"On center" means you measure from the middle of one stud to the middle of the next. The gap you'd see with the drywall off is smaller: 14 1/2″ between 16″ studs, because each stud is 1 1/2″ thick.
Why 16 and 24 inches
The numbers fit the 4×8 sheet. Drywall, plywood and OSB are 48″ wide and 96″ long, and both spacings divide into those numbers. Three 16″ bays fill 48″, and so do two 24″ bays. With studs on either layout, the end of every sheet lands in the middle of a stud where it can be nailed.
Most tape measures also have a small mark every 19.2″. That's 96 divided by 5, so five bays fill an 8-foot sheet. You'll see it in floor joists and engineered systems more than in walls.
What the code allows
IRC Table R602.3(5) sets the maximum spacing by stud size and by what the wall holds up. These are the limits for studs up to 10 feet tall.
| Wall carries | 2×4 studs | 2×6 studs |
|---|---|---|
| Roof and ceiling only | 24″ | 24″ |
| One floor, plus roof and ceiling | 16″ | 24″ |
| Two floors, plus roof and ceiling | Not allowed | 16″ |
| One floor only | 24″ | 24″ |
| Nothing (nonbearing) | 24″ | 24″ |
So a 2×4 bearing wall on the ground floor of a two-story house has to stay at 16″. Lower-grade Utility studs have tighter limits, and your local code can be stricter than the IRC, so check with your building department before you frame anything that holds up a house.
16 or 24 on a new wall
Spacing at 24″ saves lumber. A 14-foot wall is 168″ long. At 16″ that's 168 ÷ 16 = 10.5, which rounds up to 11 spaces and 12 studs. At 24″ it's 7 spaces and 8 studs. The wall uses four fewer studs and comes out slightly springier.
For a garage or a shed, 24″ is usually fine where the table allows it. Inside a house, plenty of people stay at 16″ anyway because the wall feels more solid and there's more to screw into. Check the instructions for your siding, sheathing and any tile backer before going wider, since they set their own limits. You can run both spacings through the framing calculator and compare the stud count for your wall.
Layout is the same either way. Hook your tape on the end of the plate and mark from there, so the sheets land on the stud centers. In practice most framers pull the first mark back 3/4″, which puts the edge of the first stud at 15 1/4″ and every edge after that at 16″ intervals. There's a full walk-through in how to frame a wall.
Finding studs in a finished wall
A stud finder works, but it can read pipes and wires too, so back it up with one of these:
- Outlets and switches. Electrical boxes are usually fastened to the side of a stud. Find which side and you've got a starting point.
- A strong magnet. Slide it across the wall until it grabs a drywall screw. A line of screws running up and down is a stud.
- Baseboard nails. Look along the trim for filled nail holes. Trim gets nailed into studs, so they show you the layout.
- A finish nail. Tap one through the wall somewhere a patch won't show, like behind the baseboard. You'll feel it bite into wood or slide through.
Once you've found one, measure over 16″ and test for the next. If that fails, try 24″. Expect extra studs next to windows and doors and at corners, because they're off the regular layout. Pipes and wires run through studs as well, so go carefully when drilling near fixtures and outlets.