  We estimate the parameters of the donor of the accreting black-hole binary
MAXI J1820+070. The measured values of the binary period, rotational and radial
velocities and constraints on the orbital inclination imply the donor is a
subgiant with the mass of $M_2\approx 0.49^{+0.10}_{-0.10}M_\odot$ and the
radius of $R_2\approx 1.19^{+0.08}_{-0.08}R_\odot$. We re-analyze the
previously obtained optical spectrum from the Gran Telescopio Canarias, and
found it yields a strict lower limit on the effective temperature of $T>4200$
K. We compile optical and infrared fluxes observed during the quiescence of
this system. From the minima $r$ and $i$-band fluxes found in Pan-STARSS1 Data
Release 2 pre-discovery imaging and for a distance of $D\approx3$ kpc,
reddening of $E(B$--$V)=0.23$ and $R_2\approx{1.11R_\odot}$, we find
$T\lesssim4230$ K, very close to the above lower limit. For a larger distance,
the temperature can be higher, up to about 4500 K (corresponding to a K5
spectral type, preferred by previous studies) at $D=3.5$ kpc, allowed by the
Gaia parallax. We perform evolutionary calculations for the binary system and
compare them to the observational constraints. Our model fitting the above
temperature and radius constraints at $D\approx 3$ kpc has the mass of
$0.4M_\odot$, $T\approx4200$ K and solar metallicity. Two alternative models
require $D\gtrsim 3.3$--3.4 kpc at $0.4 M_\odot$, $T\approx4500$ K and half
solar metallicity, and $0.5M_\odot$, $T\approx4300$ K and solar metallicity.
These models yield mass transfer rates of $\sim\!\!10^{-10}M_\odot$/yr,
compatible with those based on the estimated accreted mass of $\approx\!2\times
10^{25}$ g and the time between the 2018 discovery and the 1934 historical
outburst.
