Garter stitch is the first pattern most people learn, and it's also the one that trips people up when it comes to counting, because the fabric doesn't behave the way a lot of beginners expect it to. You knit every single row, no purling, and the result is a bumpy, ridged fabric that looks the same on both sides. That symmetry is exactly what causes the confusion: it's not obvious where one row ends and the next begins just by glancing at it.
The rule you need is this: one visible ridge equals two rows, not one. Garter stitch forms a ridge every time you complete a knit row followed by another knit row, because knitting into a knit row on the right side actually produces a purl bump on the next pass. Those bumps stack in pairs, and the raised ridge you see is the result of two rows working together, not a single row on its own.
How to actually count the ridges
Hold your knitting so you're looking at the fabric edge-on, with the needle at the top and the cast-on edge at the bottom. You'll see a series of horizontal ridges running across the piece, each one slightly raised, with a smoother valley between them. Count only the raised ridges — not the valleys, and not the loops on the needle. Each ridge you count represents two completed rows.
So if you count eight ridges, you've knit sixteen rows, plus however many rows you've worked since the last complete ridge formed (which will be zero or one, since a ridge only appears after a pair). This is where a lot of people go wrong: they count ridges and then forget to double, and end up thinking they're at row 20 when they're actually at row 40. If a pattern tells you to work until the piece measures a certain length, or asks for an exact row count for shaping, this distinction actually matters.
Why this trips people up more than stockinette
In stockinette, the right side and wrong side look completely different, so your eye naturally has a reference point — smooth V's on one side, bumpy purl bars on the other. Garter stitch removes that visual cue entirely, since both sides look identical. There's no 'right side' to orient yourself against, which means the ridge-counting method isn't just easier, it's really the only reliable one that doesn't involve tracking every single row as you go.
The other place people lose the thread is at the very start of a piece. The cast-on edge itself can look like a ridge if you're not used to identifying it, and counting it as your first row will throw your count off by one for the rest of the project. Cast on, then knit your first row — that first row won't produce a ridge on its own, because a ridge needs the second row to complete it. Don't count anything until you see an actual raised bump.
A faster, more reliable method
The ridge-counting method works fine for checking your position after the fact, but it's slow if you're trying to hit an exact row count for a pattern, and it gets genuinely difficult once you're past 60 or 80 rows and the ridges start to blur together, especially in a busy print yarn where the color pattern camouflages the texture.
This is really the scenario a row counter is built for. Instead of reconstructing your position by studying ridges under good light, you tap once every time you finish a row, and the number on screen is just correct. It doesn't matter if your garter panel is fifteen rows or three hundred, or whether the yarn is a solid color or a busy variegated skein that hides texture entirely — the count isn't something you have to see, it's something you already know.
If you're working garter stitch stripes or garter ridges as a border on an otherwise stockinette piece, keeping a running project note of exactly where the garter section started and ended also saves you from having to recount later, which is a common situation when you set a project down mid-border and come back a week later unsure whether you finished the last ridge or not.