The T-shirt is the most common knit garment, and its fabric consumption is usually calculated with a simple body block method. It treats the body and sleeves as rectangles, adds allowances, and converts area to weight using the fabric's GSM.
The formula#
| Symbol | Meaning |
|---|---|
| BL | Body length (high point shoulder to hem), cm |
| SL | Sleeve length, cm |
| a_L | Length allowances for hems and seams, cm |
| ½C | Half chest (chest measurement across the front), cm |
| a_W | Width allowances for side seams, cm |
| GSM | Finished fabric weight, g/m² |
The factor 2 accounts for front and back (and two sleeves side by side in the same width). 12 converts to a dozen, and 10⁷ converts cm² to m² and grams to kilograms.
Why the method works#
In a basic T-shirt, the front and back body panels are each about half chest wide, and the two sleeves laid side by side take roughly the same width. So body length plus sleeve length, times half chest, times two, is a fair estimate of the fabric area. It is an estimate: sleeve shape, neck shape and style details change the real area.
Worked example#
Size M: body length 70 cm, sleeve length 20 cm, half chest 52 cm. Length allowance 8 cm (hem and shoulder seam), width allowance 4 cm (side seams). Fabric 160 GSM.
- Length = 70 + 20 + 8 = 98 cm
- Width = 52 + 4 = 56 cm
- Area × 2 = 98 × 56 × 2 = 10,976 cm²
- kg/dz = 10,976 × 160 × 12 / 10⁷ = 2.107 kg/dz
- With 10% wastage: 2.107 × 1.10 = 2.318 kg/dz
Per garment: 2.318 / 12 = 0.193 kg, or about 193 g.
Neck rib#
Rib for the neck is a different fabric and is calculated separately:
A neck rib 46 cm long and 4 cm wide (cut, folded), in 220 GSM rib: 46 × 4 × 220 × 12 / 10⁷ = 0.049 kg/dz before wastage.
Size ratio#
Consumption varies by size. For an order with a size ratio, calculate consumption for each size and take the weighted average:
| Size | Ratio | kg/dz |
|---|---|---|
| S | 2 | 1.95 |
| M | 4 | 2.11 |
| L | 4 | 2.28 |
| XL | 2 | 2.46 |
Weighted average = (2 × 1.95 + 4 × 2.11 + 4 × 2.28 + 2 × 2.46) / 12 = 2.20 kg/dz before wastage.
Checking the estimate#
Once patterns are made, run a CAD marker and calculate consumption from the marker and GSM. Compare with the body block estimate. If the difference is large, find out why before booking fabric: a different sleeve shape, a wide tube width, or a wrong GSM are common reasons.
Common mistakes#
- Using full chest instead of half chest.
- Using greige GSM instead of finished GSM.
- Forgetting sleeve hem allowance or neck rib.
- Not adding wastage, or adding the same wastage to every fabric regardless of tube width and cutting method.
Use the T-shirt consumption calculator, which shows the full working. For other knits, see knit fabric consumption.
Related guides#
- Fabric consumption: the overall method for knits and wovens.
- Garment costing: using the result in a garment quotation.
- Fabric GSM: measuring the finished GSM used in the formula.
- Single jersey fabric: the fabric most T-shirts are made from.
- Bill of materials: booking fabric and rib from the calculated consumption.
Frequently asked questions#
Does the method work for long sleeve T-shirts? Yes, use the long sleeve length. Check the width too: long sleeves may be wider than half chest at the top, which the rectangle method may not capture.
What about tubular fabric? If the body is cut from tubular fabric without side seams, width allowance changes, and the tube diameter must match the body width. Check with the knitter.