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We derive the first closed‐form optimal refinancing rule: refinance when the current mortgage interest rate falls below the original rate by at least 1ψ[φ+W−exp−φ].In this formula W(.) is (the principal branch of) the Lambert W‐function, ψ=2ρ+λσ, φ=1+ψρ+λκ/M(1−τ),where ρ is the real discount rate, λ is the expected real rate of exogenous mortgage repayment, σ is the standard deviation of the mortgage rate, κ/M is the ratio of the tax‐adjusted refinancing cost and the remaining mortgage value, and τ is the marginal tax rate. This expression is derived by solving a tractable class of refinancing problems. Our quantitative results closely match those reported by researchers using numerical methods.