb540487a • Reviewed by Fidamen Standards Committee
Calculate subtotal, tax amount, and total when you know price, quantity, and the tax rate as a percentage.
Inputs
Results
Subtotal
$0.00
Tax amount
$0.00
Total
$0.00
| Output | Value | Unit |
|---|---|---|
| Subtotal | $0.00 | USD |
| Tax amount | $0.00 | USD |
| Total | $0.00 | USD |
Visualization
Methodology
Numeric computations use standard arithmetic and explicit conversions between percent and decimal forms. Rounding follows common financial practice: round monetary results to two decimal places unless your accounting rules require otherwise.
For software and calculation correctness we follow numeric handling principles consistent with IEEE 754 floating-point guidance and recommend system-level rounding to avoid cumulative floating-point error.
This calculator does not perform jurisdiction lookups automatically. When using a custom jurisdiction rate, enter the combined rate verified from your local tax authority. Tax determination (nexus, product taxability, exemptions) is a legal determination outside the scope of this calculator.
Worked examples
Example 1 — Standard: price 20.00, quantity 3, tax rate 8.25 percent produces subtotal 60.00, tax 4.95, total 64.95
Example 2 — Inclusive: total inclusive 107.50 with tax rate 7.5 percent yields pre-tax amount 100.00 and tax 7.50
F.A.Q.
Can this calculator determine the correct tax rate for my address?
No. This tool does not perform address-to-rate lookups. Use your jurisdiction's official rate tables or certified rate services to obtain combined state and local rates, then enter that rate here.
How are results rounded?
Monetary outputs should be rounded to two decimal places for reporting. For internal calculations, systems should follow IEEE 754 guidance and use consistent rounding rules to avoid small discrepancies.
Does this calculator determine taxability or nexus?
No. Determinations of whether an item is taxable, or whether you have tax nexus in a jurisdiction, are legal determinations and require review of statute, regulation, or an advisor. This tool only computes amounts from supplied rates.
Is this tool compliant with regulatory standards?
This calculator implements transparent arithmetic and advises practices aligned with numeric standards and quality-management guidance, but it is not a substitute for official tax guidance or certified software for filing.
How accurate are results?
Results are mathematically accurate given the supplied inputs. Accuracy depends on correct rates and inputs. Always verify rates with official sources before reporting or remitting tax.
Sources & citations
- IRS guidance on sales tax and deductions — https://www.irs.gov/credits-deductions/individuals/use-the-sales-tax-deduction-calculator
- IEEE Standard for Floating-Point Arithmetic (IEEE 754) — https://standards.ieee.org/standard/754-2019.html
- NIST — general standards and measurement traceability — https://www.nist.gov
- ISO 9001 — Quality management systems — https://www.iso.org/iso-9001-quality-management.html
- OSHA — guidance on workplace practices (organizational quality and safety context) — https://www.osha.gov
- IRS Publication 15-T — Federal Income Tax Withholding Methods — https://www.irs.gov/publications/p15t
- IRS Publication 505 — Tax Withholding and Estimated Tax — https://www.irs.gov/publications/p505
- IRS Tax Withholding Estimator — https://www.irs.gov/individuals/tax-withholding-estimator
Further resources
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
Record ID: 88deb540487aWhat changed (latest)
v1.0.0 • 2025-11-19 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
What changed (latest)
v1.0.0 • 2025-11-19 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Engine
v1.0.0
Data
Baseline (no external datasets)
Content
v1.0.0
UI
v1.0.0
Governance
Last updated: Nov 19, 2025
Reviewed by: Fidamen Standards Committee (Review board)
Credentials: Internal QA
Risk level: low
Reviewer profile (entity)
Fidamen Standards Committee
Review board
Internal QA
Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person
Semantic versioning
- MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
- MINOR: New features or fields that do not change existing outputs for the same inputs.
- PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.
Review protocol
- Verify formulas and unit definitions against primary standards or datasets.
- Run golden-case regression suite and edge-case suite.
- Record reviewer sign-off with credentials and scope.
- Document assumptions, limitations, and jurisdiction applicability.
Assumptions & limitations
- Uses exact unit definitions from the Fidamen conversion library.
- Internal calculations use double precision; display rounding follows the unit's configured decimal places.
- Not a substitute for calibrated instruments in regulated contexts.
- Jurisdiction-specific rules may require official guidance.
Change log
v1.0.0 • 2025-11-19 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: 450a58b1e7cf
- https://standards.ieee.org/standard/754-2019.html
- https://www.irs.gov/credits-deductions/individuals/use-the-sales-tax-deduction-calculator
- https://www.irs.gov/individuals/tax-withholding-estimator
- https://www.irs.gov/publications/p15t
- https://www.irs.gov/publications/p505
- https://www.iso.org/iso-9001-quality-management.html
- https://www.nist.gov
- https://www.osha.gov