Where the number on your plan came from
If you are holding a set of drawings, the wall thickness is already decided, and three things decided it. How much dirt will stand against the wall, which is really the height from the top of the footer up to finished grade. What is landing on top of it, meaning the floor system, the walls above and the roof with snow on it. And what the soil behind the wall does when it gets wet.
That last one carries more weight here than people expect. A lot of the ground in Carroll County is red clay over rock. Clay holds water and swells as it takes it up, and swelling soil pushes on a basement wall harder than well drained ground does. That shows up as thickness, as steel, and as a reason to take the drain and the backfill behind the wall seriously.
Those calls belong to whoever stamped your drawings. We do not size structural steel or set wall thickness ourselves, and a crew willing to settle either one off the top of their head standing in your driveway is not doing you a favor.
Thickness alone is not what makes a wall strong
A basement wall almost never fails by being crushed. Concrete is enormously strong in compression, and the load coming down from the house above is not the thing putting the wall at risk. What puts it at risk is the ground pushing sideways, trying to bend the wall inward.
Concrete is weak in tension, so bending is where rebar earns its keep. Bar size, spacing, lap lengths and the extra bar that gets added around window and door openings are all specified on the structural sheet, and the position of that bar within the thickness changes how much work it can do. It has to be tied tight enough that the first surge of concrete cannot move it, and stood back off the inside face so the finished wall wraps around it. Bar that ends up touching a form face is bar you paid for and did not get.
So an eight inch wall with the right bar in the right place will outperform a thicker wall with the bar in the wrong place. Thickness and reinforcing are one answer, not two separate ones.
The wall is narrower than what it stands on, on purpose
People are often surprised that the footer under an eight inch wall is wider than eight inches. That is the design, not an overbuild. The wall arrives at the ground as a narrow line of concentrated load, and the footer exists to widen that line out until the soil under it can carry the building without moving.
The top of that footer is not left flat and featureless either. Either a keyway gets pressed into it while the concrete is still wet, or dowels get stood up out of it, so the wall has something to lock onto rather than sitting on a smooth surface. When you hear that a foundation is two pours, this is the connection between them, and it is placed on the first day for the sake of the second.
What extra thickness changes on pour day
Thickness is also a volume question, and volume is what gets ordered. Concrete is figured the same way for a wall as for anything else. Length times width times thickness, all in feet, divided by 27, which gives you cubic yards. Going from eight inches to ten on a long basement wall is a quarter more concrete before anything else about the job has changed.
Ready mix arrives in ten cubic yard loads, and we order about ten percent over the calculated volume, because coming up short at the top of a wall is a far worse problem than having a little left over. Thicker walls also mean more ties, more bracing and more pressure inside the forms while the pour is running, which is one more reason a wall gets filled in lifts around the whole perimeter instead of in one spot at a time.
A wall pour is also the pour most likely to want the pump. You are aiming into a slot a few inches across at the top of the forms, usually down in a hole with the excavator's spoil piled beside it, and no ready mix chute is getting there. We run our own trailer mounted line pump on 2.5 inch and 3 inch line, so the truck stays out on firm ground and the hose walks the top of the forms.
Measuring a basement wall that is already standing
If the wall is in and you are trying to find out what you have, go to an opening. A basement window is the easiest, because the concrete is exposed on both faces and you can run a tape straight through it. A walkout door, or the hole where a pipe or a duct passes through, works the same way.
Measure the concrete itself and not the finish. Framing, furring strips, foam board and drywall on the inside face will add inches that are not wall. If the window has a buck or a return around it, measure to the concrete behind that rather than to the trim.
What you find is useful for planning and it is not a verdict on the wall. A thickness by itself says nothing about the steel inside, the footer under it, or whether the water behind it has anywhere to go. If you are measuring because something about the wall is bothering you, describe what you are actually seeing when you call instead of leading with the number.