Circles, Arcs and Curved Primitives

6 min readUpdated Aug 2026

Use the CURVES group, the second of the sketch toolbar's three, to place anything round: three ways to place a circle, two ways to place an arc, and four composite primitives that would each take a handful of lines and constraints to build by hand. Reach for it inside a sketch whenever the outline you need is not made of straight edges.

Everything the CURVES group draws, in one sketch.

Three circles

All three draw the same circle, starting from three different things you might already know about it.

Which circle to reach for

Tool	Clicks	Use it when
Circle	2 — centre, then the rim	You know where the middle goes
Circle across a diameter	2 — two opposite points	You know what it has to touch, not where its middle is
Circle through 3 points	3 — points on the rim	You know three things it must pass through

The first is the default face of the button, and the other two sit behind its caret. All three end up as the same primitive, a centre point and a radius, so you can still dimension a circle placed by three points by its diameter afterwards.

Circle through three pointsThree points on the rim, and the one circle that satisfies them.

While you drag a circle, an R field follows the cursor. Type a radius, press Enter, and the circle commits at exactly that size. The 2-point circle offers a ⌀ field instead, because a diameter is what that gesture is about.

Two arcs

Which arc to reach for

Tool	Clicks	Use it when
Arc through 3 points	3 — both ends first, then the bulge	You know where it starts and finishes
Arc from centre	3 — centre, start, end	You know the radius

Watch the click order of the 3-point arc. The third click sets the shape, not an endpoint. Draw the two ends, then pull the curve through where you want it.

Arc through three pointsEnds first, bulge last.

Slot

Three clicks: the centre of each end, then the width. What lands in the sketch is two arcs and two rails, already tangent to each other and with the arcs constrained equal. Use it for the shape a screw slides along instead of building it out of four pieces and three constraints.

SlotTwo clicks put the end centres down, and the third sets the width. What lands is two arcs and two rails, already tangent to each other.

Ellipse

Click the centre, then a corner of its bounding box. The longer of the two distances becomes the major axis, which decides the orientation you get. It starts axis-aligned. To turn it, constrain its major-axis line, which the tool draws as real geometry for exactly that purpose.

Unlike a polyline approximation, this extrudes as a true elliptical surface. Nothing downstream has to know it was ever an ellipse.

Rounded rectangle

Find this one with the straight-edge tools, behind the Rectangle caret in SHAPES, because you draw it like a rectangle: two diagonal corners, then a third click for the corner radius, clamped to whatever fits inside the box. It belongs in this article because what you get is a curved primitive: one thing with one radius, rather than four lines and four fillets.

Rounded rectangleThe two corners first, then the radius, dragged until it reads right. It arrives as one primitive with one radius to change later, not as four lines and four arcs.

Water drop

Use the water drop for a round hole you will print on its side.

A round hole printed on its side needs support: past about 45° from vertical, the top of the bore has nothing under it and sags. A teardrop-shaped hole does not. Its walls never exceed 45°, so it bridges itself.

Three clicks: centre, radius, and the direction the tip points. The tip always lands at r·√2 from the centre, which is exactly what makes the walls come out at 45°. You choose where it points, never how far it goes.

Water dropCentre, radius, and the direction of the tip.

It owns a construction circle, which you will not see drawn, because the drop's round side runs exactly along it. It is real geometry, so dimension it by radius or diameter like any other hole. It also owns a construction axis from centre to tip, drawn dashed, which you can constrain to aim it. Drag the tip and the drop rotates. Read Supports and bridging in the printing track for when a hole needs this treatment.

What each tool leaves behind

Curved tools at a glance

Tool	Key	Emits
Circle	C	A circle: centre point plus radius
Circle across a diameter	—	The same, from two rim points
Circle through 3 points	—	The same, fitted to three points
Arc through 3 points	A	An arc, its centre and two endpoints
Arc from centre	—	The same, defined the other way round
Slot	S	2 arcs + 2 rails, equal-radius and tangent
Ellipse	E	A true ellipse plus its two axis lines
Rounded rectangle	—	One derived primitive with a corner radius
Water drop	—	A derived drop, its construction circle and its tip axis

Lines, Rectangles and Polygons covers the straight-edge half of the toolbar, and text, artwork and gears are in Text, Artwork and Gears.

See also

Lines, Rectangles and Polygons: points, lines, rectangles and polygons
Text, Artwork and Gears: text, artwork and gears
Editing Sketch Geometry: construction geometry and the offset tool

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