Fidamen

Property Tax Calculator

This estimator produces a reproducible estimate of property tax liability using common ad valorem rules: taxable value multiplied by a combined tax rate, plus any fixed local assessments. It supports a California Prop 13 mode and a prorated/supplemental scenario for partial-year ownership.

The tool is an estimator only. Official tax bills, reassessments, exemptions, and supplemental levies are determined by local assessor and tax collector offices. Use the calculation results for planning and comparison; contact your county assessor for official values.

Updated Nov 13, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 5fce2c5996fc • Reviewed by Fidamen Standards Committee

Generic ad valorem calculation: taxable value times combined tax rate plus fixed local assessments. Use for general-purpose estimates where local rules are not applied.

Inputs

Results

Updates as you type

Estimated annual property tax

$6,250.00

OutputValueUnit
Estimated annual property tax$6,250.00USD
Primary result$6,250.00

Visualization

Methodology

Primary calculation follows ad valorem principles: taxable value equals assessed value minus allowable exemptions, and annual tax equals taxable value multiplied by the combined tax rate (expressed as a percent). Local fixed assessments are added to produce a total annual estimate.

California mode includes common elements used in public guidance: measurement of taxable value and inclusion of typical supplemental amounts. This tool does not perform legal reassessment, Prop 8 temporary reductions, or full statutory compliance checks.

Software quality, data handling, and security practices used to develop and maintain this estimator follow applicable industry standards for assurance and engineering: ISO 9001 for quality management considerations, IEEE best-practice guidance for software engineering, and NIST recommendations for secure configuration and data protection. Operational safety and workplace compliance reference OSHA guidance where relevant.

Worked examples

Example 1: Assessed value $500,000, tax rate 1.25%, no exemptions, no special assessments → Annual tax = $500,000 × 0.0125 = $6,250.

Example 2 (proration): Same property sold with 6 months owned → Prorated tax ≈ $6,250 × (6/12) = $3,125.

Example 3 (California): Assessed value $800,000, exemptions $100,000, tax rate 1.10%, supplemental $2,000 → Annual estimate = ($800,000 − $100,000) × 0.011 + $2,000 = $8,700 + $2,000 = $10,700.

F.A.Q.

How accurate is this calculator?

This estimator provides an approximate calculation based on inputs you supply. It does not access official assessor databases and does not apply all local rules, exemptions, or appeals. Expect differences from official bills when local special assessments, bond rates, rollback levies, or statutory caps apply. Use results for budgeting only.

Does this tool calculate Prop 13 reassessments or Prop 8 reductions?

No. This tool provides a simplified California mode that illustrates common elements (1% base plus local levies and typical supplemental amounts). It does not make legal determinations about reassessment events, assessed value caps, or temporary reductions under Prop 8. Contact your county assessor for determinations.

Where do I find my tax rate and assessed value?

Your county assessor or tax collector posts tax rates and assessed values. If unsure, use your latest tax bill or the county assessor's public portal. Local municipal bond levies and parcel taxes can also affect the combined rate.

Why should I trust the tool's calculations?

Calculations are transparent, reproducible, and based on simple arithmetic rules used by many taxing jurisdictions. Development and data-handling practices reference recognized standards for quality and security, but the output is an estimate and not a legal tax bill.

Sources & citations

Further resources

External guidance

Versioning & Change Control

Audit record (versions, QA runs, reviewer sign-off, and evidence).

Record ID: 5fce2c5996fc

What changed (latest)

v1.0.0 • 2025-11-13 • 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 13, 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 • 2025-11-13 • 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: 145ef243a341