Hydro Flasks come in a variety of sizes and are often referred to in ounces, as they originated from Colorado in the US, so it's important to check how many milliliters each bottle can hold. The most popular sizes are the 32oz (964ml) wide mouth, the 21oz (621ml) standard mouth and the 20oz (591ml) wide mouth. The standard mouth sizes are slimmer and great for fitting in backpacks, bags and cup holders. The wide mouth styles are bigger and hold more water, however are great for hikes and trails.