i would like to like to know how to calculate the amount of water required to sufficiently provide for a 100sqm of water under floor heating.
i need some kind of formula if anyone can help?

Are you trying to work out how much Fernox to add or the length of pipe you need?

The pipe pitch can vary depending on how well insulated the house is and the floor covering. Suppose we said it was 200mm for the moment. That would mean roughly 5m  of pipe per square meter of floor. Plus the runs to and from the rooms.

I think the internal diameter of UFH varies from 12mm to 15mm depending on the make.

So for 15mm ID pipe the volume is roughly..

Pi * (ID/2)^2 * Length

3.14 x 0.0075^2  * 500 = 0.088 cubic meters or 88L plus whatever is in the boiler and manifold and runs to the room. Call it 100L ?

What was the question again?