There is so much marketing (much of it dishonest) involved in women's clothes sizing that you just have to try stuff on. I was a 34, gradually reducing to a 32 before transition. I take a size 10 usually. At least 10 is my first guess. If a maker practises size inflation, I may find an 8 fits me better. Or shrunk second-hand clothes might require a 12.
If you are not out and want to find something that fits without drawing attention to yourself, try this. Measure your waist. (Mine was 34.) Divide by two (=17 for me). Take a piece of string and tie two knots in it that distance apart (i.e. 17" for me). Carry the string in your pocket. When examining a piece of clothing, hold it up on the hanger with the waistband fastened and hold the string next to the waistband. If the width matches the knots in the string, go try it on!
Unlike men's clothes, where pants have a width and a height, women's sizes do not have a separate height. Women are all assumed to be proportional. (!) You may find some clothes marked "tall" or "petite": those refer to non-standard lengths. If it doesn't say either, then a 10 is just a 10.
I have leaned never to buy anything without trying it on. No more going to the store, picking up a pair of 34x30s and taking it straight to the cashier. If I like it, and if the size label says it should fit or if it looks like it could fit, I go and try it. That goes double for bra sizes, by the way!
