How heavy is the Tardis?

We all know that the Tardis is bigger on the inside than on the outside, but what makes up it's weight? It's outside appearance or what is inside.


On the outside, it is just a wooden box with room inside for a phone.

But on the inside it is fully decked out with a swimming pool, a library and numerous other rooms, which each individual Doctor changes to suit his needs.

If the weight of the Tardis was the combination of it's internal features, then it would weigh a great deal indeed.

When seen 'landing' on various surfaces in the TV program, it does not sink, which would point to it not being overly heavy.

So my conclusion is that it's weight is the sum of it's outer appearance, rather than the sum of it's interior contents.

Unless of course, it has an anti-gravity field, which would solve the sinking/weight problem.