What this checker works out
This checker compares a substitute yarn's metres-per-gram against the original's, works out how many balls of the substitute you would need, and gives a verdict adjusted for how similar the fibres are.
Metres per gram is the number that matters. It describes how thick the yarn actually is, independently of what the band calls it.
Compare yarns
Important
A yarn can match by length but still behave differently because of fibre, construction, drape and washing. Always make a gauge swatch for fitted items.
Before you substitute
- Take length and weight from the ball band of both yarns. Weight categories such as DK and aran are broad and inconsistent between manufacturers.
- A difference of more than about 10% in metres per gram usually means a different gauge and a different fabric.
- Fibre matters as much as thickness. Wool has memory and blocks; cotton drapes and grows; acrylic does neither.
- Whatever the numbers say, swatch before committing.
Two yarns both labelled DK can differ by 20% or more in metres per gram, because the categories are conventions rather than standards and differ between countries. Metres per gram is measurable and comparable; 'DK' is a hint. Substituting on the label alone is the most common route to a project that comes out the wrong size.
How this calculator works
Thickness compared, balls recalculated:
Original total = original length per ball × balls required
Substitute balls = original total ÷ substitute length per ball, rounded up
Metres per gram for each yarn = length ÷ ball weight
Difference = (substitute rate ÷ original rate − 1) × 100The verdict combines the thickness difference with the fibre match. A close thickness with a very different fibre is flagged, because a cotton substitute for wool changes drape, weight and blocking behaviour even at identical gauge.
The ball count assumes the same total length is needed. In practice a thinner yarn worked to the same gauge uses somewhat more, and a thicker one somewhat less.
Worked example: a 20% difference
Using the default figures — original 100 m per 50 g, substitute 120 m per 50 g, 10 balls originally required, similar fibre:
- Original total: 1,000 m
- Substitute balls needed: 9
- Thickness difference: 20% — the substitute is finer
Fewer balls and a thinner yarn, which sounds economical and is a warning. A yarn 20% finer will not reach the pattern's gauge on the same needles, and going up a needle size to compensate produces a looser, drapier fabric than the designer intended. This is a substitution that needs a swatch before any yarn is bought in quantity.
Common mistakes
- Substituting on the weight category alone. DK to DK can be 20% out.
- Ignoring fibre content. Drape, weight and blocking all change with it.
- Buying the full quantity before swatching. The swatch is the test.
- Forgetting that a finer yarn may need more, not less. Gauge, not length, governs.
- Overlooking care requirements. A machine-washable substitute for a hand-wash yarn changes the finished item.
Frequently asked questions
What makes a good substitution?
Similar metres per gram, similar fibre behaviour, and a swatch that matches gauge on a reasonable needle or hook size. Wool for wool is straightforward; wool for cotton is a redesign, however close the numbers. Plant fibres have no elasticity and grow with wear, which changes fit on a garment substantially.
How close is close enough?
Within about 10% on metres per gram is usually workable with a needle size adjustment. Beyond 20% the fabric will be noticeably different whatever you do. Between the two, swatch and judge the fabric rather than the number — a slightly denser or looser fabric may be perfectly acceptable for an accessory and not for a garment.
Can I hold two yarns together?
Yes, and it is a common way to reach a heavier weight from finer yarns. Two strands of 4-ply approximate DK, and two of DK approximate aran, though not exactly. It also allows blending fibres and colours. Swatch as always, since the combined behaviour is not simply the sum of the parts.
What about yardage differences?
Buy by total length rather than by ball count. The checker calculates this, but round up generously — a substitution introduces enough uncertainty that the usual 10% margin is worth increasing to 20%, particularly where the yarn may be hard to obtain later.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — textile fibre labelling and composition requirements
- MoneyHelper — getting value from craft purchases
Sources are checked at publication and can change — how I choose and check references.