Measuring for a new countertop is basic. Allowances for overhang, fascia and appliances are the only variations from simple width-by-length measurements.
The concept of overhang is to provide a lip so that liquids drop from the countertop to the floor without touching the front of the cabinets. Particleboard or plywood substrate should extend at least 1/2 inch over the front of the cabinets. If you're dealing with an island or peninsula cabinet, it should extend 1/2 inch on all sides that do not terminate against a wall.
Fascia is the horizontal piece of trim attached to the substrate. Typically 3/4 inch thick and 1 1/2 inches wide, it's attached around the perimeter of the countertop. It adds 3/4 inch to the measurement.
Add the Two Measurements
Add 1/2 inch for the overhang, plus 3/4 inch for the thickness of the fascia. This adds 1 1/4 inches to the depth measurement from the wall to the front of the cabinets or, if applicable, to the side. This measurement can vary per installer or manufacturer, but it should never be less than 1 inch.
Most countertops have sinks, stoves or other appliances somewhere in the mix. In most cases, the plywood or particleboard substrate is cut out to fit around them. If you have a free-standing appliance, however, the countertop may or may not include an overhang. It is sometimes measured to fit flush with the side of the cabinet facing the appliance.
Eating bar extensions, typically found on island or peninsula cabinets, should include the 3/4-inch-thick allowance for fascia only. The 1/2-inch overhang is not necessary on eating bars.
Prefabricated Granite or Composite Countertops
If you're measuring for any type of preformed, ready-made countertop, measure the cabinets only. Don't add overhangs or fascia allowances. In this instance, measurements are typically for pricing only. If you choose to use prefabricated countertops, it's customary for the dealer to measure on-site.
Map It Out
Make a rough drawing of your cabinets from an overhead perspective. It doesn't have to be to scale.
Measure the linear or running length of the cabinets along the longest wall. Add 1/2-inch for overhang and 3/4-inch for fascia where needed. Write the measurement per location on the drawing.
Measure additional walls that join at 90 degrees or angles. Add allowances for fascia and overhang where needed. Write the measurement on the drawing.
Measure the depth of the countertop from the wall, to the front of the cabinet. Take the measurement perpendicular to the length midway on the longest wall. Take the same measurements on additional walls. Add the allowances for overhang and fascia, and write the measurement down on the plan.
Countertop thickness is almost always 3/4 inch, unless you're adding a thicker, prefabricated stone or granite countertop. This measurement is typically insignificant. But if you're cramped for space, the distance between the countertop and the bottom of the upper cabinets should be no less than 15 inches. The distance is typically 18 inches or more.
Identify the location for the sink cutout, which is typically done with templates or instructions supplied by the manufacturer. Sinks are centered left to right in the opening, usually with no less than a 3-inch border between the front of the cabinet, and the front of the sink. If you don't have a template, trace around the perimeter of the sink, and cut the opening 3/8 inch smaller than the tracing.
Drop-in or built-in range cooktops are also cut with templates or instructions per manufacturer and are centered left to right in the opening. Tolerances for this type of cutout are often rigid. Variations plus or minus 1/16 to 1/8 inch can make a difference.