We determined the fundamental properties of HD 140283 by obtaining new interferometric and spectroscopic measurements and combining them with photometry from the literature. The interferometric measurements were obtained using the visible interferometer VEGA on the CHARA array and we determined a 1D limb-darkened angular diameter of 0.353 +/- 0.013 milliarcseconds. Using photometry from the literature we derived the bolometric flux with two solutions: a zero-reddening one of Fbol = 3.890 +/- 0.066 1E-8 erg/s/cm2 and a solution with a maximum of Av = 0.1 mag, Fbol= 4.220 +/- 0.067 1E-8 erg/s/cm2. The interferometric Teff is thus 5534 +/- 103 K or 5647 +/- 105 K and its radius is R = 2.21 +/- 0.08 Rsol. Spectroscopic measurements of HD140283 were obtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of H-alpha line wings yields Teff(Halpha) = 5626 +/- 75 K. Using fine-tuned stellar models including diffusion of elements we then determined the mass M and age t of HD140283. Once the metallicity has been fixed, the age of the star depends on M, initial helium abundance Yi and mixing-length parameter alpha, only two of which are independent. We need to adjust alpha to much lower values than the solar one (~2) in order to fit the observations, and if Av = 0.0 mag then 0.5 < alpha < 1. We give an equation to estimate t from M, Yi (alpha) and Av. Establishing a reference alpha = 1.00 and adopting Yi = 0.245 we derive a mass and age of HD140283: M = 0.780 +/- 0.010 Msol and t = 13.7 +/- 0.7 Gyr (Av = 0.0) or M = 0.805 +/- 0.010 Msol and t = 12.2 +/- 0.6 Gyr (Av=0.1 mag). Our stellar models yield an initial metallicity of [Z/X]i = -1.70 and logg = 3.65 +/- 0.03. Asteroseismic observations are critical for overcoming limitations in our results.
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