How to crochet a t-shirt yarn bag from a rectangular base

February 6, 2026

Starting with a rectangular base is one of the most reliable ways to create a sturdy , proportionate, and spacious t-shirt yarn bag. In this tutorial, I'll show you the entire process: from calculating the base, crocheting it with the correct increases, building the sides without distorting the shape, shaping it (tote, crossbody, envelope, etc.), and finishing with handles and closures so it looks truly "store-bought."

This guide is a companion article to our main guide on bag knitting techniques. Go to the complete guide on bag shapes and structures .

Shortcut: If you want a simple bag, start with a box-shaped model (rectangular base + straight sides). If you're looking for a more professional finish, add a rigid base or lining.

Who is this technique for?

The rectangular base is ideal if you want:

A bag with good capacity (it fits more than a round base of the same width).
A stable result : the t-shirt yarn tends to "ask for shape", and the rectangular base helps.
Versatile styles : tote, crossbody, envelope, mini bag…

If you'd like to see other constructions (round base, oval base or bag knitted in one piece), we have it explained in the main guide: see all the ways to knit a bag .

Recommended materials

Essentials

T-shirt yarn (preferably of uniform thickness so that the base is even).
Crochet hook appropriate to the thickness of the t-shirt yarn (as a reference: 8–10 mm usually works well for t-shirt yarn).
Marker to indicate the start of a lap.
Scissors and a yarn needle for finishing.

Optional features for a "pro" finish

Rigid base (plastic canvas, thin PVC or purchased base) so that the bag maintains its shape.
Handles (adjustable shoulder strap, hand handles, rings, carabiners).
Lining (cotton fabric or fine canvas) if you're looking for more security and structure.
Closure : magnetic button, zipper, snap, flap…

Idealium Tip: If you notice your t-shirt yarn "opening up" at the base, try crocheting a little tighter or using a smaller crochet hook size. If the base is arching upwards, you're either crocheting too tightly or you need to add more increases at the corners.

How to calculate the measurements of a rectangular base

Before you start knitting, decide on the type of bag and translate that into measurements:

Indicative measurements according to style

Mini bag / envelope type: base 18–22 cm (length) × 6–8 cm (width).
Medium shoulder bag: base 24–28 cm × 8–10 cm.
Tote / large handbag: base 30–36 cm × 10–12 cm.

Quick rule for “beautiful proportions”

A rectangular base is usually balanced when the length is approximately 2.5–3 times the width . For example: 27 × 9, 30 × 10, 33 × 11…

Step by step: knit the rectangular base

The classic rectangular base is constructed as follows: you make a chain and then crochet around that chain, increasing at the ends to form the corner "curves." With t-shirt yarn, the single crochet stitch usually works particularly well for a compact and stable base.

diagram for how to knit a rectangular base

1) Cast the initial chain (the length measurement)

Make a chain that gives you the desired length of the base (not counting the height of the stitch).
For reference, measure the chain flat on the table to check centimeters.

2) Round 1: knit one side, increase at the end and return along the other side

Crochet single crochet stitches along the chain (one in each chain).
At the last point of the chain, crochet 3 single crochet stitches in the same place (this forms the first “corner”).
Turn the work over and crochet single crochet stitches on the other side of the chain (in the “holes” of the chain).
When you reach the other end, knit 3 single crochet stitches in the same stitch (second corner).
Join with a slip stitch if you are knitting in the round, or continue in a spiral marking the beginning of the round.

3) Rounds 2 onwards: increases only at corners

Here's the secret to getting a flat base: increase only at the corners . The sides of the body are knitted "normally," and at each corner you make evenly spaced increases.

Easy option (very common): at each corner, increase by knitting 3 stitches in the center stitch of the corner on each round.
Finer option (distributed): in each corner, distribute increases: for example, in the curved area you knit (2 stitches in 1) + 1 + (2 stitches in 1) .

The important thing: the base should lie flat when placed on the table. If it wavy, there are too many increases. If it arches like a "bowl," there aren't enough increases.

4) When do I stop growing?

By the time your base reaches the desired width (8 cm, 10 cm, 12 cm…). The length is determined by the initial chain: the base grows mainly in width with each round.

Visual trick: lay the base on and look at the corners. They should look “rounded but taut.” If you see “flowering” or waves, reduce the increases in the next round.

Raise the walls without opening the base

Once the base is the right size, it's time to "lift" it. This step determines whether the bag looks sturdy or opens up and loses its shape.

Option A: Backstitch Transition Loop (BLO)

Work a full round using single crochet stitches in the back loop only. This creates a neat "fold" and raises the wall. It's one of the most effective techniques for working with t-shirt yarn.

Option B: slip stitch round

A slip stitch round also creates a very sharp cut (and reinforces), but it can be stiffer.

Next: straight walls

From there, knit in the round (or spiral) without increases to build height. For a medium-sized bag, 18–22 cm in height usually works well, but style is key.

How to shape the bag (straight, trapezoid, envelope)

Straight “box” style bag

The simplest method: after the transition round, knit straight walls without increases or decreases. Result: a stable and very useful bag.

knitted bag with t-shirt yarn, rectangular base

Trapezoid-shaped bag (wider at the bottom, narrower at the top)

To narrow, make decreases distributed every X rounds (for example, 4 decreases per round, every 2-3 rounds), always symmetrical (front/back/sides).

Envelope-style bag (shorter, with a flap)

Knit a shorter length and finish with a firm turn. The flap can be a separate piece or an extension of the back panel.

Tips for added structure (rigid base, reinforcements)

Rigid base inside

If your bag is going to carry weight (phone, wallet, keys, etc.), a rigid base makes all the difference. You can:

Cut out a base (plastic canvas/thin PVC) of the exact size and put it inside.
Cover it with fabric so that the yarn doesn't rub against it.
Secure it with a few hand stitches or by leaving it "fitted" with the lining.

Perforated base for bags

Reinforcement of the upper edge

To prevent it from opening, finish with 1-2 firmer rounds (tight single crochet or slip stitch) and, if the bag requires it, add a strip of cord or ribbon inside.

Handles: where to place them and how to attach them (tote and shoulder bag)

Handbag handles are "half aesthetics and half engineering." A beautiful handbag can be uncomfortable if the handle isn't properly positioned or secured.

Shoulder bag (long strap)

Placement: It usually looks best on the sides, a little below the top edge (1–2 cm).
Fixing: with rings + carabiner, or sewn with reinforcement (several firm passes).
Tip: If the bag is heavy, add a small piece of reinforcement inside (a strip of strong fabric or leather).

Tote bag handles

Placement: front and back, centered, leaving room so that the bag does not "close" when picked up.
Fixing: ideal with hardware (rings) or with reinforced stitching + inner piece.

Tip: Before sewing the final stitches, secure the handles with markers or safety pins, try on the bag, and see how it hangs. Adjusting 1 cm now saves you from having to unpick it later.

Closure and lining: when it's worth it

Easy closures

Magnetic button: quick and clean, ideal for totes.
Button + loop: very nice on an envelope bag.
Zipper: more laborious, but provides total security.

Hand-crocheted clutch bag made with t-shirt yarn

Lining yes or no?

If the stitching is very loose or the bag will be used for small items, a lining is highly recommended. It also helps the bag maintain its shape.

Professional finishes

Discreet finishing: hide threads on the inside with a tapestry needle, following the direction of the fabric.
Firm top edge: 1-2 tighter turns so it doesn't give way.
Label: If you use a sewn-in label, place it before the lining to hide internal seams.
Gentle blocking: in t-shirt yarn, it is usually enough to "shape" it with your hands and let it rest, without excess water.

Common mistakes and how to avoid them

1) The base is wavy

This usually happens due to too many increases at the corners. Solution: reduce the increases in the next round or distribute them more evenly.

2) The base becomes a bowl

You're not adding enough increases (or you're knitting too tightly). Solution: Add corner increases in the next round or loosen the tension.

3) The walls “open up” and the bag loses its shape

A transition round (BLO or slip stitch) is missing, or the top edge is loose. Solution: Add a transition round and reinforce the edge.

4) The bag tilts or becomes deformed to one side

This happens if the decreases/increases are not symmetrical or if you work in a spiral without controlling the start of the turn. Solution: Use a stitch marker and distribute the changes across 4 "cardinal" points.

Frequently Asked Questions

Can I make it with cord instead of t-shirt yarn?

Yes, but the cord usually creates a more defined fabric and less "volume." The structure is achieved the same way, although you might need more turns for the same height.

Which spot is best for the base and walls?

For the base, single crochet is usually the most stable stitch . For walls, single crochet or half double crochet (if you want to speed things up) are good options. If you're looking for a texture with pronounced "channels," try crocheting several rows in the back loop only.

How do I know if my bag will hold weight?

The key is: a compact base, a smooth transition, a reinforced top edge, and securely attached handles. Adding a rigid base and lining further increases the durability.

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