Lets assume we're dealing with a superior planet, which is to say, the planet is orbiting at a greater distance than the Earth. This effectively ensures that there is no planetary phase to deal with.

Now, the Sun has a luminosity of $L_{sun}$ such that the Solar flux as seen by the planet is
$$
f_{planet} = \frac{ L_{sun} }{ 4 \pi d_s^2 }.
$$

The cross section of the planet is about $A = \pi r_p^2$, so for a given albedo $a_p$ the reflective luminosity of the planet will be
\begin{aligned}
L_{planet} &= a_p f_{planet} A \\
&= a_p \pi r_p^2 \frac{ L_{sun} }{ 4 \pi d_s^2 }.
\end{aligned}

Given that we know the absolute magnitude of the Sun, the absolute magnitude of the planet follows as
\begin{aligned}
V_{planet} &= -2.5 \log_{10}\left[ \frac{ L_{planet} }{ L_{sun} } \right] - V_{sun} \\
&= -2.5 \log_{10}\left[ a_p \frac{ r_p^2 }{ 4 d_s^2 } \right] - V_{sun}
\end{aligned}

The apparent magnitude of the planet as seen from Earth can then be calculated as
$$
m_{planet} = V_{planet} + 5 \log_{10}\left[ d_{e-p} \right] - 5
$$
where the distance between Earth and the planet, $d_{e-p}$, should be in parsecs and of course depends on the orbital phase of the two respective planets.

So the one additional parameter required was the Solar luminosity. There are probably a bunch of subtle effects with the albedo and the geometry of the system that are not taken into account here, but it should be a fair approximation.

Planetary magnitudes vary not only according to the Sun’s luminosity, their own average albedo, and their distance from the Earth, but also from:

• Variations in their albedo across their surface.


• Their phase angle, for planets that we sometimes see as a crescent.


• Their inclination, for planets like Saturn and Uranus that have a different albedo at their equator than at their poles.


• Neptune keeps getting brighter. No one knows why.

All of these effects are detailed in an informative paper Computing Apparent Planetary Magnitudes for The Astronomical Almanac by Mallama and Hilton, revised in 2018:

https://arxiv.org/pdf/1808.01973.pdf

Their source code for computing planetary magnitudes given all of these effects can be found here:

https://sourceforge.net/projects/planetary-magnitudes/

If you want correct values, you have to take into account the effects mentioned in Brandon Rhodes' answer. Nevertheless, here's how to do a quick-and-dirty calculation.

The absolute magnitude of a planet is defined as the apparent magnitude if the Sun-planet and planet-observer distances are 1 au, at opposition.

Assuming a diffuse disc reflector model, the absolute magnitude $H$ of a planet of diameter $D_{\rm p}$ is given by

$$H = 5 \log_{10} \left( \frac{D_0}{D_{\rm p} \sqrt{a_p}} \right)$$

Where $a_p$ is the planet's geometric albedo, and $D_0$ is given by

$$D_0 = 2\,{\rm au} \times 10^{H_\ast / 5}$$

Where $H_\ast$ is the absolute magnitude defined at a reference distance of 1 au, which can be calculated from the usual 10 parsec absolute magnitude $M_\ast$ as follows:

$$H_\ast = M_\ast + 5 \log_{10} \left( \frac{1 \rm \ au}{10 \rm \ pc} \right) \approx M_\ast - 31.57$$

For the Sun, this results in $D_0 \approx 1329\ \rm km$.

To get the apparent magnitude of the planet, you can then use

$$m_{\rm p} = H + 5 \log_{10} \left( \frac{d_{\rm p\ast} d_{\rm po}}{1 \mathrm{\ au}^2} \right) - 2.5 \log_{10} q(\alpha)$$

Where $d_{\rm p\ast}$ is the distance between the planet and the star, $d_{\rm po}$ is the distance between the planet and the observer, and $q(\alpha)$ is the phase integral at the phase angle $\alpha$.

For the diffuse disc reflector, $q(\alpha) = \cos \alpha$. For a Lambertian sphere,

$$q(\alpha) = \frac{2}{3} \left( \left(1 - \frac{\alpha}{\pi} \right) \cos \alpha + \frac{1}{\pi} \sin \alpha \right)$$

At opposition, $\alpha = 0$, giving phase integrals of $q(0) = 1$ for the diffuse disc, and $q(0) = \tfrac{2}{3}$ for the Lambertian sphere.

Real planets have more complex phase functions which need to be determined empirically (see Brandon Rhodes' answer).

As a quick check, let's plug in values for Jupiter, taken from the NASA Jupiter Fact Sheet. With a (volumetric mean) diameter of 139,822 km and a geometric albedo of 0.538, the computed absolute magnitude is -9.44, versus the actual value of -9.40.

Using the computed value of -9.44, a distance from Jupiter of 5.204 au to the Sun and 4.204 au to the Earth, and using the Lambertian sphere $q(0) = \tfrac{2}{3}$, the apparent magnitude works out at -2.30, while for a diffuse disc $q(0) = 1$ the apparent magnitude works out at -2.74. The brightest the real planet gets is around -2.94. Fortunately for me, the value comes out in the right ballpark, despite the gross approximations being made to the reflection behaviour of real planets.

My computed values using $q(0) = 1$ for Saturn, Uranus and Neptune are 0.54, 5.57 and 7.75 respectively, which aren't too far off the real values either.