Is size 27 waist equal to a size 5 or 7?

A 27" waist generally equals a size 5.

However,it really depends on both the brand of clothing, and your body type. For example, some clothing brands are vanity sized. Or if your hips are slim in proportion to your waist, so hip hugging jeans may require a smaller size. Always check the size charts online or try clothing on in stores.