Typically 12 to 18 feet across the finished box, most commonly 14 or 16, though those are market conventions rather than legal ones: the HUD code sets a floor of 8 body feet and no ceiling at all, and what limits the width in practice is the oversize permit the states along the route are willing to issue.
There is no legal answer to this question, which is why the ranges quoted for it never quite agree. Manufacturers build to widths the highway departments will let them tow and the parks are laid out to accept, and those conventions have settled into a short list of numbers. Knowing where the numbers come from tells you which ones to trust when a listing quotes a size.
The numbers in the market
Single wides are sold as 12, 14, 16 and 18 wides, and 14 and 16 are the volume. Length usually runs 60 to 80 feet. That gives roughly 600 to 1,300 square feet, and the width is what decides whether the plan can carry a second full bathroom, a usable second living space, or a bedroom that takes a queen bed across rather than along the wall. Two feet of width across the whole length of a home is worth more room than it sounds.
Older homes narrow as they get older. Pre-1976 mobile homes were commonly 8, 10 and 12 feet wide, because the towing rules and the plants of the day were built around those sizes. A very narrow home in a park is usually an old home, and old here has consequences for lending and insurance rather than only for taste.
What the code says, which is a minimum
24 CFR 3280.2 defines a manufactured home as a structure transportable in one or more sections which in the traveling mode is 8 body feet or more in width or 40 body feet or more in length, or 320 or more square feet when erected on site. Every number in that sentence is a floor. Nothing in the federal code caps how wide a section may be, and nothing in it uses the words single wide or double wide; the code counts sections and leaves the dimensions to the manufacturer.
The code does say how to measure. Floor area is calculated from exterior dimensions taken at the largest horizontal projections when the home is erected on site, including expandable rooms. That is the measurement to use when you are checking a listing's square footage against the home in front of you.
The real limit is the road
The ordinary legal width for a vehicle on a highway is 8 feet 6 inches, and every manufactured home section is wider than that, so every one of them travels under an oversize permit issued by each state it crosses. The permit is what sets the practical ceiling: it fixes the route, and above certain widths it adds pilot cars, daylight-only travel, restricted days, and sometimes a police escort. The wider the section, the more of that applies and the more the haul costs.
That is why 18 feet is roughly where single sections stop. Beyond it the permitting and escort requirements in most states make a second section cheaper than a wider one, which is exactly the calculation that produced the double wide.
The road also explains the older, narrower homes. When towing rules and plant tooling were built around 8 and 10 foot loads, that was the width of the product, and the market widened as the permits and the equipment did.
Measuring the one in front of you
Do not trust the advertised number on a used home. Measure the exterior wall to exterior wall at the widest point, and note separately whether the eaves, a bay window, a tag section or an added room push past that line, because those affect what a park space will take and what a mover has to permit.
Then read the data plate, the paper sheet glued inside a cabinet door, a closet or the electrical panel. It names the manufacturer, the date of manufacture, the model, and the wind and thermal zones the home was built for. Between the tape and the data plate you have the two numbers an insurer, a park manager and a transporter will each ask for.
Questions people ask
How wide is a 16 wide mobile home really?
The finished box is usually a little under 16 feet, because the nominal size is a market label rather than a measured dimension. Manufacturers publish the exact box width in the model specifications, and the number that matters for a park space or a haul is the widest horizontal projection including eaves.
What is the widest a single section can be?
There is no federal maximum. In practice 18 feet is about where single sections stop, because above that the oversize permits, pilot cars and route restrictions in most states cost more than building the home as two sections and joining it on site.
How many square feet is a single wide?
Commonly 600 to 1,300 square feet, from a 12 by 60 at the small end to an 18 by 80 at the large. The HUD code's own floor is 320 square feet, measured from the exterior dimensions at the largest horizontal projections once the home is set.