Microwave manufacturers typically market their ovens by specifying the volume of a given model's interior in cubic feet. While this is a good indicator of the oven's relative capacity, it's not the only meaningful dimension that will affect your choice of which microwave is right for you. Depending on the space you have available in your kitchen, a model's exterior dimensions may be as important to you as the oven's volume designation.
Countertop microwaves are standalone units meant, perhaps obviously, to sit on top of a kitchen counter. Unlike built-in units, this style of microwave takes up counter space that could otherwise be used as work or storage space, but countertop units also offer flexibility in terms of placement that built-in ovens don't. They are also generally less expensive than built-in models and don't require installation.
Countertop microwaves are available in a wide range of volumes. The most common models have capacities between 1 and 1.9 cubic feet, but compact models with capacities as low as 0.5 cubic feet and large models with capacities up to 2.2 cubic feet are available.
Some compact countertop models are less than 18 inches wide on the exterior, while mid-size and large models may be 24 inches wide or more. Some compact models are not much more than 12 inches deep, but more common depths range between 18 and 22 inches. Typical heights range between about 10 inches and 18 inches.
Over-the-range microwaves are designed to fit into the space above a cooktop, and they usually have a built-in venting system so that they can function as an exhaust hood for the range. They free up counter space, but they are generally more expensive than countertop models and may require professional installation.
Because they need to fit into the space above a standard cooktop, over-the-range microwaves offer less variation than countertop models in terms of size. Most models have a width of between 29 and 30 inches, a depth of between 15 and 16 inches, and a height of between 16 and 18 inches. Typical capacities range between 1.5 and 2.2 cubic feet, although compact models are available.
Built-in microwaves are designed to be installed inside custom cabinetry or mounted under cabinets and above the countertop. Like over-the-range models, built-ins don't take up counter space, but they also don't take up the space above the cooktop, allowing the installation of a more effective vent hood. Built-ins are typically the most expensive type of microwave, and they require professional installation.
Because built-ins are not constrained by the dimensions of the cooktop, they're available in a wider range of sizes than over-the-range models. As with countertop models, the most common built-in models have capacities between 1 and 1.9 cubic feet, but capacities of up to 2.2 cubic feet are available as well. The range of exterior dimensions for built-in models is similar to that of countertop models with comparable capacities.