1 gram of one harvest/strain can look like much less or much more compared to 1 gram of another, based on things like presence of seeds, density of the buds, amount of leafy material left untrimmed, etc. Your best bet is to take however much you have and try to get the right portion of it - so if you have an eighth, get a bit more than a quarter of it and you'll have a gram.

If you don't know how much you have and have no scale, well.... good luck.