The basic difference
Flat knitting means you work back and forth in rows: knit to the end, turn your work, knit back across. You work the right side, then the wrong side, alternating until the piece is complete. This produces a flat piece of fabric that is typically seamed to other flat pieces to create a three-dimensional garment.
Circular knitting (or knitting in the round) means you work continuously in one direction, always working the right side. The fabric forms a tube as you go. There are no seams in the traditional sense — the tube is the garment, or part of it.
Both methods produce excellent knitting. Neither is objectively better. The right choice depends on the project, your preferences, and what you want to get out of the process.
When flat knitting makes sense
Traditional construction methods
Many classic sweater patterns — particularly British, Scandinavian, and mid-20th century American patterns — are written as flat pieces: front, back, two sleeves, seamed together. If you're working from a heritage pattern or following a traditional construction, flat knitting is often the assumed method.
Seaming as a feature
Some knitters genuinely enjoy finishing — the process of blocking each piece flat, seaming shoulder seams with Kitchener stitch, setting in sleeves with mattress stitch. The seams add structure to the garment: set-in sleeve seams hold the sleeve cap in place; shoulder seams prevent stretching over time. For tailored, structured garments, seamed construction is sometimes preferable.
Easier stitch pattern management for some
A few stitch patterns are arguably easier to work flat. Garter stitch is always "knit every row" on flat fabric. Moss stitch and seed stitch on flat fabric follow a simple rule: k the purls, p the knits. In the round, these same patterns require working against the grain on every round, which some knitters find confusing.
Easier tension calibration
Many knitters' gauge swatches are worked flat because it's simpler to measure. If you have a significant gauge difference between your knit rows and purl rows (common in beginners), flat knitting gives you a more consistent average tension than in-the-round knitting.
When circular knitting makes sense
Tube-shaped items
Hats, socks, mittens, cowls, and tubular bag bodies are all naturally cylindrical. Working them in the round means no seams to create and close — you simply knit a tube and shape it as required. The construction logic is simpler.
Seamless garment construction
Modern seamless sweater constructions — bottom-up or top-down, worked in the round — are enormously popular. The entire body is a tube, sleeves are tubes, and the only "seaming" is joining underarm stitches with Kitchener or a three-needle bind off. The result is a garment with no visible seam lines, which appeals to knitters who dislike finishing.
Stocking stitch knitters who prefer to knit
In the round, stocking stitch means knitting every round — no purling back. Many knitters find their knit-only gauge is faster and more even than their alternating-rows gauge. For a large stocking stitch project like a sweater body, this can mean a noticeably quicker and more consistent result.
Colourwork (Fair Isle, stranded)
Stranded colourwork is much easier in the round than flat. When you're carrying two or more colours, always working the right side means you can always see the pattern forming and always manage the strands from the same side. Knitting colourwork flat means working the chart in reverse on wrong-side rows — more cognitive load and more opportunities for error.
The honest trade-offs
Seaming
Flat knitting requires seaming. If you hate finishing, this is a genuine obstacle — many knitters have boxes of flat-knitted pieces waiting for seams that never get sewn. In-the-round construction eliminates seaming (mostly). If you love finishing, seaming is not a problem.
Needle management
Working in the round requires circular needles or DPNs — you have to be comfortable managing the cable or the double-pointed set. This is a small learning curve, but a real one for beginners.
Tension differences
Some knitters have noticeably different tension for knit vs purl stitches. In the round (for stocking stitch), they knit every row and their tension is consistent. Working flat, the alternating rows can create slight horizontal striping. If this is you, in-the-round stocking stitch will look more even.
Fitting adjustments
Some knitters find it easier to try on and adjust a garment in progress when it's worked in the round (you can put the live stitches on a long thread and slip it over your head). Others find flat pieces easier to lay out and measure. This is personal preference.
The verdict
There is no correct answer. Many experienced knitters do both — flat construction for tailored garments, in-the-round for hats, socks, and casual sweaters. Some knitters work almost exclusively in the round because they dislike seaming; others work almost entirely flat because they find in-the-round needle management fiddly. Try both, notice what feels easier and more enjoyable, and let the project type guide the choice when the preference isn't clear.