Increase and decrease maths for spheres, eggs, cones and tubes

Updated 13 November 2026 · 8 min read · by FamCraftsShop

The simple arithmetic behind amigurumi shapes: how many increases to make, how many plain rounds to work, and how to size a piece in centimetres.

You don't need a pattern to make a ball, an egg or a cone. All amigurumi shapes come from three moves — increase, work plain, decrease — and a little arithmetic tells you how many of each you need. This guide covers the numbers, then shows how to turn a size in centimetres into rounds.

Writing an even increase round

To grow from a count A to a count B in one round, you need B − A increases, spread as evenly as you can. Divide the old count by the number of increases to find the size of each repeat:

• from 24 to 30: 6 increases, 24 ÷ 6 = 4 stitches per repeat, so each repeat is 3 sc and 1 inc: (3 sc, inc) x6 (30).
• from 30 to 36: 6 increases, 30 ÷ 6 = 5, so (4 sc, inc) x6 (36).
• from 18 to 21: 3 increases, 18 ÷ 3 = 6, so (5 sc, inc) x3 (21).

When it doesn't divide evenly, split the leftover stitches across the round. From 20 to 26 needs 6 increases; 20 ÷ 6 is 3 remainder 2. Make four repeats of 2 sc, inc and two of 3 sc, inc: that uses 4 × 3 + 2 × 4 = 20 and makes 4 × 4 + 2 × 5 = 26.

Writing an even decrease round

Decreases work the same way in reverse. To go from A down to B, you need A − B decreases. Each decrease uses two stitches and makes one.

• from 36 to 30: 6 decreases, 36 ÷ 6 = 6 stitches per repeat, so (4 sc, dec) x6 (30).
• from 24 to 18: (2 sc, dec) x6 (18).
• from 12 to 6: 6 dec (6).

A simple check: in any repeat, (sc + inc + 2 × dec) is what it uses, and (sc + 2 × inc + dec) is what it makes.

The sphere

A ball is the shape every other one is built from. In single crochet with 6 increases per round:

1. Increase every round until you reach the widest count. If you stop at 6 × n stitches, that took n rounds (counting the magic ring as round 1).
2. Work plain rounds. For a round ball, work roughly n − 1 to n + 1 plain rounds.
3. Decrease 6 per round, mirroring the increases, back down to 6, and close.

The reason: a sphere's height (half its outline, from pole to pole) is about half its circumference. With stitches as tall as they are wide, a ball whose widest round has 6n stitches should be about 3n rounds tall in total — n increasing, n plain-ish, n decreasing. Tight tension, yarn and stuffing change this a little, which is why it's a range.

R1: MR 6 (6)
R2: 6 inc (12)
R3: (sc, inc) x6 (18)
R4: (2 sc, inc) x6 (24)
R5: (3 sc, inc) x6 (30)
R6: (4 sc, inc) x6 (36)
R7-12: sc around (36)
R13: (4 sc, dec) x6 (30)
R14: (3 sc, dec) x6 (24)
R15: (2 sc, dec) x6 (18)
R16: (sc, dec) x6 (12)
R17: 6 dec (6)

The egg

An egg is a sphere with more plain rounds, and often a slower top. Work more plain rounds than increase rounds — 1.5 to 2 times as many — then decrease. For a pointier top, decrease every other round instead of every round. Penguin and bird bodies, and most "bean" characters, are eggs.

The cone

A cone grows steadily from a point. Instead of 6 increases every round, increase less often:

• 6 increases every second round gives a fairly wide cone — a gnome hat or a party hat.
• 3 increases every round gives a slim cone — a carrot, a horn, a beak.
• 3 increases every second or third round gives a long, thin spike.

The tube

Stop increasing and every round is plain: you get a cylinder. Arms, legs, tails and stems are tubes. A tube's circumference is set by the stitch count, so a 12-stitch arm is about twice as wide as a 6-stitch one. Close the end with a round of decreases, or leave it open to sew on.

From centimetres to stitches

To size a piece, you need your gauge: how wide one stitch is in your yarn and hook. A quick way to find it is to make a 20-stitch tube or a flat swatch and measure.

As a starting point, single crochet in amigurumi is roughly:

Then: widest stitch count ≈ (diameter × 3.14) ÷ stitch width. For a 6 cm ball in DK at 0.43 cm a stitch: 6 × 3.14 ÷ 0.43 ≈ 44 stitches. Round to a multiple of 6 — 42 — and you need 7 increase rounds.

StitchCheck's shape builder does this calculation: pick ball, egg, tube or cone, type a width and height in centimetres, and it writes the rounds. You can also draw an outline and it works out where the increases and decreases go.