What is the ratio of woman to men in the world?

Around 49 to 51 per cent.

It all depends on the region. Statistically, Asian countries have higher percentage of single men. However, If you look at some Eastern European countries, there is higher ration of single women. Once, I was doing a research and was browsing through female dating sites, and I found out that there are way more single ladies in Eastern Europe, but almost equal ratio of single men and women in the US.