Thinking about selling a rental property in California. Own it outright, depreciated over 20 years we have owned it. Where can I go to calculate tax consequence?

First, rental property is depreciated over 27.5 years - not 20 years.

To determine your taxable gain (a rough estimate) subtract all the depreciation already taken from what you ***ORIGINALLY*** paid for the property. That is your adjusted cost basis. The difference between your sales price and the adjusted cost basis is your taxable gain that you will pay taxes on.... both federal and state.

Try this tool https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1. Enter your regular income first to see the regular tax. Then add the sale to see the effect.
Enter the difference between the sale price and what you paid for it originally as a long term capital gain (LTCG). Enter the depreciation you've taken over the years (depreciation "recapture") as other income. Depending on how much total income you have LTCG are partially taxed at 0%, 15%, 20% and/or 23.8%. Depreciation recapture is taxed at your marginal rate, but not more than 25%*.

*The taxcaster tool does not have the ability to limit the recapture rate to 25%; so your result may be slightly high, depending on your total income.