Kegs are sacred vessels, designed not just for storing but for serving mass quantities of beer (or other beverages like cider, but most often beer). Perfect for everyone from bar owners to home brewmasters and barbecue aficionados, thick metal kegs are the easy-transportation superstar that kicks breakable glass bottles and agitation-sensitive cans to the curb. Plus, they’re also wildly eco-friendly (especially if you tap your keg into 100% compostable disposable cups). But how big are they anyway?

Kegs always contain the same fluid-ounce capacity regardless of the beer type. But there are several different common keg sizes that fans should know. The standard full keg (aka a half-barrel keg) is typically the size for which most commercial draft systems are outfitted. Full kegs hold approximately 15 ½ gallons of beer, the equivalent of 124 pints or 165 12-ounce bottles, and weigh 160 pounds when filled with beer. Likewise, a full keg contains the equivalent of 165 regular beer cans or 124 tall boys.

Pony kegs (aka quarter barrel kegs) hold a more modest 7 ¾ gallons, roughly 62 pints or 82 bottles. These stubby beauties are the same circumference as full kegs with only half the height, and hold the right amount for gatherings like office parties or potlucks. A pony keg also weighs 80 pounds, filled, which can make solo transportation possible without the help of a dolly.

Just like there’s a rule for calculating how much pizza to order for a party, there’s an equation to help hosts calculate what keg size they’ll need to serve a crowd. As a general rule, hosts should expect each guest to drink two beers (aka two 12-fluid-ounce servings) in the first hour of the party, followed by one beer per hour for the remaining duration of the event. This calculation allows for some guests to only pour a drink or two all night, while others might have more. So, if your party is going to last for roughly five hours, and you’ve invited 20 guests, 20 x (5+1) = 120, meaning the host should purchase two pony kegs or one full/half-barrel keg.

Beyond the size, it’s the price tag that varies most from one brand to the next. Full kegs tend to run in the $100-$250 range, which might seem steep before remembering that 15 ½ gallons shakes out to $1 to $2 per serving — a bargain compared to buying 165 individual bottled or canned beers. Online retailers like Five Points Bottle Shop carry a wide inventory of kegged beers, where a full keg of Guinness Stout costs $199.99, a lighter variety like Blue Moon costs $159.99, Bud Light runs for $144.99, and old reliable King of Beers Miller High Life costs just $64.99. Beyond online retailers, kegs can be purchased from liquor stores, breweries, and wholesale clubs like Costco.