The haematocrit (Hct) describes the proportion of one's blood that is made up of red blood cells. It's usual range is about 35-45% for women and 40-50% for men. The rest is almost all plasma, with a minor contribution coming from white cells and platelets.
The other figure you need is the mean corpuscular haemoglobin concentration (MCHC). As the name hints, this is the concentration of haemoglobin in a particular volume of red cells. So, you can sort of see how knowing both (i) the concentration of haemoglobin within a bunch of red cells and (ii) the proportion of the whole blood that these red cells constitute could tell you the concentration of haemoglobin for the whole blood. Indeed, the formula is simple:
Hct × MCHC = Hb
For instance, assuming a haematocrit of 40% and a MCHC of 35 g/dl, the Hb turns out to be:
0.40 × 35 = 14 g/dl
You really should have the exact MCHC for the calculations (it's almost always given as part of a complete blood count), but if you're desperate you can estimate based on the normal range of 32 to 36 g/dl.