Methodology

What we calculate

We estimate a property's Proposition 13 factored base-year value — its normal Prop 13 assessment path — then apply a county-level estimated tax rate. The sequence is:

Purchase price
  → apply official historical Prop 13 inflation factors
  → estimated Prop 13 factored base-year value
  → apply county estimated tax rate
  → estimated annual property tax  (÷ 12 → monthly)

This is not the county's official enrolled taxable value, which may be lower (e.g. a temporary Proposition 8 decline-in-value reduction) or otherwise different.

Data sources

• Proposition 13 inflation factors. Official annual California Consumer Price Index inflation factors published by the California State Board of Equalization in its Letters to Assessors. We use the authoritative “Factor” column covering assessment years 1976-77 through 2026-27 (51 years). Latest source: BOE LTA 2026-27 CCPI. See the full historical inflation factor table.
• County tax rates. A Version 1 estimate of the total ad-valorem rate applied to assessed value for each of California's 58 counties — the 1% constitutional general levy plus a county-typical amount of voter-approved ad-valorem debt. These are approximate county averages intended to be replaced with Tax Rate Area (TRA) level data in a later release. They exclude parcel-specific direct charges (Mello-Roos, parcel taxes, special assessments).

Calculation assumptions

• The purchase price becomes the base-year value.
• The first inflation factor applies on the January 1 lien date after the purchase (assessment year purchase-year + 1), then each subsequent year through 2026-27.
• Full precision is retained through the compounding; only displayed dollar amounts are rounded to the nearest dollar.
• The two important rate concepts are not confused: we estimate the rate applied to assessed value (≈1% general levy + voter-approved debt), not the effective rate on current market value.

Coverage

• Purchase years 1975–2026.
• All 58 California counties.
• Assessment years 1976-77 to 2026-27.

Known limitations

The estimate does not model Proposition 8 reductions, new construction, ownership-transfer reassessments, supplemental or escape assessments, disaster adjustments, Proposition 19 transfers, exemptions, or parcel-specific direct charges. County rates are averages, not parcel-specific. Treat all results as educational estimates.