How the estimate works
Residential rentals are depreciated over 27.5 years, straight-line. The building's value (your purchase price minus the land's share) divided by 27.5 gives your annual write-off; multiply by years owned and you get total depreciation. If you enter a known total from your returns, the calculator uses that instead.
One detail that surprises people: recapture is based on depreciation that was allowed or allowable. If you never claimed depreciation you were entitled to, the IRS still taxes you as if you had. There's no upside to skipping the deduction.